Author: saqibkhan

Recursion in java is a process in which a method calls itself continuously. A method in java that calls itself is called recursive method.

It makes the code compact but complex to understand.

Syntax:

returntype methodname(){  

//code to be executed  

methodname();//calling same method  

}

Java Recursion Example 1: Infinite times

public class RecursionExample1 {  

static void p(){  

System.out.println("hello");  

p();  

}  

  

public static void main(String[] args) {  

p();  

}  

}

Output:

hello
hello
...
java.lang.StackOverflowError

Java Recursion Example 2: Finite times

public class RecursionExample2 {  

static int count=0;  

static void p(){  

count++;  

if(count<=5){  

System.out.println("hello "+count);  

p();  

}  

}  

public static void main(String[] args) {  

p();  

}  

}  

Output:

hello 1
hello 2
hello 3
hello 4
hello 5

Java Recursion Example 3: Factorial Number

public class RecursionExample3 {  

    static int factorial(int n){      

          if (n == 1)      

            return 1;      

          else      

            return(n * factorial(n-1));      

    }      

  

public static void main(String[] args) {  

System.out.println("Factorial of 5 is: "+factorial(5));  

}  

} 

Output:

Factorial of 5 is: 120

Working of above program:

factorial(5) 
   factorial(4) 
  factorial(3) 
     factorial(2) 
        factorial(1) 
           return 1 
        return 2*1 = 2 
     return 3*2 = 6 
  return 4*6 = 24 
   return 5*24 = 120

Java Recursion Example 4: Fibonacci Series

public class RecursionExample4 {  

    static int n1=0,n2=1,n3=0;      

     static void printFibo(int count){      

        if(count>0){      

             n3 = n1 + n2;      

             n1 = n2;      

             n2 = n3;      

             System.out.print(" "+n3);     

             printFibo(count-1);      

         }      

     }        

  

public static void main(String[] args) {  

    int count=15;      

      System.out.print(n1+" "+n2);//printing 0 and 1      

      printFibo(count-2);//n-2 because 2 numbers are already printed     

}  

} 

Output:

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

The wrapper class in Java provides the mechanism to convert primitive into object and object into primitive.

Since J2SE 5.0, autoboxing and unboxing feature convert primitives into objects and objects into primitives automatically. The automatic conversion of primitive into an object is known as autoboxing and vice-versa unboxing.

Use of Wrapper classes in Java

Java is an object-oriented programming language, so we need to deal with objects many times like in Collections, Serialization, Synchronization, etc. Let us see the different scenarios, where we need to use the wrapper classes.

• Change the value in Method: Java supports only call by value. So, if we pass a primitive value, it will not change the original value. But, if we convert the primitive value in an object, it will change the original value.
• Serialization: We need to convert the objects into streams to perform the serialization. If we have a primitive value, we can convert it in objects through the wrapper classes.
• Synchronization: Java synchronization works with objects in Multithreading.
• java.util package: The java.util package provides the utility classes to deal with objects.
• Collection Framework: Java collection framework works with objects only. All classes of the collection framework (ArrayList, LinkedList, Vector, HashSet, LinkedHashSet, TreeSet, PriorityQueue, ArrayDeque, etc.) deal with objects only.

The eight classes of the java.lang package are known as wrapper classes in Java. The list of eight wrapper classes are given below:

Primitive Type	Wrapper class
boolean	Boolean
char	Character
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double

Autoboxing

The automatic conversion of primitive data type into its corresponding wrapper class is known as autoboxing, for example, byte to Byte, char to Character, int to Integer, long to Long, float to Float, boolean to Boolean, double to Double, and short to Short.

Since Java 5, we do not need to use the valueOf() method of wrapper classes to convert the primitive into objects.

