Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object. It is an important part of OOPs (Object Oriented programming system).
The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also.
Inheritance represents the IS-A relationship which is also known as a parent-child relationship.
Why use inheritance in java
For Method Overriding (so runtime polymorphism can be achieved).
For Code Reusability.
Terms used in Inheritance
Class: A class is a group of objects which have common properties. It is a template or blueprint from which objects are created.
Sub Class/Child Class: Subclass is a class which inherits the other class. It is also called a derived class, extended class, or child class.
Super Class/Parent Class: Superclass is the class from where a subclass inherits the features. It is also called a base class or a parent class.
Reusability: As the name specifies, reusability is a mechanism which facilitates you to reuse the fields and methods of the existing class when you create a new class. You can use the same fields and methods already defined in the previous class.
The syntax of Java Inheritance
class Subclass-name extends Superclass-name
{
//methods and fields
}
The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of “extends” is to increase the functionality.
In the terminology of Java, a class which is inherited is called a parent or superclass, and the new class is called child or subclass.
Java Inheritance Example
As displayed in the above figure, Programmer is the subclass and Employee is the superclass. The relationship between the two classes is Programmer IS-A Employee. It means that Programmer is a type of Employee.
class Employee{
float salary=40000;
}
class Programmer extends Employee{
int bonus=10000;
public static void main(String args[]){
Programmer p=new Programmer();
System.out.println("Programmer salary is:"+p.salary);
System.out.println("Bonus of Programmer is:"+p.bonus);
}
}
Programmer salary is:40000.0
Bonus of programmer is:10000
In the above example, Programmer object can access the field of own class as well as of Employee class i.e. code reusability.
Types of inheritance in java
On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical.
In java programming, multiple and hybrid inheritance is supported through interface only. We will learn about interfaces later.
Note: Multiple inheritance is not supported in Java through class.
When one class inherits multiple classes, it is known as multiple inheritance. For Example:
Single Inheritance Example
When a class inherits another class, it is known as a single inheritance. In the example given below, Dog class inherits the Animal class, so there is the single inheritance.
File: TestInheritance.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class TestInheritance{
public static void main(String args[]){
Dog d=new Dog();
d.bark();
d.eat();
}}
Output:
barking...
eating...
Multilevel Inheritance Example
When there is a chain of inheritance, it is known as multilevel inheritance. As you can see in the example given below, BabyDog class inherits the Dog class which again inherits the Animal class, so there is a multilevel inheritance.
File: TestInheritance2.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class BabyDog extends Dog{
void weep(){System.out.println("weeping...");}
}
class TestInheritance2{
public static void main(String args[]){
BabyDog d=new BabyDog();
d.weep();
d.bark();
d.eat();
}}
Output:
weeping...
barking...
eating...
Hierarchical Inheritance Example
When two or more classes inherits a single class, it is known as hierarchical inheritance. In the example given below, Dog and Cat classes inherits the Animal class, so there is hierarchical inheritance.
File: TestInheritance3.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class Cat extends Animal{
void meow(){System.out.println("meowing...");}
}
class TestInheritance3{
public static void main(String args[]){
Cat c=new Cat();
c.meow();
c.eat();
//c.bark();//C.T.Error
}}
Output:
meowing...
eating...
Q) Why multiple inheritance is not supported in java?
To reduce the complexity and simplify the language, multiple inheritance is not supported in java.
Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.
Since compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have same method or different, there will be compile time error.
class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
public static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}
There can be a lot of usage of Java this keyword. In Java, this is a reference variable that refers to the current object.
Usage of Java this keyword
Here is given the 6 usage of java this keyword.
this can be used to refer current class instance variable.
this can be used to invoke current class method (implicitly)
this() can be used to invoke current class constructor.
this can be passed as an argument in the method call.
this can be passed as argument in the constructor call.
this can be used to return the current class instance from the method.
Suggestion: If you are beginner to java, lookup only three usages of this keyword.
1) this: to refer current class instance variable
The this keyword can be used to refer current class instance variable. If there is ambiguity between the instance variables and parameters, this keyword resolves the problem of ambiguity.
Understanding the problem without this keyword
Let’s understand the problem if we don’t use this keyword by the example given below:
class Student{
int rollno;
String name;
float fee;
Student(int rollno,String name,float fee){
rollno=rollno;
name=name;
fee=fee;
}
void display(){System.out.println(rollno+" "+name+" "+fee);}
}
class TestThis1{
public static void main(String args[]){
Student s1=new Student(111,"ankit",5000f);
Student s2=new Student(112,"sumit",6000f);
s1.display();
s2.display();
}}
Output:
0 null 0.0
0 null 0.0
In the above example, parameters (formal arguments) and instance variables are same. So, we are using this keyword to distinguish local variable and instance variable.
Solution of the above problem by this keyword
class Student{
int rollno;
String name;
float fee;
Student(int rollno,String name,float fee){
this.rollno=rollno;
this.name=name;
this.fee=fee;
}
void display(){System.out.println(rollno+" "+name+" "+fee);}
}
class TestThis2{
public static void main(String args[]){
Student s1=new Student(111,"ankit",5000f);
Student s2=new Student(112,"sumit",6000f);
s1.display();
s2.display();
}}
Output:
111 ankit 5000.0
112 sumit 6000.0
If local variables(formal arguments) and instance variables are different, there is no need to use this keyword like in the following program:
Program where this keyword is not required
class Student{
int rollno;
String name;
float fee;
Student(int r,String n,float f){
rollno=r;
name=n;
fee=f;
}
void display(){System.out.println(rollno+" "+name+" "+fee);}
}
class TestThis3{
public static void main(String args[]){
Student s1=new Student(111,"ankit",5000f);
Student s2=new Student(112,"sumit",6000f);
s1.display();
s2.display();
}}
Output:
111 ankit 5000.0
112 sumit 6000.0
It is better approach to use meaningful names for variables. So we use same name for instance variables and parameters in real time, and always use this keyword.
2) this: to invoke current class method
You may invoke the method of the current class by using the this keyword. If you don’t use the this keyword, compiler automatically adds this keyword while invoking the method. Let’s see the example
class A{
void m(){System.out.println("hello m");}
void n(){
System.out.println("hello n");
//m();//same as this.m()
this.m();
}
}
class TestThis4{
public static void main(String args[]){
A a=new A();
a.n();
}}
Output:
hello n
hello m
3) this() : to invoke current class constructor
The this() constructor call can be used to invoke the current class constructor. It is used to reuse the constructor. In other words, it is used for constructor chaining.
