Question

Anytime I press the Back Key, my application crashes. I have tried to different ways, and they both crash throwing a NullPointerException…. any ideas?

Way 1:

public boolean onKeyDown(int keyCode, KeyEvent event) {
  // Check if the key event was the BACK key and if there's history
  if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebView.canGoBack()) {
      myWebView.goBack();
      return true;
  }
  // If it wasn't the BACK key or there's no web page history, bubble up to the default
  // system behavior (probably exit the activity)
  return super.onKeyDown(keyCode, event);
}

Way 2:

    public void onBackPressed() {
      myWebView.goBack();
      return;
}

Stack Trace:

11-19 20:13:12.425: E/AndroidRuntime(1963): FATAL EXCEPTION: main
11-19 20:13:12.425: E/AndroidRuntime(1963): java.lang.NullPointerException
11-19 20:13:12.425: E/AndroidRuntime(1963):     at com.meanwhileinwv.android.MNWVShow.onBackPressed(MNWVShow.java:27)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.app.Activity.onKeyUp(Activity.java:1983)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.view.KeyEvent.dispatch(KeyEvent.java:1518)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.app.Activity.dispatchKeyEvent(Activity.java:2163)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1747)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.view.ViewRoot.deliverKeyEventToViewHierarchy(ViewRoot.java:2702)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.view.ViewRoot.handleFinishedEvent(ViewRoot.java:2677)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1965)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.os.Looper.loop(Looper.java:143)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at android.app.ActivityThread.main(ActivityThread.java:4263)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at java.lang.reflect.Method.invokeNative(Native Method)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at java.lang.reflect.Method.invoke(Method.java:507)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
11-19 20:13:12.425: E/AndroidRuntime(1963):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

The only way to get the stack trace you are getting from the code you’ve shared is for myWebView to be null when myWebView.goBack() is called. To prove this to yourself, modify your onBackPressed():

public void onBackPressed() {
    if (myWebView != null) {
        myWebView.goBack();
    }
}

The exception should go away. (Though, obviously, you still won’t get any back behavior occurring.)

Possible causes:

• myWebView is not a member variable in your class. You may have declared and set it in another method, which means it’s not in scope of onBackPressed.
• myWebView isn’t the actual name of the variable that refers to your WebView
• myWebView used to refer to your WebView but at some point was set to null again
• The code that you think is pointing myWebView to your WebView is actually setting to null. For instance, a findViewById call that is not actually finding the WebView because you’ve specified the wrong ID will return null.
• etc.

If you’re still stuck, post the code where myWebView is declared and where it is assigned.