Category: 1. Tutorial

Java keywords are also known as reserved words. Keywords are particular words that act as a key to a code. These are predefined words by Java so they cannot be used as a variable or object name or class name.

List of Java Keywords

A list of Java keywords or reserved words are given below:

1. abstract: Java abstract keyword is used to declare an abstract class. An abstract class can provide the implementation of the interface. It can have abstract and non-abstract methods.
2. boolean: Java boolean keyword is used to declare a variable as a boolean type. It can hold True and False values only.
3. break: Java break keyword is used to break the loop or switch statement. It breaks the current flow of the program at specified conditions.
4. byte: Java byte keyword is used to declare a variable that can hold 8-bit data values.
5. case: Java case keyword is used with the switch statements to mark blocks of text.
6. catch: Java catch keyword is used to catch the exceptions generated by try statements. It must be used after the try block only.
7. char: Java char keyword is used to declare a variable that can hold unsigned 16-bit Unicode characters
8. class: Java class keyword is used to declare a class.
9. continue: Java continue keyword is used to continue the loop. It continues the current flow of the program and skips the remaining code at the specified condition.
10. default: Java default keyword is used to specify the default block of code in a switch statement.
11. do: Java do keyword is used in the control statement to declare a loop. It can iterate a part of the program several times.
12. double: Java double keyword is used to declare a variable that can hold 64-bit floating-point number.
13. else: Java else keyword is used to indicate the alternative branches in an if statement.
14. enum: Java enum keyword is used to define a fixed set of constants. Enum constructors are always private or default.
15. extends: Java extends keyword is used to indicate that a class is derived from another class or interface.
16. final: Java final keyword is used to indicate that a variable holds a constant value. It is used with a variable. It is used to restrict the user from updating the value of the variable.
17. finally: Java finally keyword indicates a block of code in a try-catch structure. This block is always executed whether an exception is handled or not.
18. float: Java float keyword is used to declare a variable that can hold a 32-bit floating-point number.
19. for: Java for keyword is used to start a for loop. It is used to execute a set of instructions/functions repeatedly when some condition becomes true. If the number of iteration is fixed, it is recommended to use for loop.
20. if: Java if keyword tests the condition. It executes the if block if the condition is true.
21. implements: Java implements keyword is used to implement an interface.
22. import: Java import keyword makes classes and interfaces available and accessible to the current source code.
23. instanceof: Java instanceof keyword is used to test whether the object is an instance of the specified class or implements an interface.
24. int: Java int keyword is used to declare a variable that can hold a 32-bit signed integer.
25. interface: Java interface keyword is used to declare an interface. It can have only abstract methods.
26. long: Java long keyword is used to declare a variable that can hold a 64-bit integer.
27. native: Java native keyword is used to specify that a method is implemented in native code using JNI (Java Native Interface).
28. new: Java new keyword is used to create new objects.
29. null: Java null keyword is used to indicate that a reference does not refer to anything. It removes the garbage value.
30. package: Java package keyword is used to declare a Java package that includes the classes.
31. private: Java private keyword is an access modifier. It is used to indicate that a method or variable may be accessed only in the class in which it is declared.
32. protected: Java protected keyword is an access modifier. It can be accessible within the package and outside the package but through inheritance only. It can’t be applied with the class.
33. public: Java public keyword is an access modifier. It is used to indicate that an item is accessible anywhere. It has the widest scope among all other modifiers.
34. return: Java return keyword is used to return from a method when its execution is complete.
35. short: Java short keyword is used to declare a variable that can hold a 16-bit integer.
36. static: Java static keyword is used to indicate that a variable or method is a class method. The static keyword in Java is mainly used for memory management.
37. strictfp: Java strictfp is used to restrict the floating-point calculations to ensure portability.
38. super: Java super keyword is a reference variable that is used to refer to parent class objects. It can be used to invoke the immediate parent class method.
39. switch: The Java switch keyword contains a switch statement that executes code based on test value. The switch statement tests the equality of a variable against multiple values.
40. synchronized: Java synchronized keyword is used to specify the critical sections or methods in multithreaded code.
41. this: Java this keyword can be used to refer the current object in a method or constructor.
42. throw: The Java throw keyword is used to explicitly throw an exception. The throw keyword is mainly used to throw custom exceptions. It is followed by an instance.
43. throws: The Java throws keyword is used to declare an exception. Checked exceptions can be propagated with throws.
44. transient: Java transient keyword is used in serialization. If you define any data member as transient, it will not be serialized.
45. try: Java try keyword is used to start a block of code that will be tested for exceptions. The try block must be followed by either catch or finally block.
46. void: Java void keyword is used to specify that a method does not have a return value.
47. volatile: Java volatile keyword is used to indicate that a variable may change asynchronously.
48. while: Java while keyword is used to start a while loop. This loop iterates a part of the program several times. If the number of iteration is not fixed, it is recommended to use the while loop.

