CODEWITHSUNDEEP
java
blekione
blekione

Reputation: 10590

What is returning class when use "return this"?

I started learning Java and I couldn't understand one of examples in "Thinking in Java" book. In this example author represent, as he state "simple use of 'this' keyword":

//Leaf.java
//simple use of the "this" keyword

public class Leaf {
    int i = 0;
    Leaf increment() {
        i++;
        return this;
    }
    void print() {
        System.out.println("i = " + i);
    }
    public static void main(String[] args) {
        Leaf x = new Leaf();
        x.increment().increment().increment().print();
    }
}

And when above code is working as indeed, I cant understand what increment() method is returning.

It's not variable i, it's not object x? I just don't get it. I tried to modify program to understand it (like replace return this with return i or print x instead of i), but compiler shows me errors.

Upvotes: 7

Views: 1152

Answers (4)

Juned Ahsan
Juned Ahsan

Reputation: 68715

return this;

will return the current object i.e. the object which you used to call that method. In your case object x of type Leaf will be returned.

Upvotes: 2

fastcodejava
fastcodejava

Reputation: 41117

return this; returns the instance of the class it is acting on. So every call of increment() is returns the same instance and then it calls increment again. You can keep on calling increment() as :

x.increment().increment().increment().increment().increment().increment()...

Upvotes: 0

Brendan Lesniak
Brendan Lesniak

Reputation: 2321

this is a keyword that is referencing the current Leaf instance. When you create your first Leaf with Leaf leaf = new Leaf () it creates a singular instance of Leaf

Esentially, you are returning the Leaf instance that is calling increment()

Upvotes: 0

developerwjk
developerwjk

Reputation: 8659

this represents the instance of the class from which the method was called. So returning this means returning this instance of the class. So, as the return type shows anyway, the increment() method is returning a Leaf and its returning the instance in which the increment() method was called.

This is why you can call:

x.increment().increment().increment().print();

Because with each call to .increment() you are getting another Leaf on which you can call all the methods inside Leaf again.

Upvotes: 0

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