Modify buttons from an array of buttons

Issue

I am trying to write a program so that whenever the button called 'front' is pressed all other buttons will change to gray except the button that equals GreenButton. I am struggling with accessing the buttons in the for loop, and keep getting a null pointer exception.

The full logcat is shown below, showing that the NPE is occurring when I am trying to access the array in the loop

09-20 11:50:54.727 5302-5302/com.example.kshah.movingbutton E/AndroidRuntime: FATAL EXCEPTION: main
  Process: com.example.kshah.movingbutton, PID: 5302
  java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setBackgroundColor(int)' on a null object reference
      at com.example.kshah.movingbutton.MainActivity$1.onTouch(MainActivity.java:108)
      at android.view.View.dispatchTouchEvent(View.java:9296)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2197)
      at com.android.internal.policy.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2403)
      at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1737)
      at android.app.Activity.dispatchTouchEvent(Activity.java:2771)
      at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:69)
      at com.android.internal.policy.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2364)
      at android.view.View.dispatchPointerEvent(View.java:9520)
      at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4230)
      at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4096)
      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
      at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695)
      at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661)
      at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3787)
      at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669)
      at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3844)
      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
      at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695)
      at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661)
      at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669)
      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
      at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5922)
      at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5896)
      at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5857)
      at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6025)
      at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
      at android.os.MessageQueue.nativePollOnce(Native Method)
      at android.os.MessageQueue.next(MessageQueue.java:323)
      at android.os.Looper.loop(Looper.java:135)
      at android.app.ActivityThread.main(ActivityThread.java:5417)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

CODE:

public class MainActivity extends AppCompatActivity {

  private Button front;
  private Button a;
  private Button b1;
  private Button b2;
  private Button b3;
  private Button b4;

  Button[] buttons = {front,a,b1,b2,b3,b4};

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    front = (Button) findViewById(R.id.front);
    a = (Button) findViewById(R.id.a);
    b1 = (Button) findViewById(R.id.b1);
    b2 = (Button) findViewById(R.id.b2);
    b3 = (Button) findViewById(R.id.b3);
    b4 = (Button) findViewById(R.id.b4);

    front.setOnTouchListener(new View.OnTouchListener() {
        @Override
        public boolean onTouch(View view, MotionEvent event) {
                Button GreenButton = b3;
            for(int i = 0; i <= buttons.length; i++) {
                if (buttons[i] == b3) {
                    buttons[i].setBackgroundColor(Color.GREEN);
                }
                else
                    buttons[i].setBackgroundColor(Color.GRAY);
            }
            return false;
        }
    });
  }
}

Solution

You can try using new Button[size], something like this:

// allocates memory for 6 button.
Button[] buttons = new Button[6]; 

Then, add the buttons to it:

front = (Button) findViewById(R.id.front);
a = (Button) findViewById(R.id.a);
b1 = (Button) findViewById(R.id.b1);
b2 = (Button) findViewById(R.id.b2);
b3 = (Button) findViewById(R.id.b3);
b4 = (Button) findViewById(R.id.b4);

buttons[0] = front;
buttons[1] = a;
buttons[2] = b1;
buttons[3] = b2;
buttons[4] = b3;
buttons[5] = b4;

---

Explanation:

When you creating the array with:

private Button front;
private Button a;
private Button b1;
private Button b2;
private Button b3;
private Button b4;

Button[] buttons = {front,a,b1,b2,b3,b4};

You're creating an array with null object elements, which is like

Button[] buttons = {null,null,null,null,null,null};

Then when you assign the Button to one of the array member with:

front = (Button) findViewById(R.id.front);

You're assigning an object to front not to buttons[0].

---

You can try the following code to understand what the above explanation means:

public class MainActivity extends AppCompatActivity {
  public static final String TAG = MainActivity.class.getSimpleName();

  private Button front;
  private Button a;
  private Button b1;
  private Button b2;
  private Button b3;
  private Button b4;

  Button[] buttons = {front,a,b1,b2,b3,b4};

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Check address:
    Log.d(TAG, "Before initializing..");
    for(int i = 0; i < buttons.length; i++) {
      Log.d(TAG, "i " + buttons[i]);
    }
    front = (Button) findViewById(R.id.front);
    a = (Button) findViewById(R.id.a);
    b1 = (Button) findViewById(R.id.b1);
    b2 = (Button) findViewById(R.id.b2);
    b3 = (Button) findViewById(R.id.b3);
    b4 = (Button) findViewById(R.id.b4);
    Log.d(TAG, "After initializing..");
    for(int i = 0; i < buttons.length; i++) {
      Log.d(TAG, "i " + buttons[i]);
    }

    Log.d(TAG, "After assigning..");
    Log.d(TAG, "front " + front);
    Log.d(TAG, "a " + a);
    Log.d(TAG, "b1 " + b1);
    Log.d(TAG, "b2 " + b2);
    Log.d(TAG, "b3 " + b3);
    Log.d(TAG, "b4 " + b4);

  }
}

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