Operator in Java is a symbol that is used to perform operations. For example: +, -, *, / etc.

There are many types of operators in Java which are given below:

• Unary Operator,
• Arithmetic Operator,
• Shift Operator,
• Relational Operator,
• Bitwise Operator,
• Logical Operator,
• Ternary Operator and
• Assignment Operator.

Java Operator Precedence

Operator Type	Category	Precedence
Unary	postfix	expr++ expr--
	prefix	++expr --expr +expr -expr ~ !
Arithmetic	multiplicative	* / %
	additive	+ -
Shift	shift	<< >> >>>
Relational	comparison	< > <= >= instanceof
	equality	== !=
Bitwise	bitwise AND	&
	bitwise exclusive OR	^
	bitwise inclusive OR	|
Logical	logical AND	&&
	logical OR	||
Ternary	ternary	? :
Assignment	assignment	= += -= *= /= %= &= ^= |= <<= >>= >>>=

Java Unary Operator

The Java unary operators require only one operand. Unary operators are used to perform various operations i.e.:

• incrementing/decrementing a value by one
• negating an expression
• inverting the value of a boolean

Java Unary Operator Example: ++ and —

public class OperatorExample{  

public static void main(String args[]){  

int x=10;  

System.out.println(x++);//10 (11)  

System.out.println(++x);//12  

System.out.println(x--);//12 (11)  

System.out.println(--x);//10  

}} 

Output:

10
12
12
10

Java Unary Operator Example 2: ++ and —

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=10;  

System.out.println(a++ + ++a);//10+12=22  

System.out.println(b++ + b++);//10+11=21  

  

}}

Output:

22
21

Java Unary Operator Example: ~ and !

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=-10;  

boolean c=true;  

boolean d=false;  

System.out.println(~a);//-11 (minus of total positive value which starts from 0)  

System.out.println(~b);//9 (positive of total minus, positive starts from 0)  

System.out.println(!c);//false (opposite of boolean value)  

System.out.println(!d);//true  

}}  

Output:

-11
9
false
true

Java Arithmetic Operators

Java arithmetic operators are used to perform addition, subtraction, multiplication, and division. They act as basic mathematical operations.

Java Arithmetic Operator Example

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=5;  

System.out.println(a+b);//15  

System.out.println(a-b);//5  

System.out.println(a*b);//50  

System.out.println(a/b);//2  

System.out.println(a%b);//0  

}} 

Output:

15
5
50
2
0

Java Arithmetic Operator Example: Expression

public class OperatorExample{  

public static void main(String args[]){  

System.out.println(10*10/5+3-1*4/2);  

}} 

Output:

21

Java Left Shift Operator

The Java left shift operator << is used to shift all of the bits in a value to the left side of a specified number of times.

Java Left Shift Operator Example

public class OperatorExample{  

public static void main(String args[]){  

System.out.println(10<<2);//10*2^2=10*4=40  

System.out.println(10<<3);//10*2^3=10*8=80  

System.out.println(20<<2);//20*2^2=20*4=80  

System.out.println(15<<4);//15*2^4=15*16=240  

}} 

Output:

40
80
80
240

Java Right Shift Operator

The Java right shift operator >> is used to move the value of the left operand to right by the number of bits specified by the right operand.

