Modify buttons from an array of buttons

Question

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;
        }
    });
  }
}

Answer 1

for(int i = 0; i <= buttons.length; i++){do something with buttons[i]} is NEVER a valid "for" loop for a simple reason. It will ALWAYS give you NullPointerException because during the last cycle of the loop when i == buttons.length(); you are trying to perform an action on buttons[buttons.length()] which simply DOES NOT EXIST.

Try for(int i = 0; i < buttons.length; i++){do something with buttons[i]}

Answer 2

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);

  }
}

Answer 3

You cannot store the references in an initializer array like this

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

but why ? let go to Array Initializers

The length of the array to be constructed is equal to the number of variable initializers immediately enclosed by the braces of the array initializer. Space is allocated for a new array of that length. If there is insufficient space to allocate the array, evaluation of the array initializer completes abruptly by throwing an OutOfMemoryError. Otherwise, a one-dimensional array is created of the specified length, and each component of the array is initialized to its default value

So here the references will just be replaced by the values mean null hence they can no longer keep the track of change in the actual reference values

so the solution is to declare the array or ArrayList with reqired length and add your references to it.

//                  {null,null,null,....}
//Button[] buttons = {front,a,b1,b2,b3,b4};
  Button[] buttons = new Button[6];

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

    buttons[0] = (Button) findViewById(R.id.front);
    buttons[1] = (Button) findViewById(R.id.a);
    buttons[2] = (Button) findViewById(R.id.b1);
    buttons[3] = (Button) findViewById(R.id.b2);
    buttons[4] = (Button) findViewById(R.id.b3);
    buttons[5] = (Button) findViewById(R.id.b4);

    buttons[0].setOnTouchListener(new View.OnTouchListener() {
        @Override
        public boolean onTouch(View view, MotionEvent event) {
                Button GreenButton = b3;
            // note < instead of <=
            for(int i = 0; i < buttons.length; i++) {
                if (buttons[i] == buttons[4]) {
                    buttons[i].setBackgroundColor(Color.GREEN);
                }
                else
                    buttons[i].setBackgroundColor(Color.GRAY);
            }
            return false;
        }
    });