this keyword in java
There can be a lot of usage of this keyword. In java, this is a reference variable that refers to the current object.Usage of this keyword
the 6 usage of java this keyword.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.problem without using this keyword
Example:
class Person{ int rollno; String name; float fee; Person(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[]){ Person p1=new Person(101,"Ramesh",8000f); Person p2=new Person(102,"Sumun",9000f); p1.display(); p2.display(); } }
Output:
0 null 0.0 0 null 0.0
Solution of the above problem by this keyword
class Person{ int rollno; String name; float fee; Person(int rollno,String name,float fee){ This.rollno=rollno; This.name=name; This.fee=fee; } void display(){System.out.println(rollno+" "+name+" "+fee);} } class TestThis1{ public static void main(String args[]){ Person p1=new Person(101,"Ramesh",8000f); Person p2=new Person(102,"Sumun",9000f); p1.display(); p2.display(); } }
Output:
101 Ramesh 8000 102 Suman 9000
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.Example:
class P{ void q() { System.out.println("Hello q java");} void r() { System.out.println("Hello r java"); //q();//same as this.q() this.q(); } } class TestThis2 { public static void main(String args[]) { P p = new P(); p.r(); } }
Output:
Hello q java Hello r java
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.Example:
class P { P() { System.out.println("Hello Catchmecoder");} P(int y) { this(); System.out.println(y); } } class TestThis3 { public static void main(String args[]) { P p = new P(15); } }
Output:
Hello Catchmecoder 15
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 handlingExample:
class P2 { void r(P2 obj) { System.out.println("passed an argument in method"); } void s() { r(this); } public static void main(String args[]) { P2 p1 = new P2(); p1.s(); } }
Output:
passed an argument in method
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.Example:
class P { R3 obj; P(R3 obj) { this.obj=obj; } void display() { System.out.println(obj.data); //using data member of R3 class } } class R3 { int data=15; R3() { P p = new P(this); p.display(); } public static void main(String args[]){ R3 r = new R3(); } }
Output:
10
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).Syntax:
return_type method_name(){ return this; }
Example:
class P { P getP() { return this; } void msg() { System.out.println("Hello Catchmecoder"); } } class Test5 { public static void main(String args[]) { new P().getP().msg(); } }
Output:
Hello Catchmecoder