Ändern von Tasten aus einem Array von Tasten

Question

Ich versuche, ein Programm zu schreiben, so dass, wenn die Taste "Front" gedrückt wird, alle anderen Tasten zu grau mit Ausnahme der Schaltfläche ändern, die GreenButton entspricht. Ich habe Probleme mit dem Zugriff auf die Schaltflächen in der for-Schleife und bekomme weiterhin eine Null-Zeiger-Ausnahme.

Der vollständige logcat ist unten dargestellt, die zeigt, dass die NPE auftritt, wenn ich das Array in der Schleife zuzugreifen versuchen

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; 
     } 
    }); 
    } 
}