Java Right Shift Operator Example

public OperatorExample{  

public static void main(String args[]){  

System.out.println(10>>2);//10/2^2=10/4=2  

System.out.println(20>>2);//20/2^2=20/4=5  

System.out.println(20>>3);//20/2^3=20/8=2  

}} 

Output:

2
5
2

Java Shift Operator Example: >> vs >>>

public class OperatorExample{  

public static void main(String args[]){  

    //For positive number, >> and >>> works same  

    System.out.println(20>>2);  

    System.out.println(20>>>2);  

    //For negative number, >>> changes parity bit (MSB) to 0  

    System.out.println(-20>>2);  

    System.out.println(-20>>>2);  

}} 

Output:

5
5
-5
1073741819

Java AND Operator Example: Logical && and Bitwise &

The logical && operator doesn’t check the second condition if the first condition is false. It checks the second condition only if the first one is true.

The bitwise & operator always checks both conditions whether first condition is true or false.

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=5;  

int c=20;  

System.out.println(a<b&&a<c);//false && true = false  

System.out.println(a<b&a<c);//false & true = false  

}} 

Output:

false
false

Java AND Operator Example: Logical && vs Bitwise &

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=5;  

int c=20;  

System.out.println(a<b&&a++<c);//false && true = false  

System.out.println(a);//10 because second condition is not checked  

System.out.println(a<b&a++<c);//false && true = false  

System.out.println(a);//11 because second condition is checked  

}} 

Output:

false
10
false
11

Java OR Operator Example: Logical || and Bitwise |

The logical || operator doesn’t check the second condition if the first condition is true. It checks the second condition only if the first one is false.

The bitwise | operator always checks both conditions whether first condition is true or false.

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=5;  

int c=20;  

System.out.println(a>b||a<c);//true || true = true  

System.out.println(a>b|a<c);//true | true = true  

//|| vs |  

System.out.println(a>b||a++<c);//true || true = true  

System.out.println(a);//10 because second condition is not checked  

System.out.println(a>b|a++<c);//true | true = true  

System.out.println(a);//11 because second condition is checked  

}} 

Output:

true
true
true
10
true
11

Java Ternary Operator

Java Ternary operator is used as one line replacement for if-then-else statement and used a lot in Java programming. It is the only conditional operator which takes three operands.

Java Ternary Operator Example

public class OperatorExample{  

public static void main(String args[]){  

int a=2;  

int b=5;  

int min=(a<b)?a:b;  

System.out.println(min);  

}} 

Output:

2

Another Example:

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=5;  

int min=(a<b)?a:b;  

System.out.println(min);  

}}  

Output:

5

Java Assignment Operator

Java assignment operator is one of the most common operators. It is used to assign the value on its right to the operand on its left.

Java Assignment Operator Example

public class OperatorExample{  

public static void main(String args[]){  

int a=10;  

int b=20;  

a+=4;//a=a+4 (a=10+4)  

b-=4;//b=b-4 (b=20-4)  

System.out.println(a);  

System.out.println(b);  

}} 

Output:

14
16

Java Assignment Operator Example

public class OperatorExample{  

public static void main(String[] args){  

int a=10;  

a+=3;//10+3  

System.out.println(a);  

a-=4;//13-4  

System.out.println(a);  

a*=2;//9*2  

System.out.println(a);  

a/=2;//18/2  

System.out.println(a);  

}}  

Output:

13
9
18
9

Java Assignment Operator Example: Adding short

public class OperatorExample{  

public static void main(String args[]){  

short a=10;  

short b=10;  

//a+=b;//a=a+b internally so fine  

a=a+b;//Compile time error because 10+10=20 now int  

System.out.println(a);  

}}

Output:

Compile time error

After type cast:

public class OperatorExample{  

public static void main(String args[]){  

short a=10;  

short b=10;  

a=(short)(a+b);//20 which is int now converted to short  

System.out.println(a);  

}} 

Output:

20

Unicode is a universal international standard character encoding that is capable of representing most of the world’s written languages.

Why java uses Unicode System?

Before Unicode, there were many language standards:

ASCII (American Standard Code for Information Interchange) for the United States.ISO 8859-1 for Western European Language.KOI-8 for Russian.GB18030 and BIG-5 for chinese, and so on.

