What does "this" mean?

Question

Possible Duplicate:
Java this.method() vs method()

I've been reading some things and doing some tutorials about android java but I still dont understand what "this" means, like in the following code.

    View continueButton = this.findViewById(R.id.continue_button);
    continueButton.setOnClickListener(this);
    View newButton = this.findViewById(R.id.new_button);
    newButton.setOnClickListener(this);

Also why is it in this example that a button is not defined with Button but with View, what is the difference?

ps. Great site!! trying to learn java and got ALLOT of answers by searching here!

You can't know. Because `this` is the receiver of the method in which this code is present. As we don't have the code, we can't answer. — Denys Séguret, Jun 06 '12 at 19:53
Here's the official Java explanation of [this](http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html). — Sam, Jun 06 '12 at 19:54
@Mark Not necessarily a dupe of that question, since the OP also asks about `setOnClickListener(this)` — Andres F., Jun 06 '12 at 20:00
read here: http://en.wikipedia.org/wiki/This_(computer_programming) and then find a good Object Oriented programming tutorial ... — rosco, Jun 06 '12 at 20:11

nhaarman · Accepted Answer · 2012-06-06T20:07:03.537

The this keyword is a reference to the current object. It is used to pass this instance of the object, and more.

For example, these two allocations are equal:

class Test{

    int a;

    public Test(){
        a = 5;
        this.a = 5;
    }

}

Sometimes you have a hidden field you want to access:

class Test{

    int a;

    public Test(int a){
        this.a = a;
    }

}

Here you assigned the field a with the value in the parameter a.

The this keyword works the same way with methods. Again, these two are the same:

this.findViewById(R.id.myid);
findViewById(R.id.myid);

Finally, say you have a class MyObject that has a method which takes a MyObject parameter:

class MyObject{

    public static void myMethod(MyObject object){
        //Do something
    }

    public MyObject(){
        myMethod(this);
    }

}

In this last example you passed a reference of the current object to a static method.

Also why is it in this example that a button is not defined with Button but with View, what is the difference?

In Android SDK, a Button is a subclass of View. You can request the Button as a View and cast the View to a Button:

Button newButton = (Button) this.findViewById(R.id.new_button);

nicholas.hauschild · Answer 2 · 2012-06-06T20:00:52.543

1

this is referring to the instance of the object that is being acted upon.

In the case that you have above, this.findViewById(R.id.continue_button) this is referring to a method in a parent class (Specifically either Activity.findViewById() or View.findViewByid(), assuming you are writing your own subclass of Activity or View!).

edited Jun 06 '12 at 20:00

answered Jun 06 '12 at 19:53

nicholas.hauschild

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That code could be placed within a custom View and it would still work perfectly. Saying its only Activity.findViewById() is not perfectly correct. – HandlerExploit Jun 06 '12 at 19:55
"this.someMethod" doesn't necessarily call the method in the parent class. In fact, `this` doesn't add information in this case, the way say, `super` would. (Unless by "parent class" you meant "the class of this instance). – Andres F. Jun 06 '12 at 19:57
1

@HandlerExploit Thank you for pointing that out. I will update accordingly. – nicholas.hauschild Jun 06 '12 at 19:57

score 0 · Answer 3 · edited Jul 06 '16 at 00:18

0

"this" is the current object instance.

class Blaa
{
   int bar=0;
   public Blaa()
   {}
   public void mogrify(int bar,Blaa that)
   {
       bar=1; //changes the local variable bar
       this.bar=1; //changes the member variable bar. 
       that.bar=1; //changes "that"'s member variable bar. 
   }

}

edited Jul 06 '16 at 00:18

Mr_Dave

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answered Jun 06 '12 at 19:53

Markus Mikkolainen

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score 0 · Answer 4 · answered Jun 06 '12 at 19:53

0

this refers to the current instance of a class

answered Jun 06 '12 at 19:53

Hunter McMillen

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`this` is NOT the current instance . its a pointer to the current instance. – Jay D Jun 06 '12 at 20:36
@JayD notice the word 'refers' in my answer above, there are _no_ pointers in Java only references. – Hunter McMillen Jun 06 '12 at 21:23
check this link for this keyword.. http://www.javabeat.net/qna/16-what-is-super-and-this-keyword-in-java/ – swiftBoy Jun 09 '12 at 07:07

score 0 · Answer 5 · answered Jun 06 '12 at 19:55

this in Java is a reference to the current object instance. So if you are writing a method for class MyClass, this is the current instance of MyClass.

Note that in your case, writing this.findViewById(...) isn't really necessary, and may be considered bad style.

score 0 · Answer 6 · answered Jun 06 '12 at 20:14

"this" in object oriented languages such as java, c# is a reference to the object on which you are invoking the method or whose data you are accessing.

See if this link is helpful for you in understanding the "this" more -

http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html

What does "this" mean?

6 Answers6

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