when and how to use this keyword in a method

[Vonique Leary]

class Zoo {
public String coolMethod() {
return "Wow baby";
}
}
class Moo extends Zoo {
public void useMyCoolMethod() {
// Does an instance of Moo inherit the coolMethod()?
System.out.println("Moo says, " + this.coolMethod());
// The preceding line works because Moo can inherit the
// public method
// Can an instance of Moo invoke coolMethod() on an
// instance of Zoo?
Zoo z = new Zoo();
System.out.println("Zoo says, " + z.coolMethod());
// coolMethod() is public, so Moo can invoke it on a Zoo
//reference
}
}

What is the point of the this keyword in this example from Bates and Sierra's book? I tried the example both with and without it and got the same result. So now I'm confused about when to use this inside a method. Can someone give me an example that actually makes a difference in the output?

Thanks, Vonique

[Kemal Sokolovic]

Read Using the this Keyword, it contains good explanation on usage of this with some examples.

[Campbell Ritchie]

Hello again, VL.

There is a 3rd use of this, which that link misses out. It can mean the whole object you happen to be in. You cna have a serialising method which writes the entire object to a file with an object output stream or similar:-
oos.writeObject(this);

[Junilu Lacar]

The keyword can also help you access same-named variables that have been shadowed in the current scope. In the example below, the 'message' field declared on line 3 is shadowed by the parameter on line 10. Using 'this' on line 11 allows you to access the field instead of the parameter. I think this is kind of where the example you cited was going. Note that this is just an example and you should avoid situations where names are shadowed in real-world code.

public class ThisExample {

    private String message = "Member value";
    
    public static void main(String[] args) {
    	ThisExample example = new ThisExample();
    	example.show("Parameter value");
    }
    
    private void show(String message) {
        System.out.println("this.message=" + this.message);
        System.out.println("parameter message=" + message);
    }
}

[Vonique Leary]

In my above example the this keyword doesn't seem necessary because if you take it out you get the same results. Why would the writers include it? And also, I understand that this.coolMethod() refers to the current object that is calling it so it returns "Wow baby" but can anyone explain in which situation would it return something different? I guess that is what I am struggling to understand.

Hi again to you Campbell!

[Vonique Leary]

Okay, I think I just figured out my answer. If I alter the code like this:

public class Moo extends Zoo {  
    	
    	public String coolMethod() {
    		return "Hi baby";

and override the method, then the second coolMethod() inside Moo returns "Hi baby."

Got it. The little snippet in the SCJP book uses this.coolMethod() when it wasn't even necessary and that is what threw me off.

Thanks, all

Vonique