Wrapper class Example: Primitive to Wrapper

//Java program to convert primitive into objects  

//Autoboxing example of int to Integer  

public class WrapperExample1{  

public static void main(String args[]){  

//Converting int into Integer  

int a=20;  

Integer i=Integer.valueOf(a);//converting int into Integer explicitly  

Integer j=a;//autoboxing, now compiler will write Integer.valueOf(a) internally  

  

System.out.println(a+" "+i+" "+j);  

}}  

Output:

20 20 20

Unboxing

The automatic conversion of wrapper type into its corresponding primitive type is known as unboxing. It is the reverse process of autoboxing. Since Java 5, we do not need to use the intValue() method of wrapper classes to convert the wrapper type into primitives.

Wrapper class Example: Wrapper to Primitive

//Java program to convert object into primitives  

//Unboxing example of Integer to int  

public class WrapperExample2{    

public static void main(String args[]){    

//Converting Integer to int    

Integer a=new Integer(3);    

int i=a.intValue();//converting Integer to int explicitly  

int j=a;//unboxing, now compiler will write a.intValue() internally    

    

System.out.println(a+" "+i+" "+j);    

}}    

Output:

3 3 3

Java Wrapper classes Example

//Java Program to convert all primitives into its corresponding   

//wrapper objects and vice-versa  

public class WrapperExample3{  

public static void main(String args[]){  

byte b=10;  

short s=20;  

int i=30;  

long l=40;  

float f=50.0F;  

double d=60.0D;  

char c='a';  

boolean b2=true;  

  

//Autoboxing: Converting primitives into objects  

Byte byteobj=b;  

Short shortobj=s;  

Integer intobj=i;  

Long longobj=l;  

Float floatobj=f;  

Double doubleobj=d;  

Character charobj=c;  

Boolean boolobj=b2;  

  

//Printing objects  

System.out.println("---Printing object values---");  

System.out.println("Byte object: "+byteobj);  

System.out.println("Short object: "+shortobj);  

System.out.println("Integer object: "+intobj);  

System.out.println("Long object: "+longobj);  

System.out.println("Float object: "+floatobj);  

System.out.println("Double object: "+doubleobj);  

System.out.println("Character object: "+charobj);  

System.out.println("Boolean object: "+boolobj);  

  

//Unboxing: Converting Objects to Primitives  

byte bytevalue=byteobj;  

short shortvalue=shortobj;  

int intvalue=intobj;  

long longvalue=longobj;  

float floatvalue=floatobj;  

double doublevalue=doubleobj;  

char charvalue=charobj;  

boolean boolvalue=boolobj;  

  

//Printing primitives  

System.out.println("---Printing primitive values---");  

System.out.println("byte value: "+bytevalue);  

System.out.println("short value: "+shortvalue);  

System.out.println("int value: "+intvalue);  

System.out.println("long value: "+longvalue);  

System.out.println("float value: "+floatvalue);  

System.out.println("double value: "+doublevalue);  

System.out.println("char value: "+charvalue);  

System.out.println("boolean value: "+boolvalue);  

}}  

Output:

---Printing object values---
Byte object: 10
Short object: 20
Integer object: 30
Long object: 40
Float object: 50.0
Double object: 60.0
Character object: a
Boolean object: true
---Printing primitive values---
byte value: 10
short value: 20
int value: 30
long value: 40
float value: 50.0
double value: 60.0
char value: a
boolean value: true

Custom Wrapper class in Java

Java Wrapper classes wrap the primitive data types, that is why it is known as wrapper classes. We can also create a class which wraps a primitive data type. So, we can create a custom wrapper class in Java.

//Creating the custom wrapper class  

class Javatpoint{  

private int i;  

Javatpoint(){}  

Javatpoint(int i){  

this.i=i;  

}  

public int getValue(){  

return i;  

}  

public void setValue(int i){  

this.i=i;  

}  

@Override  

public String toString() {  

  return Integer.toString(i);  

}  

}  

//Testing the custom wrapper class  

public class TestJavatpoint{  

public static void main(String[] args){  

Javatpoint j=new Javatpoint(10);  

System.out.println(j);  

}}  

Output:

10