Array of buttons in Android

Question

I want to map the buttons to an array of buttons, and during compilation the code has no errors, but when it starts, the force closes:

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 

Answer 1

Your array is null and you are trying to get an index into it. That is what causes the NullPointerException . Your array must be initialized before you can use it to store your buttons.

If you need an array of nine buttons, change this line:

 Button buttons[] = null; 

For this:

 Button buttons[] = new Button[9]; 

In addition, you have a member of the Button buttons[] class and a local function variable, also called Button buttons[] . If this is intentional, then be sure to continue. Otherwise, you will need to change your line as follows:

 buttons[] = new Button[9]; 

Answer 2

This is usually better if you don't need to hard code constants like 9 into your code. And you usually don't need to.

You can, for example, put identifiers in an array and build on their basis the dynamic size of List

 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); } } 

Answer 3

  Button buttons[] = null; 

Need to create a button

using the new operator:

  Button buttons[] = new Button[9]; 

Answer 4

Try using 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); } 

Answer 5

EXAMPLE OF USE:

 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 the id button in your layout is button1, button2, button3, .... etc.

Answer 6

I had this situation, I chose a different approach. I stored id in an integer array.

 Int[] btnarr = new int[3]; btn[0]=R.id.button1; // give your ID Button btn = (Button).findViewById(cc[i]);