Calling default constructor from parameterized constructor:
class A{
A(){System.out.println("hello a");}
A(int x){
this();
System.out.println(x);
}
}
class TestThis5{
public static void main(String args[]){
A a=new A(10);
}}
Output:
hello a
10
Calling parameterized constructor from default constructor:
class A{
A(){
this(5);
System.out.println("hello a");
}
A(int x){
System.out.println(x);
}
}
class TestThis6{
public static void main(String args[]){
A a=new A();
}}
Output:
5
hello a
Real usage of this() constructor call
The this() constructor call should be used to reuse the constructor from the constructor. It maintains the chain between the constructors i.e. it is used for constructor chaining. Let’s see the example given below that displays the actual use of this keyword.
Compile Time Error: Call to this must be first statement in constructor
4) this: to pass as an argument in the method
The this keyword can also be passed as an argument in the method. It is mainly used in the event handling. Let’s see the example:
class S2{
void m(S2 obj){
System.out.println("method is invoked");
}
void p(){
m(this);
}
public static void main(String args[]){
S2 s1 = new S2();
s1.p();
}
}
Output:
method is invoked
Application of this that can be passed as an argument:
In event handling (or) in a situation where we have to provide reference of a class to another one. It is used to reuse one object in many methods.
5) this: to pass as argument in the constructor call
We can pass the this keyword in the constructor also. It is useful if we have to use one object in multiple classes. Let’s see the example:
class B{
A4 obj;
B(A4 obj){
this.obj=obj;
}
void display(){
System.out.println(obj.data);//using data member of A4 class
}
}
class A4{
int data=10;
A4(){
B b=new B(this);
b.display();
}
public static void main(String args[]){
A4 a=new A4();
}
}
Output:10
6) this keyword can be used to return current class instance
We can return this keyword as an statement from the method. In such case, return type of the method must be the class type (non-primitive). Let’s see the example:
Syntax of this that can be returned as a statement
return_type method_name(){
return this;
}
Example of this keyword that you return as a statement from the method
class A{
A getA(){
return this;
}
void msg(){System.out.println("Hello java");}
}
class Test1{
public static void main(String args[]){
new A().getA().msg();
}
}
Output:
Hello java
Proving this keyword
Let’s prove that this keyword refers to the current class instance variable. In this program, we are printing the reference variable and this, output of both variables are same.
class A5{
void m(){
System.out.println(this);//prints same reference ID
}
public static void main(String args[]){
A5 obj=new A5();
System.out.println(obj);//prints the reference ID
obj.m();
}
}
The static keyword in Java is used for memory management mainly. We can apply static keyword with variables, methods, blocks and nested classes. The static keyword belongs to the class than an instance of the class.
The static can be:
Variable (also known as a class variable)
Method (also known as a class method)
Block
Nested class
1) Java static variable
If you declare any variable as static, it is known as a static variable.
The static variable can be used to refer to the common property of all objects (which is not unique for each object), for example, the company name of employees, college name of students, etc.
The static variable gets memory only once in the class area at the time of class loading.
Advantages of static variable
It makes your program memory efficient (i.e., it saves memory).
Understanding the problem without static variable
class Student{
int rollno;
String name;
String college="ITS";
}
Suppose there are 500 students in my college, now all instance data members will get memory each time when the object is created. All students have its unique rollno and name, so instance data member is good in such case. Here, “college” refers to the common property of all objects. If we make it static, this field will get the memory only once.
Java static property is shared to all objects.
Example of static variable
//Java Program to demonstrate the use of static variable
class Student{
int rollno;//instance variable
String name;
static String college ="ITS";//static variable
//constructor
Student(int r, String n){
rollno = r;
name = n;
}
//method to display the values
void display (){System.out.println(rollno+" "+name+" "+college);}
}
//Test class to show the values of objects
public class TestStaticVariable1{
public static void main(String args[]){
Student s1 = new Student(111,"Karan");
Student s2 = new Student(222,"Aryan");
//we can change the college of all objects by the single line of code
//Student.college="BBDIT";
s1.display();
s2.display();
}
}
Output:
111 Karan ITS
222 Aryan ITS
Program of the counter without static variable
In this example, we have created an instance variable named count which is incremented in the constructor. Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable. If it is incremented, it won’t reflect other objects. So each object will have the value 1 in the count variable.
//Java Program to demonstrate the use of an instance variable
//which get memory each time when we create an object of the class.
class Counter{
int count=0;//will get memory each time when the instance is created
Counter(){
count++;//incrementing value
System.out.println(count);
}
public static void main(String args[]){
//Creating objects
Counter c1=new Counter();
Counter c2=new Counter();
Counter c3=new Counter();
}
}
Output:
1
1
1
Program of counter by static variable
As we have mentioned above, static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value.
//Java Program to illustrate the use of static variable which
//is shared with all objects.
class Counter2{
static int count=0;//will get memory only once and retain its value
Counter2(){
count++;//incrementing the value of static variable
System.out.println(count);
}
public static void main(String args[]){
//creating objects
Counter2 c1=new Counter2();
Counter2 c2=new Counter2();
Counter2 c3=new Counter2();
}
}
Output:
1
2
3
2) Java static method
If you apply static keyword with any method, it is known as static method.
A static method belongs to the class rather than the object of a class.
A static method can be invoked without the need for creating an instance of a class.
A static method can access static data member and can change the value of it.
Example of static method
//Java Program to demonstrate the use of a static method.
class Student{
int rollno;
String name;
static String college = "ITS";
//static method to change the value of static variable
static void change(){
college = "BBDIT";
}
//constructor to initialize the variable
Student(int r, String n){
rollno = r;
name = n;
}
//method to display values
void display(){System.out.println(rollno+" "+name+" "+college);}
}
//Test class to create and display the values of object
public class TestStaticMethod{
public static void main(String args[]){
Student.change();//calling change method
//creating objects
Student s1 = new Student(111,"Karan");
Student s2 = new Student(222,"Aryan");
Student s3 = new Student(333,"Sonoo");
//calling display method
s1.display();
s2.display();
s3.display();
}
}
Output:
111 Karan BBDIT
222 Aryan BBDIT
333 Sonoo BBDIT
Another example of a static method that performs a normal calculation
//Java Program to get the cube of a given number using the static method
class Calculate{
static int cube(int x){
return x*x*x;
}
public static void main(String args[]){
int result=Calculate.cube(5);
System.out.println(result);
}
}
Output:125
Restrictions for the static method
There are two main restrictions for the static method. They are:
The static method can not use non static data member or call non-static method directly.
this and super cannot be used in static context.
class A{
int a=40;//non static
public static void main(String args[]){
System.out.println(a);
}
}
Output:Compile Time Error
Q) Why is the Java main method static?