Problem

This caused two problems:A particular code value corresponds to different letters in the various language standards.The encodings for languages with large character sets have variable length.Some common characters are encoded as single bytes, other require two or more byte.

Solution

To solve these problems, a new language standard was developed i.e. Unicode System.

In unicode, character holds 2 byte, so java also uses 2 byte for characters.

lowest value:\u0000

highest value:\uFFFF

Data types specify the different sizes and values that can be stored in the variable. There are two types of data types in Java:

1. Primitive data types: The primitive data types include boolean, char, byte, short, int, long, float and double.
2. Non-primitive data types: The non-primitive data types include Classes, Interfaces, and Arrays.

Java Primitive Data Types

In Java language, primitive data types are the building blocks of data manipulation. These are the most basic data types available in Java language.

Java is a statically-typed programming language. It means, all variables must be declared before its use. That is why we need to declare variable’s type and name.

There are 8 types of primitive data types:

• boolean data type
• byte data type
• char data type
• short data type
• int data type
• long data type
• float data type
• double data type

Data Type	Default Value	Default size
boolean	false	1 bit
char	‘\u0000’	2 byte
byte	0	1 byte
short	0	2 byte
int	0	4 byte
long	0L	8 byte
float	0.0f	4 byte
double	0.0d	8 byte

Boolean Data Type

The Boolean data type is used to store only two possible values: true and false. This data type is used for simple flags that track true/false conditions.

The Boolean data type specifies one bit of information, but its “size” can’t be defined precisely.

Example:

Boolean one = false  

Byte Data Type

The byte data type is an example of primitive data type. It isan 8-bit signed two’s complement integer. Its value-range lies between -128 to 127 (inclusive). Its minimum value is -128 and maximum value is 127. Its default value is 0.

The byte data type is used to save memory in large arrays where the memory savings is most required. It saves space because a byte is 4 times smaller than an integer. It can also be used in place of “int” data type.

Example:

byte a = 10, byte b = -20  

Short Data Type

The short data type is a 16-bit signed two’s complement integer. Its value-range lies between -32,768 to 32,767 (inclusive). Its minimum value is -32,768 and maximum value is 32,767. Its default value is 0.

The short data type can also be used to save memory just like byte data type. A short data type is 2 times smaller than an integer.

Example:

short s = 10000, short r = -5000  

Int Data Type

The int data type is a 32-bit signed two’s complement integer. Its value-range lies between – 2,147,483,648 (-2^31) to 2,147,483,647 (2^31 -1) (inclusive). Its minimum value is – 2,147,483,648and maximum value is 2,147,483,647. Its default value is 0.

The int data type is generally used as a default data type for integral values unless if there is no problem about memory.

Example:

int a = 100000, int b = -200000  

Long Data Type

The long data type is a 64-bit two’s complement integer. Its value-range lies between -9,223,372,036,854,775,808(-2^63) to 9,223,372,036,854,775,807(2^63 -1)(inclusive). Its minimum value is – 9,223,372,036,854,775,808and maximum value is 9,223,372,036,854,775,807. Its default value is 0. The long data type is used when you need a range of values more than those provided by int.

Example:

long a = 100000L, long b = -200000L  

Float Data Type

The float data type is a single-precision 32-bit IEEE 754 floating point.Its value range is unlimited. It is recommended to use a float (instead of double) if you need to save memory in large arrays of floating point numbers. The float data type should never be used for precise values, such as currency. Its default value is 0.0F.

Example:

float f1 = 234.5f  

Double Data Type

The double data type is a double-precision 64-bit IEEE 754 floating point. Its value range is unlimited. The double data type is generally used for decimal values just like float. The double data type also should never be used for precise values, such as currency. Its default value is 0.0d.

Example:

double d1 = 12.3  

Char Data Type

The char data type is a single 16-bit Unicode character. Its value-range lies between ‘\u0000’ (or 0) to ‘\uffff’ (or 65,535 inclusive).The char data type is used to store characters.

Example:

char letterA = 'A'  

Why char uses 2 byte in java and what is \u0000 ?

