Now that the buttons have resource IDs, you can access them in QuizActivity. The first step is to add two member variables.
Type the following code into QuizActivity.java. (Do not use code completion; type it in yourself.) After you save the file, it will report two errors.
Listing 1.7 Adding member variables (QuizActivity.java)
public class QuizActivity extends AppCompatActivity {
private Button mTrueButton;
private Button mFalseButton;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
}
}
You will fix the errors in just a second. First, notice the m prefix on the two member (instance) variable names. This prefix is an Android naming convention that we will follow throughout this book.
Now mouse over the red error indicators. They report the same problem: Cannot resolve symbol 'Button'.
These errors are telling you that you need to import the android.widget.Button class into QuizActivity.java. You could type the following import statement at the top of the file:
import android.widget.Button;
Or you can do it the easy way and let Android Studio do it for you. Just press Option+Return (or Alt+Enter) to let the IntelliJ magic under the hood amaze you. The new import statement now appears with the others at the top of the file. This shortcut is generally useful when something is not correct with your code. Try it often!
This should get rid of the errors. (If you still have errors, check for typos in your code and XML.)
Now you can wire up your button widgets. This is a two-step process:
-
get references to the inflated View objects
-
set listeners on those objects to respond to user actions
In an activity, you can get a reference to an inflated widget by calling the following Activity method:
public View findViewById(int id)
This method accepts a resource ID of a widget and returns a View object.
In QuizActivity.java, use the resource IDs of your buttons to retrieve the inflated objects and assign them to your member variables. Note that you must cast the returned View to Button before assigning it.
Listing 1.8 Getting references to widgets (QuizActivity.java)
public class QuizActivity extends AppCompatActivity {
private Button mTrueButton;
private Button mFalseButton;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
mTrueButton = (Button) findViewById(R.id.true_button);
mFalseButton = (Button) findViewById(R.id.false_button);
}
}
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(Bundle) just after the variable assignment.
Listing 1.9 Setting a listener for the 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.
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() { @Override public 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 you 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 Setting a listener for the 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!
}
});
}