Ans) It is because the object is not required to call a static method. If it were a non-static method, JVM creates an object first then call main() method that will lead the problem of extra memory allocation.
3) Java static block
Is used to initialize the static data member.
It is executed before the main method at the time of classloading.
Example of static block
class A2{
static{System.out.println("static block is invoked");}
public static void main(String args[]){
System.out.println("Hello main");
}
}
Output:static block is invoked
Hello main
Q) Can we execute a program without main() method?
Ans) No, one of the ways was the static block, but it was possible till JDK 1.6. Since JDK 1.7, it is not possible to execute a Java class without the main method.
class A3{
static{
System.out.println("static block is invoked");
System.exit(0);
}
}
Output:
static block is invoked
Since JDK 1.7 and above, output would be:
Error: Main method not found in class A3, please define the main method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling constructor, memory for the object is allocated in the memory.
It is a special type of method which is used to initialize the object.
Every time an object is created using the new() keyword, at least one constructor is called.
It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.
There are two types of constructors in Java: no-arg constructor, and parameterized constructor.
Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn’t have any.
Rules for creating Java constructor
There are two rules defined for the constructor.
Constructor name must be the same as its class name
A Constructor must have no explicit return type
A Java constructor cannot be abstract, static, final, and synchronized
Note: We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected, public or default constructor in Java.
Types of Java constructors
There are two types of constructors in Java:
Default constructor (no-arg constructor)
Parameterized constructor
Java Default Constructor
A constructor is called “Default Constructor” when it doesn’t have any parameter.
Syntax of default constructor:
<class_name>(){}
Example of default constructor
In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation.
//Java Program to create and call a default constructor
class Bike1{
//creating a default constructor
Bike1(){System.out.println("Bike is created");}
//main method
public static void main(String args[]){
//calling a default constructor
Bike1 b=new Bike1();
}
}
Output:
Bike is created
Rule: If there is no constructor in a class, compiler automatically creates a default constructor.
Q) What is the purpose of a default constructor?
The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.
Example of default constructor that displays the default values
//Let us see another example of default constructor
//which displays the default values
class Student3{
int id;
String name;
//method to display the value of id and name
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
//creating objects
Student3 s1=new Student3();
Student3 s2=new Student3();
//displaying values of the object
s1.display();
s2.display();
}
}
Output:
0 null
0 null
Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.
Java Parameterized Constructor
A constructor which has a specific number of parameters is called a parameterized constructor.
Why use the parameterized constructor?
The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.
Example of parameterized constructor
In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.
//Java Program to demonstrate the use of the parameterized constructor.
class Student4{
int id;
String name;
//creating a parameterized constructor
Student4(int i,String n){
id = i;
name = n;
}
//method to display the values
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
//creating objects and passing values
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
//calling method to display the values of object
s1.display();
s2.display();
}
}
Output:
111 Karan
222 Aryan
Constructor Overloading in Java
In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.
Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.
Example of Constructor Overloading
//Java program to overload constructors
class Student5{
int id;
String name;
int age;
//creating two arg constructor
Student5(int i,String n){
id = i;
name = n;
}
//creating three arg constructor
Student5(int i,String n,int a){
id = i;
name = n;
age=a;
}
void display(){System.out.println(id+" "+name+" "+age);}
public static void main(String args[]){
Student5 s1 = new Student5(111,"Karan");
Student5 s2 = new Student5(222,"Aryan",25);
s1.display();
s2.display();
}
}
Output:
111 Karan 0
222 Aryan 25
Difference between constructor and method in Java
There are many differences between constructors and methods. They are given below.
Java Constructor
Java Method
A constructor is used to initialize the state of an object.
A method is used to expose the behavior of an object.
A constructor must not have a return type.
A method must have a return type.
The constructor is invoked implicitly.
The method is invoked explicitly.
The Java compiler provides a default constructor if you don’t have any constructor in a class.
The method is not provided by the compiler in any case.
The constructor name must be same as the class name.
The method name may or may not be same as the class name.
Java Copy Constructor
There is no copy constructor in Java. However, we can copy the values from one object to another like copy constructor in C++.
There are many ways to copy the values of one object into another in Java. They are:
By constructor
By assigning the values of one object into another
By clone() method of Object class
In this example, we are going to copy the values of one object into another using Java constructor.
//Java program to initialize the values from one object to another object.
class Student6{
int id;
String name;
//constructor to initialize integer and string
Student6(int i,String n){
id = i;
name = n;
}
//constructor to initialize another object
Student6(Student6 s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student6 s1 = new Student6(111,"Karan");
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}
Output:
111 Karan
111 Karan
Copying values without constructor
We can copy the values of one object into another by assigning the objects values to another object. In this case, there is no need to create the constructor.
class Student7{
int id;
String name;
Student7(int i,String n){
id = i;
name = n;
}
Student7(){}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student7 s1 = new Student7(111,"Karan");
Student7 s2 = new Student7();
s2.id=s1.id;
s2.name=s1.name;
s1.display();
s2.display();
}
}
In general, a method is a way to perform some task. Similarly, the method in Java is a collection of instructions that performs a specific task. It provides the reusability of code. We can also easily modify code using methods. In this section, we will learn what is a method in Java, types of methods, method declaration, and how to call a method in Java.
What is a method in Java?
A method is a block of code or collection of statements or a set of code grouped together to perform a certain task or operation. It is used to achieve the reusability of code. We write a method once and use it many times. We do not require to write code again and again. It also provides the easy modification and readability of code, just by adding or removing a chunk of code. The method is executed only when we call or invoke it.
The most important method in Java is the main() method. If you want to read more about the main() method, go through the link https://www.javatpoint.com/java-main-method.
