问题
I want to map the buttons to an array of buttons and the code has no errors while compiling but there is force close when i run it:
Button buttons[];
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.game_board_view);
// Set OnClick listeners
Button buttons[] = null;
buttons[0] = (Button)findViewById(R.id.buttonOne);
buttons[1] = (Button)findViewById(R.id.buttonTwo);
buttons[2] = (Button)findViewById(R.id.buttonThree);
buttons[3] = (Button)findViewById(R.id.buttonFour);
buttons[4] = (Button)findViewById(R.id.buttonFive);
buttons[5] = (Button)findViewById(R.id.buttonSix);
buttons[6] = (Button)findViewById(R.id.buttonSeven);
buttons[7] = (Button)findViewById(R.id.buttonEight);
buttons[8] = (Button)findViewById(R.id.buttonMid);
}
LogCat:
03-26 21:42:51.455: D/dalvikvm(1156): GC_EXTERNAL_ALLOC freed 55K, 53% free 2566K/5379K, external 1625K/2137K, paused 98ms
03-26 21:42:54.323: D/AndroidRuntime(1156): Shutting down VM
03-26 21:42:54.323: W/dalvikvm(1156): threadid=1: thread exiting with uncaught exception (group=0x40015560)
03-26 21:42:54.343: E/AndroidRuntime(1156): FATAL EXCEPTION: main
03-26 21:42:54.343: E/AndroidRuntime(1156): java.lang.RuntimeException: Unable to start activity ComponentInfo{edu.project.superwordwheel/edu.project.superwordwheel.GameView}: java.lang.NullPointerException
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.os.Handler.dispatchMessage(Handler.java:99)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.os.Looper.loop(Looper.java:123)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread.main(ActivityThread.java:3683)
03-26 21:42:54.343: E/AndroidRuntime(1156): at java.lang.reflect.Method.invokeNative(Native Method)
03-26 21:42:54.343: E/AndroidRuntime(1156): at java.lang.reflect.Method.invoke(Method.java:507)
03-26 21:42:54.343: E/AndroidRuntime(1156): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
03-26 21:42:54.343: E/AndroidRuntime(1156): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
03-26 21:42:54.343: E/AndroidRuntime(1156): at dalvik.system.NativeStart.main(Native Method)
03-26 21:42:54.343: E/AndroidRuntime(1156): Caused by: java.lang.NullPointerException
03-26 21:42:54.343: E/AndroidRuntime(1156): at edu.project.superwordwheel.GameView.onCreate(GameView.java:43)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-26 21:42:54.343: E/AndroidRuntime(1156): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
03-26 21:42:54.343: E/AndroidRuntime(1156): ... 11 more
回答1:
Your array is null and you're trying to get an index into it. That is what's causing the NullPointerException. Your array must be initialized before you can use it to store your buttons.
If you want an array of nine buttons then change this line:
Button buttons[] = null;
To this:
Button buttons[] = new Button[9];
Also, you have a class member Button buttons[] and a local function variable that is also named Button buttons[]. If this is intentional then by all means carry on. Otherwise, you'll want to further change your line to this:
buttons[] = new Button[9];
回答2:
It's usually better if you don't have to hardcode constants like a 9 into your code. And you usually don't need to.
You can for example put the ids into an array and build a dynamically sized List based on them
private List<Button> buttons;
private static final int[] BUTTON_IDS = {
R.id.buttonOne,
R.id.buttonTwo,
R.id.buttonThree,
R.id.buttonFour,
R.id.buttonFive,
R.id.buttonSix,
R.id.buttonSeven,
R.id.buttonEight,
R.id.buttonMid,
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.game_board_view);
buttons = new ArrayList<Button>();
// or slightly better
// buttons = new ArrayList<Button>(BUTTON_IDS.length);
for(int id : BUTTON_IDS) {
Button button = (Button)findViewById(id);
button.setOnClickListener(this); // maybe
buttons.add(button);
}
}
回答3:
Button buttons[] = null;
button has to be created, using the new operator:
Button buttons[] = new Button[9];
回答4:
Try the following code:
private int objectLength = 9; //Array elements
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.game_board_view);
Button[] buttons = new Button[objectLength];
buttons[0] = (Button)findViewById(R.id.buttonOne);
buttons[1] = (Button)findViewById(R.id.buttonTwo);
buttons[2] = (Button)findViewById(R.id.buttonThree);
buttons[3] = (Button)findViewById(R.id.buttonFour);
buttons[4] = (Button)findViewById(R.id.buttonFive);
buttons[5] = (Button)findViewById(R.id.buttonSix);
buttons[6] = (Button)findViewById(R.id.buttonSeven);
buttons[7] = (Button)findViewById(R.id.buttonEight);
buttons[8] = (Button)findViewById(R.id.buttonMid);
}
回答5:
EXAMPLE USAGE:
Button[] buttons = initializeButtons(3);
buttons[1].setText("I am button1");
buttons[2].setText("I am button2");
buttons[3].setText("I am button3");
FUNCTION:
public Button[] initializeButtons(int x) {
Resources res = getResources();
Button[] buttons = new Button[x];
for (int i = 0; i < x; i++) {
String b = "button" + i;
buttons[i] = (Button) findViewById(res.getIdentifier(b, "id", getPackageName()));
} return buttons;//to prevent array out of bounds exception start from 0
}
NOTE: Make sure in your layout the button id is button1, button2, button3, .. .. etc'
回答6:
I had situation like this, I chose different approach. I stored id into integer array.
Int[] btnarr = new int[3];
btn[0]=R.id.button1; // give your ID
Button btn = (Button).findViewById(cc[i]);
来源：https://stackoverflow.com/questions/15642104/array-of-buttons-in-android