title: Change EditText according to another EditText
body: <p>I'm new to Android development. I have two EditTexts, and I'm trying to change value of ET1 according to ET2, and also value of ET2 according to ET1, automatically during the typing. </p>
  
  <p>Sum of their values must be 200. So if I want to type 56 in ET1, when I type 5, ET2 should display 195. When I add 6, ET2 should display 144 (200 - 56). Everything should happen while focus is still on ET1.</p>
  
  <pre><code>scoreWe.setOnFocusChangeListener(new View.OnFocusChangeListener() {
          @Override
          public void onFocusChange(View v, boolean hasFocus) {
              if (v.hasFocus()) {
                  scoreYou.addTextChangedListener(null);
                  scoreWe.addTextChangedListener(new TextWatcher() {
                      @Override
                      public void beforeTextChanged(CharSequence s, int start, int count, int after) {
  
                      }
  
                      @Override
                      public void onTextChanged(CharSequence s, int start, int before, int count) {
  
                      }
  
                      @Override
                      public void afterTextChanged(Editable s) {
                          int scoreW;
                          try {
                              scoreW = Integer.parseInt(scoreWe.getText().toString());
                          } catch (NumberFormatException e) {
                              scoreW = 0;
                          }
                          int scoreY = 200 - scoreW;
                          String scY = Integer.toString(scoreY);
                          scoreYou.setText(scY);
                      }
                  });
              }
          }
      });
  </code></pre>
  
  <p>-</p>
  
  <pre><code>scoreYou.setOnFocusChangeListener(new View.OnFocusChangeListener() {
          @Override
          public void onFocusChange(View v, boolean hasFocus) {
              if (v.hasFocus()) {
                  scoreWe.addTextChangedListener(null);
                  scoreYou.addTextChangedListener(new TextWatcher() {
                      @Override
                      public void beforeTextChanged(CharSequence s, int start, int count, int after) {
  
                      }
  
                      @Override
                      public void onTextChanged(CharSequence s, int start, int before, int count) {
  
                      }
  
                      @Override
                      public void afterTextChanged(Editable s) {
                          int scoreY;
                          try {
                              scoreY = Integer.parseInt(scoreYou.getText().toString());
                          } catch (NumberFormatException e) {
                              scoreY = 0;
                          }
                          int scoreW = 200 - scoreY;
                          String scW = Integer.toString(scoreW);
                          scoreWe.setText(scW);
                      }
                  });
              }
          }
      });
  </code></pre>
  
  <p>Those are 2 OnFocusChangeListeners, and after I try to input something in any of the EditTexts, app crashes instantly.
  Log:</p>
  
  <pre><code>09-18 16:50:22.362 17764-17764/com.example.dvido.bela E/AndroidRuntime: FATAL EXCEPTION: main
                                                                      Process: com.example.dvido.bela, PID: 17764
                                                                      java.lang.NullPointerException: Attempt to invoke interface method 'void android.text.TextWatcher.beforeTextChanged(java.lang.CharSequence, int, int, int)' on a null object reference
                                                                          at android.widget.TextView.sendBeforeTextChanged(TextView.java:7941)
                                                                          at android.widget.TextView.setText(TextView.java:4242)
                                                                          at android.widget.TextView.setText(TextView.java:4199)
                                                                          at android.widget.EditText.setText(EditText.java:84)
                                                                          at android.widget.TextView.setText(TextView.java:4174)
                                                                          at com.example.dvido.bela.InputGame$1$1.afterTextChanged(InputGame.java:89)
                                                                          at android.widget.TextView.sendAfterTextChanged(TextView.java:8007)
                                                                          at android.widget.TextView$ChangeWatcher.afterTextChanged(TextView.java:10168)
                                                                          at android.text.SpannableStringBuilder.sendAfterTextChanged(SpannableStringBuilder.java:1043)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:560)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:492)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:491)
                                                                          at android.text.method.NumberKeyListener.onKeyDown(NumberKeyListener.java:121)
                                                                          at android.widget.TextView.doKeyDown(TextView.java:6098)
                                                                          at android.widget.TextView.onKeyDown(TextView.java:5911)
                                                                          at android.view.KeyEvent.dispatch(KeyEvent.java:2640)
                                                                          at android.view.View.dispatchKeyEvent(View.java:9234)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1640)
                                                                          at com.android.internal.policy.PhoneWindow$DecorView.superDispatchKeyEvent(PhoneWindow.java:2395)
                                                                          at com.android.internal.policy.PhoneWindow.superDispatchKeyEvent(PhoneWindow.java:1727)
                                                                          at android.app.Activity.dispatchKeyEvent(Activity.java:2725)
                                                                          at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:543)
                                                                          at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:53)
                                                                          at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:312)
                                                                          at com.android.internal.policy.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2310)
                                                                          at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:4127)
                                                                          at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4089)
                                                                          at android.view.ViewRootImpl$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$InputStage.onDeliverToNext(ViewRootImpl.java:3695)
                                                                          at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661)
                                                                          at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3820)
                                                                          at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3981)
                                                                          at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2253)
                                                                          at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1874)
  </code></pre>
  
  <p>09-18 16:50:36.917 17764-17764/com.example.dvido.bela I/Process: Sending signal. PID: 17764 SIG: 9</p>