Method Declaration
The method declaration provides information about method attributes, such as visibility, return-type, name, and arguments. It has six components that are known as method header, as we have shown in the following figure.
Method Signature: Every method has a method signature. It is a part of the method declaration. It includes the method name and parameter list.
Access Specifier: Access specifier or modifier is the access type of the method. It specifies the visibility of the method. Java provides four types of access specifier:
Public: The method is accessible by all classes when we use public specifier in our application.
Private: When we use a private access specifier, the method is accessible only in the classes in which it is defined.
Protected: When we use protected access specifier, the method is accessible within the same package or subclasses in a different package.
Default: When we do not use any access specifier in the method declaration, Java uses default access specifier by default. It is visible only from the same package only.
Return Type: Return type is a data type that the method returns. It may have a primitive data type, object, collection, void, etc. If the method does not return anything, we use void keyword.
Method Name: It is a unique name that is used to define the name of a method. It must be corresponding to the functionality of the method. Suppose, if we are creating a method for subtraction of two numbers, the method name must be subtraction(). A method is invoked by its name.
Parameter List: It is the list of parameters separated by a comma and enclosed in the pair of parentheses. It contains the data type and variable name. If the method has no parameter, left the parentheses blank.
Method Body: It is a part of the method declaration. It contains all the actions to be performed. It is enclosed within the pair of curly braces.
Naming a Method
While defining a method, remember that the method name must be a verb and start with a lowercase letter. If the method name has more than two words, the first name must be a verb followed by adjective or noun. In the multi-word method name, the first letter of each word must be in uppercase except the first word. For example:
It is also possible that a method has the same name as another method name in the same class, it is known as method overloading.
Types of Method
There are two types of methods in Java:
Predefined Method
User-defined Method
Predefined Method
In Java, predefined methods are the method that is already defined in the Java class libraries is known as predefined methods. It is also known as the standard library method or built-in method. We can directly use these methods just by calling them in the program at any point. Some pre-defined methods are length(), equals(), compareTo(), sqrt(), etc. When we call any of the predefined methods in our program, a series of codes related to the corresponding method runs in the background that is already stored in the library.
Each and every predefined method is defined inside a class. Such as print() method is defined in the java.io.PrintStream class. It prints the statement that we write inside the method. For example, print(“Java”), it prints Java on the console.
Let’s see an example of the predefined method.
Demo.java
public class Demo
{
public static void main(String[] args)
{
// using the max() method of Math class
System.out.print("The maximum number is: " + Math.max(9,7));
}
}
Output:
The maximum number is: 9
In the above example, we have used three predefined methods main(), print(), and max(). We have used these methods directly without declaration because they are predefined. The print() method is a method of PrintStream class that prints the result on the console. The max() method is a method of the Math class that returns the greater of two numbers.
We can also see the method signature of any predefined method by using the link https://docs.oracle.com/. When we go through the link and see the max() method signature, we find the following:
In the above method signature, we see that the method signature has access specifier public, non-access modifier static, return type int, method name max(), parameter list (int a, int b). In the above example, instead of defining the method, we have just invoked the method. This is the advantage of a predefined method. It makes programming less complicated.
Similarly, we can also see the method signature of the print() method.
User-defined Method
The method written by the user or programmer is known as a user-defined method. These methods are modified according to the requirement.
How to Create a User-defined Method
Let’s create a user defined method that checks the number is even or odd. First, we will define the method.
//user defined method
public static void findEvenOdd(int num)
{
//method body
if(num%2==0)
System.out.println(num+" is even");
else
System.out.println(num+" is odd");
}
We have defined the above method named findevenodd(). It has a parameter num of type int. The method does not return any value that’s why we have used void. The method body contains the steps to check the number is even or odd. If the number is even, it prints the number is even, else prints the number is odd.
How to Call or Invoke a User-defined Method
Once we have defined a method, it should be called. The calling of a method in a program is simple. When we call or invoke a user-defined method, the program control transfer to the called method.
import java.util.Scanner;
public class EvenOdd
{
public static void main (String args[])
{
//creating Scanner class object
Scanner scan=new Scanner(System.in);
System.out.print("Enter the number: ");
//reading value from the user
int num=scan.nextInt();
//method calling
findEvenOdd(num);
}
In the above code snippet, as soon as the compiler reaches at line findEvenOdd(num), the control transfer to the method and gives the output accordingly.
Let’s combine both snippets of codes in a single program and execute it.
EvenOdd.java
import java.util.Scanner;
public class EvenOdd
{
public static void main (String args[])
{
//creating Scanner class object
Scanner scan=new Scanner(System.in);
System.out.print("Enter the number: ");
//reading value from user
int num=scan.nextInt();
//method calling
findEvenOdd(num);
}
//user defined method
public static void findEvenOdd(int num)
{
//method body
if(num%2==0)
System.out.println(num+" is even");
else
System.out.println(num+" is odd");
}
}
Output 1:
Enter the number: 12
12 is even
Output 2:
Enter the number: 99
99 is odd
Let’s see another program that return a value to the calling method.
In the following program, we have defined a method named add() that sum up the two numbers. It has two parameters n1 and n2 of integer type. The values of n1 and n2 correspond to the value of a and b, respectively. Therefore, the method adds the value of a and b and store it in the variable s and returns the sum.
Addition.java
public class Addition
{
public static void main(String[] args)
{
int a = 19;
int b = 5;
//method calling
int c = add(a, b); //a and b are actual parameters
System.out.println("The sum of a and b is= " + c);
}
//user defined method
public static int add(int n1, int n2) //n1 and n2 are formal parameters
{
int s;
s=n1+n2;
return s; //returning the sum
}
}
Output:
The sum of a and b is= 24
Static Method
A method that has static keyword is known as static method. In other words, a method that belongs to a class rather than an instance of a class is known as a static method. We can also create a static method by using the keyword static before the method name.
The main advantage of a static method is that we can call it without creating an object. It can access static data members and also change the value of it. It is used to create an instance method. It is invoked by using the class name. The best example of a static method is the main() method.
Example of static method
Display.java
public class Display
{
public static void main(String[] args)
{
show();
}
static void show()
{
System.out.println("It is an example of static method.");
}
}
Output:
It is an example of a static method.
