Android applications are typically event driven. Unlike command-line programs or scripts, event-driven applications start and then wait for an event, such as the user pressing a button. (Events can also be initiated by the OS or another application, but user-initiated events are the most obvious.) When your application is waiting for a specific event, we say that it is “listening for” that event. The object that you create to respond to an event is called a listener, and the listener implements a listener interface for that event.
The Android SDK comes with listener interfaces for various events, so you do not have to write your own. In this case, the event you want to listen for is a button being pressed (or “clicked”), so your listener will implement the View.OnClickListener interface. Start with the True button. In QuizActivity.java, add the following code to onCreate(…) just after the variable assignment.
Listing 1.9 Set listener for True button (QuizActivity.java)
...
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
mTrueButton = (Button) findViewById(R.id.true_button);
mTrueButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Does nothing yet, but soon!
}
});
mFalseButton = (Button) findViewById(R.id.false_button);
}
}
(If you have a View cannot be resolved to a type error, try using Option+Return (Alt+Enter) to import the View class.) In Listing 1.9, you set a listener to inform you when the Button known as mTrueButton has been pressed. The setOnClickListener(OnClickListener) method takes a listener as its argument. In particular, it takes an object that implements OnClickListener.
Using anonymous inner classes
This listener is implemented as an anonymous inner class. The syntax is a little tricky, but it helps to remember that everything within the outermost set of parentheses is passed into setOnClickListener(OnClickListener). Within these parentheses, you create a new, nameless class and pass its entire implementation.
mTrueButton.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {// Does nothing yet, but soon!}});All of the listeners in this book will be implemented as anonymous inner classes. Doing so puts the implementations of the listeners’ methods right where you want to see them. And there is no need for the overhead of a named class because the class will be used in one place only. Because your anonymous class implements OnClickListener, it must implement that interface’s sole method, onClick(View). You have left the implementation of onClick(View) empty for now, and the compiler is OK with that. A listener interface requires you to implement onClick(View), but it makes no rules about how to implement it. (If your knowledge of anonymous inner classes, listeners, or interfaces is rusty, you may want to review some Java before continuing or at least keep a reference nearby.)
Set a similar listener for the False button.
Listing 1.10 Set listener for False button (QuizActivity.java)
...
mTrueButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Does nothing yet, but soon!
}
});
mFalseButton = (Button) findViewById(R.id.false_button);
mFalseButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Does nothing yet, but soon!
}
});
}