It is because java uses Unicode system not ASCII code system. The \u0000 is the lowest range of Unicode system. To get detail explanation about Unicode visit next page.

A variable is a container which holds the value while the Java program is executed. A variable is assigned with a data type.

Variable is a name of memory location. There are three types of variables in java: local, instance and static.

There are two types of data types in Java: primitive and non-primitive.

Variable

A variable is the name of a reserved area allocated in memory. In other words, it is a name of the memory location. It is a combination of “vary + able” which means its value can be changed.

int data=50;//Here data is variable  

Types of Variables

There are three types of variables in Java:

• local variable
• instance variable
• static variable

1) Local Variable

A variable declared inside the body of the method is called local variable. You can use this variable only within that method and the other methods in the class aren’t even aware that the variable exists.

A local variable cannot be defined with “static” keyword.

2) Instance Variable

A variable declared inside the class but outside the body of the method, is called an instance variable. It is not declared as static.

It is called an instance variable because its value is instance-specific and is not shared among instances.

3) Static variable

A variable that is declared as static is called a static variable. It cannot be local. You can create a single copy of the static variable and share it among all the instances of the class. Memory allocation for static variables happens only once when the class is loaded in the memory.

Example to understand the types of variables in java

public class A  

{  

    static int m=100;//static variable  

    void method()  

    {    

        int n=90;//local variable    

    }  

    public static void main(String args[])  

    {  

        int data=50;//instance variable    

    }  

}//end of class 

Java Variable Example: Add Two Numbers

public class Simple{    

public static void main(String[] args){    

int a=10;    

int b=10;    

int c=a+b;    

System.out.println(c);    

}  

}  

Output:

20

Java Variable Example: Widening

public class Simple{  

public static void main(String[] args){  

int a=10;  

float f=a;  

System.out.println(a);  

System.out.println(f);  

}}

Output:

10
10.0

Java Variable Example: Narrowing (Typecasting)

public class Simple{  

public static void main(String[] args){  

float f=10.5f;  

//int a=f;//Compile time error  

int a=(int)f;  

System.out.println(f);  

System.out.println(a);  

}} 

Output:

10.5
10

Java Variable Example: Overflow

class Simple{  

public static void main(String[] args){  

//Overflow  

int a=130;  

byte b=(byte)a;  

System.out.println(a);  

System.out.println(b);  

}}

Output:

130
-126

Java Variable Example: Adding Lower Type

class Simple{  

public static void main(String[] args){  

byte a=10;  

byte b=10;  

//byte c=a+b;//Compile Time Error: because a+b=20 will be int  

byte c=(byte)(a+b);  

System.out.println(c);  

}} 

Output:

20

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.

JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).

What is JVM

It is:

1. A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Oracle and other companies.
2. An implementation Its implementation is known as JRE (Java Runtime Environment).
3. Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

What it does

The JVM performs following operation:

• Loads code
• Verifies code
• Executes code
• Provides runtime environment

JVM provides definitions for the:

• Memory area
• Class file format
• Register set
• Garbage-collected heap
• Fatal error reporting etc.

JVM Architecture

Let’s understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.

1) Classloader

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

1. Bootstrap ClassLoader: This is the first classloader which is the super class of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes etc.
2. Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loades the jar files located inside $JAVA_HOME/jre/lib/ext directory.
3. System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the classfiles from classpath. By default, classpath is set to current directory. You can change the classpath using “-cp” or “-classpath” switch. It is also known as Application classloader.

//Let's see an example to print the classloader name  

public class ClassLoaderExample  

{  

    public static void main(String[] args)  

    {  

        // Let's print the classloader name of current class.   

        //Application/System classloader will load this class  

        Class c=ClassLoaderExample.class;  

        System.out.println(c.getClassLoader());  

        //If we print the classloader name of String, it will print null because it is an  

        //in-built class which is found in rt.jar, so it is loaded by Bootstrap classloader  

        System.out.println(String.class.getClassLoader());  

    }  

}

Output:

sun.misc.Launcher$AppClassLoader@4e0e2f2a
null

These are the internal classloaders provided by Java. If you want to create your own classloader, you need to extend the ClassLoader class.