Instance Method
The method of the class is known as an instance method. It is a non-static method defined in the class. Before calling or invoking the instance method, it is necessary to create an object of its class. Let’s see an example of an instance method.
InstanceMethodExample.java
public class InstanceMethodExample
{
public static void main(String [] args)
{
//Creating an object of the class
InstanceMethodExample obj = new InstanceMethodExample();
//invoking instance method
System.out.println("The sum is: "+obj.add(12, 13));
}
int s;
//user-defined method because we have not used static keyword
public int add(int a, int b)
{
s = a+b;
//returning the sum
return s;
}
}
Output:
The sum is: 25
There are two types of instance method:
Accessor Method
Mutator Method
Accessor Method: The method(s) that reads the instance variable(s) is known as the accessor method. We can easily identify it because the method is prefixed with the word get. It is also known as getters. It returns the value of the private field. It is used to get the value of the private field.
Example
public int getId()
{
return Id;
}
Mutator Method: The method(s) read the instance variable(s) and also modify the values. We can easily identify it because the method is prefixed with the word set. It is also known as setters or modifiers. It does not return anything. It accepts a parameter of the same data type that depends on the field. It is used to set the value of the private field.
Example
public void setRoll(int roll)
{
this.roll = roll;
}
Example of accessor and mutator method
Student.java
public class Student
{
private int roll;
private String name;
public int getRoll() //accessor method
{
return roll;
}
public void setRoll(int roll) //mutator method
{
this.roll = roll;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public void display()
{
System.out.println("Roll no.: "+roll);
System.out.println("Student name: "+name);
}
}
Abstract Method
The method that does not has method body is known as abstract method. In other words, without an implementation is known as abstract method. It always declares in the abstract class. It means the class itself must be abstract if it has abstract method. To create an abstract method, we use the keyword abstract.
Syntax
abstractvoid method_name();
Example of abstract method
Demo.java
abstract class Demo //abstract class
{
//abstract method declaration
abstract void display();
}
public class MyClass extends Demo
{
//method impelmentation
void display()
{
System.out.println("Abstract method?");
}
public static void main(String args[])
{
//creating object of abstract class
Demo obj = new MyClass();
//invoking abstract method
obj.display();
}
}
In this page, we will learn about Java objects and classes. In object-oriented programming technique, we design a program using objects and classes.
An object in Java is the physical as well as a logical entity, whereas, a class in Java is a logical entity only.
What is an object in Java
An entity that has state and behavior is known as an object e.g., chair, bike, marker, pen, table, car, etc. It can be physical or logical (tangible and intangible). The example of an intangible object is the banking system.
An object has three characteristics:
State: represents the data (value) of an object.
Behavior: represents the behavior (functionality) of an object such as deposit, withdraw, etc.
Identity: An object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. However, it is used internally by the JVM to identify each object uniquely.
For Example, Pen is an object. Its name is Reynolds; color is white, known as its state. It is used to write, so writing is its behavior.
An object is an instance of a class. A class is a template or blueprint from which objects are created. So, an object is the instance(result) of a class.
Object Definitions:
An object is a real-world entity.
An object is a runtime entity.
The object is an entity which has state and behavior.
The object is an instance of a class.
What is a class in Java
A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can’t be physical.
A class in Java can contain:
Fields
Methods
Constructors
Blocks
Nested class and interface
Syntax to declare a class:
class <class_name>{
field;
method;
}
Instance variable in Java
A variable which is created inside the class but outside the method is known as an instance variable. Instance variable doesn’t get memory at compile time. It gets memory at runtime when an object or instance is created. That is why it is known as an instance variable.
Method in Java
In Java, a method is like a function which is used to expose the behavior of an object.
Advantage of Method
Code Reusability
Code Optimization
new keyword in Java
The new keyword is used to allocate memory at runtime. All objects get memory in Heap memory area.
Object and Class Example: main within the class
In this example, we have created a Student class which has two data members id and name. We are creating the object of the Student class by new keyword and printing the object’s value.
Here, we are creating a main() method inside the class.
File: Student.java
//Java Program to illustrate how to define a class and fields
//Defining a Student class.
class Student{
//defining fields
int id;//field or data member or instance variable
String name;
//creating main method inside the Student class
public static void main(String args[]){
//Creating an object or instance
Student s1=new Student();//creating an object of Student
//Printing values of the object
System.out.println(s1.id);//accessing member through reference variable
System.out.println(s1.name);
}
}
Output:
0
null
Object and Class Example: main outside the class
In real time development, we create classes and use it from another class. It is a better approach than previous one. Let’s see a simple example, where we are having main() method in another class.
We can have multiple classes in different Java files or single Java file. If you define multiple classes in a single Java source file, it is a good idea to save the file name with the class name which has main() method.
File: TestStudent1.java
//Java Program to demonstrate having the main method in
//another class
//Creating Student class.
class Student{
int id;
String name;
}
//Creating another class TestStudent1 which contains the main method
class TestStudent1{
public static void main(String args[]){
Student s1=new Student();
System.out.println(s1.id);
System.out.println(s1.name);
}
}
Output:
0
null
3 Ways to initialize object
There are 3 ways to initialize object in Java.
By reference variable
By method
By constructor
1) Object and Class Example: Initialization through reference
Initializing an object means storing data into the object. Let’s see a simple example where we are going to initialize the object through a reference variable.
File: TestStudent2.java
class Student{
int id;
String name;
}
class TestStudent2{
public static void main(String args[]){
Student s1=new Student();
s1.id=101;
s1.name="Sonoo";
System.out.println(s1.id+" "+s1.name);//printing members with a white space
}
}
Output:
101 Sonoo
We can also create multiple objects and store information in it through reference variable.
File: TestStudent3.java
class Student{
int id;
String name;
}
class TestStudent3{
public static void main(String args[]){
//Creating objects
Student s1=new Student();
Student s2=new Student();
//Initializing objects
s1.id=101;
s1.name="Sonoo";
s2.id=102;
s2.name="Amit";
//Printing data
System.out.println(s1.id+" "+s1.name);
System.out.println(s2.id+" "+s2.name);
}
}
Output:
101 Sonoo
102 Amit
2) Object and Class Example: Initialization through method
In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.
