Implements Keyword In Android

In Android I am seeing code like this: public class Now extends Activity implements View.OnClickListener Is this legal in java? What exactly is View.OnClickListener representing h

Solution 1:

The question seems to be about the use of View.x. As @Vicente_Plata nicely put it in the comments:

OnClickListener is an interface declared inside the View class. Like an "inner class" but, rather, an "inner interface

---

Old answer:

---

View.OnclickListener is an interface. It defines methods that your activity must implement, in this case an OnClickListener(). Anyone checking wants to know if there is such a function in your class, so they can use Now as an OnclickListener.

An interface does not provide an implementation.

Also, from this page

Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

Solution 2:

OnClickListener is the interface used to receive click events. You need to override the onClick() method and implement your own code to deal with it. http://developer.android.com/reference/android/view/View.OnClickListener.html

You can take a look at the View class structure here: http://developer.android.com/reference/android/view/View.html#nestedclasses

And browse the implementation here: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.1_r2/android/view/View.java

You may want to read the android Dev Guide (specially the topic to handle UI events): http://developer.android.com/guide/topics/ui/ui-events.html

The following two are equivalent:

import android.view.View;
publicclassNowextendsActivityGroupimplementsOnClickListener {

import android.view.*;
publicclassNowextendsActivityGroupimplementsView.OnClickListener {

---

The following are not needed after the clarification

And perhaps the Oracle's java interface tutorial:

http://download.oracle.com/javase/tutorial/java/concepts/interface.html

http://download.oracle.com/javase/tutorial/java/IandI/createinterface.html

Solution 3:

As others already pointed out, implements View.OnClickListener specifies that your class implements the methods defined in the View.OnClickListener interface. If the dot notation makes you uncomfortable, you can import the interface explicitly:

import android.view.View.OnClickListener;

publicclassNowextendsActivityimplementsOnClickListener// ...