this keyword in java

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'this'keyword refers to the current Object (-the object whose method or constructor is being called), the most common use of 'this' keyword is to remove ambiguity when a method level variable is hiding a class level variable.

Usage of this keyword:

• 'this' keyword in Java is used to refer to current Object/Instance within an instance method or constructor
• t can be used to access Instance variable within a method or constructor
• It can be used to call one constructor from another one
• It can be used to pass current Object/Instance as parameter in a method
• It can be used to return current object from a method

this keyword with Field (Instance variable):

This is the most common use of 'this' keyword, to remove ambiguity when a method level variable is hiding a class level variable.

package com.meritmix.blog.java;

public class ThisKeyword {
private int a=10;

public void print(int a) {
 // now method level variable/variable passed in parameter hides variable 'a' at class level
 System.out.println("print loal variable: "+a);
 System.out.println("print class level variable: "+this.a);
}
 public static void main(String[] args) {
  new ThisKeyword().print(5);

 }

}

Output:

print loal variable: 5
print class level variable: 10

this keyword with Constructor:

'this' keyword can be used to call another overloaded constructor from a constructor of an object. This is called explicit constructor invocation. This occurs when class has one or more overloaded constructor.

package com.meritmix.blog.java;

public class ThisKeywordConstructor {

 public ThisKeywordConstructor() {
  this(5);
  System.out.println("invoke parameterized constructor(int version) from defauld constructor");
  //this(5); Error: Constructor call must be the first statement in a constructor
 }

 public ThisKeywordConstructor(int a) {
  System.out.println("int :"+a);
 }

 public ThisKeywordConstructor(String str) {
  this();
  System.out.println("invoke defauld constructor from parameterized constructor(String version)");
 }
 public static void main(String[] args) {
  ThisKeywordConstructor obj = new ThisKeywordConstructor("test");//invoke String version

 }
}

Output:

int :5
invoke parameterized constructor(int version) from defauld constructor
invoke defauld constructor from parameterized constructor(String version)

Note:

• 'this' keyword can only be the first statement in Constructor
• A constructor can have either 'this' or 'super' keyword but not both

this keyword with method:

'this' keyword can be used to call another method from a method of of same class.

Example:

package com.meritmix.blog.java;

public class ThisKeywordMethod {

 public ThisKeywordMethod() {
 }

 public void demo() {
  System.out.println("demo method");
  this.print(5);// this is same as calling print(5)
 }

 public void print(int a) {
  System.out.println("value: "+a);
 }
 public static void main(String[] args) {
  ThisKeywordMethod obj = new ThisKeywordMethod();//invoke String version
  obj.demo();
 }

}

Output:

demo method
value: 5

this keyword as parameter:

package com.meritmix.blog.java;

public class ThisAsMehodParameter {
 int a=0;
 public void print() {
  print(this);
 }
 public void print(ThisAsMehodParameter obj) {
  System.out.println(obj.a);
 }

 public static void main(String[] args) {
  ThisAsMehodParameter obj =new ThisAsMehodParameter();
  obj.a =20;
  obj.print();

 }

}

this keyword as return type:

package com.meritmix.blog.java;

public class ThisAsMehodReturnType {
 int a=0;
 public ThisAsMehodReturnType getObject() {
  return this; //return current object
 }
 public void print() {
  System.out.println(getObject().a);
 }

 public static void main(String[] args) {
  ThisAsMehodReturnType obj =new ThisAsMehodReturnType();
  obj.print();

 }

}

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