File: TestStudent4.java
class Student{
int rollno;
String name;
void insertRecord(int r, String n){
rollno=r;
name=n;
}
void displayInformation(){System.out.println(rollno+" "+name);}
}
class TestStudent4{
public static void main(String args[]){
Student s1=new Student();
Student s2=new Student();
s1.insertRecord(111,"Karan");
s2.insertRecord(222,"Aryan");
s1.displayInformation();
s2.displayInformation();
}
}
Output:
111 Karan
222 Aryan
As you can see in the above figure, object gets the memory in heap memory area. The reference variable refers to the object allocated in the heap memory area. Here, s1 and s2 both are reference variables that refer to the objects allocated in memory.
3) Object and Class Example: Initialization through a constructor
We will learn about constructors in Java later.
Object and Class Example: Employee
Let’s see an example where we are maintaining records of employees.
File: TestEmployee.java
class Employee{
int id;
String name;
float salary;
void insert(int i, String n, float s) {
id=i;
name=n;
salary=s;
}
void display(){System.out.println(id+" "+name+" "+salary);}
}
public class TestEmployee {
public static void main(String[] args) {
Employee e1=new Employee();
Employee e2=new Employee();
Employee e3=new Employee();
e1.insert(101,"ajeet",45000);
e2.insert(102,"irfan",25000);
e3.insert(103,"nakul",55000);
e1.display();
e2.display();
e3.display();
}
}
There is given another example that maintains the records of Rectangle class.
File: TestRectangle1.java
class Rectangle{
int length;
int width;
void insert(int l, int w){
length=l;
width=w;
}
void calculateArea(){System.out.println(length*width);}
}
class TestRectangle1{
public static void main(String args[]){
Rectangle r1=new Rectangle();
Rectangle r2=new Rectangle();
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}
Output:
55
45
What are the different ways to create an object in Java?
There are many ways to create an object in java. They are:
By new keyword
By newInstance() method
By clone() method
By deserialization
By factory method etc.
We will learn these ways to create object later.
Anonymous object
Anonymous simply means nameless. An object which has no reference is known as an anonymous object. It can be used at the time of object creation only.
If you have to use an object only once, an anonymous object is a good approach. For example:
new Calculation();//anonymous object
Calling method through a reference:
Calculation c=new Calculation();
c.fact(5);
Calling method through an anonymous object
new Calculation().fact(5);
Let’s see the full example of an anonymous object in Java.
class Calculation{
void fact(int n){
int fact=1;
for(int i=1;i<=n;i++){
fact=fact*i;
}
System.out.println("factorial is "+fact);
}
public static void main(String args[]){
new Calculation().fact(5);//calling method with anonymous object
}
}
Output:
Factorial is 120
Creating multiple objects by one type only
We can create multiple objects by one type only as we do in case of primitives.
Initialization of primitive variables:
int a=10, b=20;
Initialization of refernce variables:
Rectangle r1=new Rectangle(), r2=new Rectangle();//creating two objects
Let’s see the example:
//Java Program to illustrate the use of Rectangle class which
//has length and width data members
class Rectangle{
int length;
int width;
void insert(int l,int w){
length=l;
width=w;
}
void calculateArea(){System.out.println(length*width);}
}
class TestRectangle2{
public static void main(String args[]){
Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}
Output:
55
45
Real World Example: Account
File: TestAccount.java
//Java Program to demonstrate the working of a banking-system
//where we deposit and withdraw amount from our account.
//Creating an Account class which has deposit() and withdraw() methods
class Account{
int acc_no;
String name;
float amount;
//Method to initialize object
void insert(int a,String n,float amt){
acc_no=a;
name=n;
amount=amt;
}
//deposit method
void deposit(float amt){
amount=amount+amt;
System.out.println(amt+" deposited");
}
//withdraw method
void withdraw(float amt){
if(amount<amt){
System.out.println("Insufficient Balance");
}else{
amount=amount-amt;
System.out.println(amt+" withdrawn");
}
}
//method to check the balance of the account
void checkBalance(){System.out.println("Balance is: "+amount);}
//method to display the values of an object
void display(){System.out.println(acc_no+" "+name+" "+amount);}
}
//Creating a test class to deposit and withdraw amount
class TestAccount{
public static void main(String[] args){
Account a1=new Account();
a1.insert(832345,"Ankit",1000);
a1.display();
a1.checkBalance();
a1.deposit(40000);
a1.checkBalance();
a1.withdraw(15000);
a1.checkBalance();
}}
Output:
832345 Ankit 1000.0
Balance is: 1000.0
40000.0 deposited
Balance is: 41000.0
15000.0 withdrawn
Balance is: 26000.0
Java naming convention is a rule to follow as you decide what to name your identifiers such as class, package, variable, constant, method, etc.
But, it is not forced to follow. So, it is known as convention not rule. These conventions are suggested by several Java communities such as Sun Microsystems and Netscape.
All the classes, interfaces, packages, methods and fields of Java programming language are given according to the Java naming convention. If you fail to follow these conventions, it may generate confusion or erroneous code.
Advantage of Naming Conventions in Java
By using standard Java naming conventions, you make your code easier to read for yourself and other programmers. Readability of Java program is very important. It indicates that less time is spent to figure out what the code does.
Naming Conventions of the Different Identifiers
The following table shows the popular conventions used for the different identifiers.
Identifiers Type
Naming Rules
Examples
Class
It should start with the uppercase letter. It should be a noun such as Color, Button, System, Thread, etc. Use appropriate words, instead of acronyms.
public class Employee { //code snippet }
Interface
It should start with the uppercase letter. It should be an adjective such as Runnable, Remote, ActionListener. Use appropriate words, instead of acronyms.
interface Printable { //code snippet }
Method
It should start with lowercase letter. It should be a verb such as main(), print(), println(). If the name contains multiple words, start it with a lowercase letter followed by an uppercase letter such as actionPerformed().
It should start with a lowercase letter such as id, name. It should not start with the special characters like & (ampersand), $ (dollar), _ (underscore). If the name contains multiple words, start it with the lowercase letter followed by an uppercase letter such as firstName, lastName. Avoid using one-character variables such as x, y, z.
class Employee { // variable int id; //code snippet }
Package
It should be a lowercase letter such as java, lang. If the name contains multiple words, it should be separated by dots (.) such as java.util, java.lang.
