‘This’ Keyword usage

Question

class S2{
void m(S2 obj){
System.out.println("Hello");
}
void p(){
m(this);
}
public static void main(String args[]){
S2 s1 = new S2();
s1.p();
}
}// output: Hello

class A{
A get(){return this;}
}

class B1 extends A{
B1 get(){return this;}
void message(){System.out.println("welcome to covariant return type");}

public static void main(String args[]){
new B1().get().message();
}
} // output: welcome to covariant return type

Iam not understanding the purpose of ‘this’ keyword in both programs. Is This keyword is used to call class or obj? Can anyone plz explain

Answer 1

"This" is used as a reference of an object who uses a method or a variable.

For example the first program the m function needs an object of the class S2, when you call the function p it calls the function m and pass "this" as parameter, referring to the object who activate it.

This is more visible if you print in console a variable of the object instead a constant message.

Have a nice day 🧏🏼‍♀️🧏🏼‍♀️🧏🏼‍♀️

Answer 2

//interesting Sololearn runs both main() methods in one code :)

modify your code for print object and this object:
class S2 {
void m(S2 obj) {
System.out.println(obj+" this obj");
}
..
..main(..) {
S2 s1 = new S2();
System.out.println(s1 +" s1");
s1.p();
class B1 ..
..main(..)
B1 b1 = new B1();
System.out.println(b1 +" b1");
System.out.println(b1.get()+" this obj of B1" );