2) Class(Method) Area

Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.

3) Heap

It is the runtime data area in which objects are allocated.

4) Stack

Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return.

Each thread has a private JVM stack, created at the same time as thread.

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.

5) Program Counter Register

PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.

6) Native Method Stack

It contains all the native methods used in the application.

7) Execution Engine

It contains:

1. A virtual processor
2. Interpreter: Read bytecode stream then execute the instructions.
3. Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here, the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

8) Java Native Interface

Java Native Interface (JNI) is a framework which provides an interface to communicate with another application written in another language like C, C++, Assembly etc. Java uses JNI framework to send output to the Console or interact with OS libraries.

We must understand the differences between JDK, JRE, and JVM before proceeding further to Java. See the brief overview of JVM here.

If you want to get the detailed knowledge of Java Virtual Machine, move to the next page. Firstly, let’s see the differences between the JDK, JRE, and JVM.

JVM

JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn’t physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed. It can also run those programs which are written in other languages and compiled to Java bytecode.

JVMs are available for many hardware and software platforms. JVM, JRE, and JDK are platform dependent because the configuration of each OS is different from each other. However, Java is platform independent. There are three notions of the JVM: specification, implementation, and instance.

The JVM performs the following main tasks:

• Loads code
• Verifies code
• Executes code
• Provides runtime environment

JRE

JRE is an acronym for Java Runtime Environment. It is also written as Java RTE. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

The implementation of JVM is also actively released by other companies besides Sun Micro Systems.

JDK

JDK is an acronym for Java Development Kit. The Java Development Kit (JDK) is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools.

JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:

• Standard Edition Java Platform
• Enterprise Edition Java Platform
• Micro Edition Java Platform

The JDK contains a private Java Virtual Machine (JVM) and a few other resources such as an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc), etc. to complete the development of a Java Application.

The path is required to be set for using tools such as javac, java, etc.

If you are saving the Java source file inside the JDK/bin directory, the path is not required to be set because all the tools will be available in the current directory.

However, if you have your Java file outside the JDK/bin folder, it is necessary to set the path of JDK.

There are two ways to set the path in Java:

1. Temporary
2. Permanent

1) How to set the Temporary Path of JDK in Windows

To set the temporary path of JDK, you need to follow the following steps:

• Open the command prompt
• Copy the path of the JDK/bin directory
• Write in command prompt: set path=copied_path

For Example:

set path=C:\Program Files\Java\jdk1.6.0_23\bin

Let’s see it in the figure given below:

2) How to set Permanent Path of JDK in Windows

For setting the permanent path of JDK, you need to follow these steps:

• Go to MyComputer properties -> advanced tab -> environment variables -> new tab of user variable -> write path in variable name -> write path of bin folder in variable value -> ok -> ok -> ok

For Example:

1) Go to MyComputer properties

2) Click on the advanced tab

3) Click on environment variables

4) Click on the new tab of user variables

5) Write the path in the variable name

6) Copy the path of bin folder

7) Paste path of bin folder in the variable value

8) Click on ok button

9) Click on ok button

Now your permanent path is set. You can now execute any program of java from any drive.

Setting Java Path in Linux OS

Setting path in Linux OS is the same as setting the path in the Windows OS. But, here we use the export tool rather than set. Let’s see how to set path in Linux OS:

export PATH=$PATH:/home/jdk1.6.01/bin/

Here, we have installed the JDK in the home directory under Root (/home).

In the previous section, we have created Java Hello World program and learn how to compile and run a Java program. In this section, we are going to learn, what happens while we compile and run the Java program. Moreover, we will see some questions based on the first program.

What happens at compile time?

At compile time, the Java file is compiled by Java Compiler (It does not interact with OS) and converts the Java code into bytecode.

What happens at runtime?

At runtime, the following steps are performed:

Classloader: It is the subsystem of JVM that is used to load class files.

Bytecode Verifier: Checks the code fragments for illegal code that can violate access rights to objects.

Interpreter: Read bytecode stream then execute the instructions.

Q) Can you save a Java source file by another name than the class name?