//package package com.javatpoint; class Employee { //code snippet }
Constant
It should be in uppercase letters such as RED, YELLOW. If the name contains multiple words, it should be separated by an underscore(_) such as MAX_PRIORITY. It may contain digits but not as the first letter.
class Employee { //constant static final int MIN_AGE = 18; //code snippet }
CamelCase in Java naming conventions
Java follows camel-case syntax for naming the class, interface, method, and variable.
If the name is combined with two words, the second word will start with uppercase letter always such as actionPerformed(), firstName, ActionEvent, ActionListener, etc.
In this page, we will learn about the basics of OOPs. Object-Oriented Programming is a paradigm that provides many concepts, such as inheritance, data binding, polymorphism, etc.
Simula is considered the first object-oriented programming language. The programming paradigm where everything is represented as an object is known as a truly object-oriented programming language.
Smalltalk is considered the first truly object-oriented programming language.
The popular object-oriented languages are Java, C#, PHP, Python, C++, etc.
The main aim of object-oriented programming is to implement real-world entities, for example, object, classes, abstraction, inheritance, polymorphism, etc.
OOPs (Object-Oriented Programming System)
Object means a real-world entity such as a pen, chair, table, computer, watch, etc. Object-Oriented Programming is a methodology or paradigm to design a program using classes and objects. It simplifies software development and maintenance by providing some concepts:
Object
Class
Inheritance
Polymorphism
Abstraction
Encapsulation
Apart from these concepts, there are some other terms which are used in Object-Oriented design:
Coupling
Cohesion
Association
Aggregation
Composition
Object
Any entity that has state and behavior is known as an object. For example, a chair, pen, table, keyboard, bike, etc. It can be physical or logical.
An Object can be defined as an instance of a class. An object contains an address and takes up some space in memory. Objects can communicate without knowing the details of each other’s data or code. The only necessary thing is the type of message accepted and the type of response returned by the objects.
Example: A dog is an object because it has states like color, name, breed, etc. as well as behaviors like wagging the tail, barking, eating, etc.
Class
Collection of objects is called class. It is a logical entity.
A class can also be defined as a blueprint from which you can create an individual object. Class doesn’t consume any space.
Inheritance
When one object acquires all the properties and behaviors of a parent object, it is known as inheritance. It provides code reusability. It is used to achieve runtime polymorphism.
Polymorphism
If one task is performed in different ways, it is known as polymorphism. For example: to convince the customer differently, to draw something, for example, shape, triangle, rectangle, etc.
In Java, we use method overloading and method overriding to achieve polymorphism.
Another example can be to speak something; for example, a cat speaks meow, dog barks woof, etc.
Abstraction
Hiding internal details and showing functionality is known as abstraction. For example phone call, we don’t know the internal processing.
In Java, we use abstract class and interface to achieve abstraction.
Encapsulation
Binding (or wrapping) code and data together into a single unit are known as encapsulation. For example, a capsule, it is wrapped with different medicines.
A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members are private here.
Coupling
Coupling refers to the knowledge or information or dependency of another class. It arises when classes are aware of each other. If a class has the details information of another class, there is strong coupling. In Java, we use private, protected, and public modifiers to display the visibility level of a class, method, and field. You can use interfaces for the weaker coupling because there is no concrete implementation.
Cohesion
Cohesion refers to the level of a component which performs a single well-defined task. A single well-defined task is done by a highly cohesive method. The weakly cohesive method will split the task into separate parts. The java.io package is a highly cohesive package because it has I/O related classes and interface. However, the java.util package is a weakly cohesive package because it has unrelated classes and interfaces.
Association
Association represents the relationship between the objects. Here, one object can be associated with one object or many objects. There can be four types of association between the objects:
One to One
One to Many
Many to One, and
Many to Many
Let’s understand the relationship with real-time examples. For example, One country can have one prime minister (one to one), and a prime minister can have many ministers (one to many). Also, many MP’s can have one prime minister (many to one), and many ministers can have many departments (many to many).
Association can be undirectional or bidirectional.
Aggregation
Aggregation is a way to achieve Association. Aggregation represents the relationship where one object contains other objects as a part of its state. It represents the weak relationship between objects. It is also termed as a has-a relationship in Java. Like, inheritance represents the is-a relationship. It is another way to reuse objects.
Composition
The composition is also a way to achieve Association. The composition represents the relationship where one object contains other objects as a part of its state. There is a strong relationship between the containing object and the dependent object. It is the state where containing objects do not have an independent existence. If you delete the parent object, all the child objects will be deleted automatically.
Advantage of OOPs over Procedure-oriented programming language
1) OOPs makes development and maintenance easier, whereas, in a procedure-oriented programming language, it is not easy to manage if code grows as project size increases.
2) OOPs provides data hiding, whereas, in a procedure-oriented programming language, global data can be accessed from anywhere.
Figure: Data Representation in Procedure-Oriented Programming
Figure: Data Representation in Object-Oriented Programming
3) OOPs provides the ability to simulate real-world event much more effectively. We can provide the solution of real word problem if we are using the Object-Oriented Programming language.
What is the difference between an object-oriented programming language and object-based programming language?
Object-based programming language follows all the features of OOPs except Inheritance. JavaScript and VBScript are examples of object-based programming languages.
The Java comments are the statements in a program that are not executed by the compiler and interpreter.
Why do we use comments in a code?
Comments are used to make the program more readable by adding the details of the code.
It makes easy to maintain the code and to find the errors easily.
The comments can be used to provide information or explanation about the variable, method, class, or any statement.
It can also be used to prevent the execution of program code while testing the alternative code.
Types of Java Comments
There are three types of comments in Java.
Single Line Comment
Multi Line Comment
Documentation Comment
1) Java Single Line Comment
The single-line comment is used to comment only one line of the code. It is the widely used and easiest way of commenting the statements.
Single line comments starts with two forward slashes (//). Any text in front of // is not executed by Java.
Syntax:
//This is single line comment
Let’s use single line comment in a Java program.
CommentExample1.java
public class CommentExample1 {
public static void main(String[] args) {
int i=10; // i is a variable with value 10
System.out.println(i); //printing the variable i
}
}
Output:
10
2) Java Multi Line Comment
The multi-line comment is used to comment multiple lines of code. It can be used to explain a complex code snippet or to comment multiple lines of code at a time (as it will be difficult to use single-line comments there).
