Wrapper classes

Wrapper classes provide a way to use primitive data types (int, boolean, etc..) as objects.

Primitive data type	Constructors	Wrapper Class
char	char	Character
byte	byte, String	Byte
short	short, String	Short
int	int, String	Integer
long	long, String	Long
float	double, float, String	Float
double	double, String	Double
boolean	boolean, String	Boolean

Putting the primitive value inside the object is called wrapping or boxing and taking out the value from object is called unwrapping or unboxing. 

Methods:

• toString();  Whenever,  we are printing reference variable, internally it calls toString() present in object class.

class Test{

       public static void main(String[] args) {

             Test t = new Test();

             System.out.println(t);

             System.out.println(t.toString());

             

             Integer i = new Integer(100);

             System.out.println(i);

             System.out.println(i.toString()); //actual i.e. calls toString method implicitly

       }

}

Output

com.cerotid.oo.principles.Test@15db9742

com.cerotid.oo.principles.Test@15db9742

100

100

toString() belongs to the object class and it returns class-name@hashcode (unique identification number of object generated by JVM to identify the object uniquely)  but in all wrapper classes, toString() is overriding to returns the content of the all wrapper class object. 

Example 

Converting a primitive type to Wrapper object

public class Wrapper1 {

    public static void main(String[] args) {

        int num = 100;

        Integer obj = Integer.valueOf(num);

        System.out.println(num);

        System.out.println(obj);

    }

}

Output:

100

100

Example 

Converting Wrapper class object to Primitive

package com.cerotid.methodconcept;

public class Wrapper1 {

    public static void main(String[] args) {

        Integer obj = new Integer(100); // 100 here is object value

        int num = obj.intValue();

        System.out.println(num);

        System.out.println(obj);

    }

}

Output:

100 (primitive)

100 (object)