Yes, if the class is not public. It is explained in the figure given below:

To compile:	javac Hard.java
To execute:	java Simple

Observe that, we have compiled the code with file name but running the program with class name. Therefore, we can save a Java program other than class name.

Q) Can you have multiple classes in a java source file?

Yes, like the figure given below illustrates:

In this section, we will learn how to write the simple program of Java. We can write a simple hello Java program easily after installing the JDK.

To create a simple Java program, you need to create a class that contains the main method. Let’s understand the requirement first.

The requirement for Java Hello World Example

For executing any Java program, the following software or application must be properly installed.

• Install the JDK if you don’t have installed it, download the JDK and install it.
• Set path of the jdk/bin directory. http://www.javatpoint.com/how-to-set-path-in-java
• Create the Java program
• Compile and run the Java program

Creating Hello World Example

Let’s create the hello java program:

class Simple{  

    public static void main(String args[]){  

     System.out.println("Hello Java");  

    }  

}

Save the above file as Simple.java.

To compile:	javac Simple.java
To execute:	java Simple

Output:

Hello Java

Compilation Flow:

When we compile Java program using javac tool, the Java compiler converts the source code into byte code.

Parameters used in First Java Program

Let’s see what is the meaning of class, public, static, void, main, String[], System.out.println().

• class keyword is used to declare a class in Java.
• public keyword is an access modifier that represents visibility. It means it is visible to all.
• static is a keyword. If we declare any method as static, it is known as the static method. The core advantage of the static method is that there is no need to create an object to invoke the static method. The main() method is executed by the JVM, so it doesn’t require creating an object to invoke the main() method. So, it saves memory.
• void is the return type of the method. It means it doesn’t return any value.
• main represents the starting point of the program.
• String[] args or String args[] is used for command line argument. We will discuss it in coming section.
• System.out.println() is used to print statement. Here, System is a class, out is an object of the PrintStream class, println() is a method of the PrintStream class. We will discuss the internal working of System.out.println() statement in the coming section.

To write the simple program, you need to open notepad by start menu -> All Programs -> Accessories -> Notepad and write a simple program as we have shownbelow:

As displayed in the above diagram, write the simple program of Java in notepad and saved it as Simple.java. In order to compile and run the above program, you need to open the command prompt by start menu -> All Programs -> Accessories -> command prompt. When we have done with all the steps properly, it shows the following output:

To compile and run the above program, go to your current directory first; my current directory is c:\new. Write here:

To compile:	javac Simple.java
To execute:	java Simple

In how many ways we can write a Java program?

There are many ways to write a Java program. The modifications that can be done in a Java program are given below:

1) By changing the sequence of the modifiers, method prototype is not changed in Java.

Let’s see the simple code of the main method.

static public void main(String args[])  

2) The subscript notation in the Java array can be used after type, before the variable or after the variable.

Let’s see the different codes to write the main method.

public static void main(String[] args)  

public static void main(String []args)  

public static void main(String args[])

3) You can provide var-args support to the main() method by passing 3 ellipses (dots)

Let’s see the simple code of using var-args in the main() method. We will learn about var-args later in the Java New Features chapter.

public static void main(String... args)  

4) Having a semicolon at the end of class is optional in Java.

Let’s see the simple code.

class A{  

static public void main(String... args){  

System.out.println("hello java4");  

}  

};

Valid Java main() method signature

public static void main(String[] args)  
public static void main(String []args)  
public static void main(String args[])  
public static void main(String... args)  
static public void main(String[] args)  
public static final void main(String[] args)  
final public static void main(String[] args)  
final strictfp public static void main(String[] args)    

Invalid Java main() method signature

public void main(String[] args)  

static void main(String[] args)  

public void static main(String[] args)  

abstract public static void main(String[] args)

Resolving an error “javac is not recognized as an internal or external command”?

If there occurs a problem like displayed in the below figure, you need to set a path. Since DOS doesn’t recognize javac and java as internal or external command. To overcome this problem, we need to set a path. The path is not required in a case where you save your program inside the JDK/bin directory. However, it is an excellent approach to set the path. Click here for How to set path in java.