Multi-line comments are placed between /* and */. Any text between /* and */ is not executed by Java.
Syntax:
/*
This
is
multi line
comment
*/
Let’s use multi-line comment in a Java program.
CommentExample2.java
public class CommentExample2 {
public static void main(String[] args) {
/* Let's declare and
print variable in java. */
int i=10;
System.out.println(i);
/* float j = 5.9;
float k = 4.4;
System.out.println( j + k ); */
}
}
Output:
10
Note: Usually // is used for short comments and /* */ is used for longer comments.
3) Java Documentation Comment
Documentation comments are usually used to write large programs for a project or software application as it helps to create documentation API. These APIs are needed for reference, i.e., which classes, methods, arguments, etc., are used in the code.
To create documentation API, we need to use the javadoc tool. The documentation comments are placed between /** and */.
Syntax:
/**
*
*We can use various tags to depict the parameter
*or heading or author name
*We can also use HTML tags
*
*/
javadoc tags
Some of the commonly used tags in documentation comments:
Tag
Syntax
Description
{@docRoot}
{@docRoot}
to depict relative path to root directory of generated document from any page.
@author
@author name – text
To add the author of the class.
@code
{@code text}
To show the text in code font without interpreting it as html markup or nested javadoc tag.
@version
@version version-text
To specify “Version” subheading and version-text when -version option is used.
@since
@since release
To add “Since” heading with since text to generated documentation.
@param
@param parameter-name description
To add a parameter with given name and description to ‘Parameters’ section.
@return
@return description
Required for every method that returns something (except void)
Let’s use the Javadoc tag in a Java program.
Calculate.java
import java.io.*;
/**
* <h2> Calculation of numbers </h2>
* This program implements an application
* to perform operation such as addition of numbers
* and print the result
* <p>
* <b>Note:</b> Comments make the code readable and
* easy to understand.
*
* @author Anurati
* @version 16.0
* @since 2021-07-06
*/
public class Calculate{
/**
* This method calculates the summation of two integers.
* @param input1 This is the first parameter to sum() method
* @param input2 This is the second parameter to the sum() method.
* @return int This returns the addition of input1 and input2
*/
public int sum(int input1, int input2){
return input1 + input2;
}
/**
* This is the main method uses of sum() method.
* @param args Unused
* @see IOException
*/
public static void main(String[] args) {
Calculate obj = new Calculate();
int result = obj.sum(40, 20);
System.out.println("Addition of numbers: " + result);
}
}
Compile it by javac tool:
Create Document
Create documentation API by javadoc tool:
Now, the HTML files are created for the Calculate class in the current directory, i.e., abcDemo. Open the HTML files, and we can see the explanation of Calculate class provided through the documentation comment.
Are Java comments executable?
Ans: As we know, Java comments are not executed by the compiler or interpreter, however, before the lexical transformation of code in compiler, contents of the code are encoded into ASCII in order to make the processing easy.
Test.java
public class Test{
public static void main(String[] args) {
//the below comment will be executed
// \u000d System.out.println("Java comment is executed!!");
}
}
Output:
The above code generate the output because the compiler parses the Unicode character \u000d as a new line before the lexical transformation, and thus the code is transformed as shown below:
Test.java
public class Test{
public static void main(String[] args) {
//the below comment will be executed
//
System.out.println("Java comment is executed!!");
}
}
The continue statement is used in loop control structure when you need to jump to the next iteration of the loop immediately. It can be used with for loop or while loop.
The Java continue statement is used to continue the loop. It continues the current flow of the program and skips the remaining code at the specified condition. In case of an inner loop, it continues the inner loop only.
We can use Java continue statement in all types of loops such as for loop, while loop and do-while loop.
Syntax:
jump-statement;
continue;
Java Continue Statement Example
ContinueExample.java
//Java Program to demonstrate the use of continue statement
//inside the for loop.
public class ContinueExample {
public static void main(String[] args) {
//for loop
for(int i=1;i<=10;i++){
if(i==5){
//using continue statement
continue;//it will skip the rest statement
}
System.out.println(i);
}
}
}
Output:
1
2
3
4
6
7
8
9
10
As you can see in the above output, 5 is not printed on the console. It is because the loop is continued when it reaches to 5.
Java Continue Statement with Inner Loop
It continues inner loop only if you use the continue statement inside the inner loop.
ContinueExample2.java
//Java Program to illustrate the use of continue statement
//inside an inner loop
public class ContinueExample2 {
public static void main(String[] args) {
//outer loop
for(int i=1;i<=3;i++){
//inner loop
for(int j=1;j<=3;j++){
if(i==2&&j==2){
//using continue statement inside inner loop
continue;
}
System.out.println(i+" "+j);
}
}
}
}
Output:
1 1
1 2
1 3
2 1
2 3
3 1
3 2
3 3
Java Continue Statement with Labelled For Loop
We can use continue statement with a label. This feature is introduced since JDK 1.5. So, we can continue any loop in Java now whether it is outer loop or inner.
Example:
ContinueExample3.java
//Java Program to illustrate the use of continue statement
//with label inside an inner loop to continue outer loop
public class ContinueExample3 {
public static void main(String[] args) {
aa:
for(int i=1;i<=3;i++){
bb:
for(int j=1;j<=3;j++){
if(i==2&&j==2){
//using continue statement with label
continue aa;
}
System.out.println(i+" "+j);
}
}
}
}
Output:
1 1
1 2
1 3
2 1
3 1
3 2
3 3
Java Continue Statement in while loop
ContinueWhileExample.java
//Java Program to demonstrate the use of continue statement
//inside the while loop.
public class ContinueWhileExample {
public static void main(String[] args) {
//while loop
int i=1;
while(i<=10){
if(i==5){
//using continue statement
i++;
continue;//it will skip the rest statement
}
System.out.println(i);
i++;
}
}
}
Output:
1
2
3
4
6
7
8
9
10
Java Continue Statement in do-while Loop
ContinueDoWhileExample.java
//Java Program to demonstrate the use of continue statement
//inside the Java do-while loop.
public class ContinueDoWhileExample {
public static void main(String[] args) {
//declaring variable
int i=1;
//do-while loop
do{
if(i==5){
//using continue statement
i++;
continue;//it will skip the rest statement
}
System.out.println(i);
i++;
}while(i<=10);
}
}
