OnLongClickListener导致运行时错误

Question

I have been attempting to use a OnLongClickListener in my Android app. I am trying to use the OnLongClickListener to increment a number while the user has a button pressed. I had taken OnClickListener code that is working elsewhere in my application and merely updated it OnLongClickListener however when I try to run my application runtime errors occur. Here is my code:

public void onCreate(Bundle savedInstanceState)                                     //performed when application is first run        
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);                                             //link to activity_main.xml
    calculateElapsedTime();     
    Button myButton = (Button)findViewById(R.id.delayUp);                   
    myButton.setOnLongClickListener(new View.OnLongClickListener() {                    
        public boolean onLongClick(View v){                                             
            System.out.println("Hello World");
            return true;
        }
    });
}

Could anyone please provide some insight on why this code is not working. Note that I have "Hello World" in the code strictly for testing purposes however since I get run-time errors when the application loads obviously this line of code is never processed.

Here is the errors from LogCat:

09-16 17:22:34.982: D/dalvikvm(275): GC_EXTERNAL_ALLOC freed 750 objects / 55632 bytes in 179ms
09-16 17:22:35.113: D/AndroidRuntime(275): Shutting down VM
09-16 17:22:35.124: W/dalvikvm(275): threadid=1: thread exiting with uncaught exception  (group=0x4001d800)
09-16 17:22:35.203: E/AndroidRuntime(275): FATAL EXCEPTION: main
09-16 17:22:35.203: E/AndroidRuntime(275): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.morinnic.intervaltimer/com.morinnic.intervaltimer.MainActivity}: java.lang.ClassCastException: android.widget.ImageButton
09-16 17:22:35.203: E/AndroidRuntime(275):  at   android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
09-16 17:22:35.203: E/AndroidRuntime(275):  at  android.os.Handler.dispatchMessage(Handler.java:99)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.os.Looper.loop(Looper.java:123)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.ActivityThread.main(ActivityThread.java:4627)
09-16 17:22:35.203: E/AndroidRuntime(275):  at java.lang.reflect.Method.invokeNative(Native Method)
09-16 17:22:35.203: E/AndroidRuntime(275):  at java.lang.reflect.Method.invoke(Method.java:521)
09-16 17:22:35.203: E/AndroidRuntime(275):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-16 17:22:35.203: E/AndroidRuntime(275):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-16 17:22:35.203: E/AndroidRuntime(275):  at dalvik.system.NativeStart.main(Native Method)
09-16 17:22:35.203: E/AndroidRuntime(275): Caused by: java.lang.ClassCastException: android.widget.ImageButton
09-16 17:22:35.203: E/AndroidRuntime(275):  at com.morinnic.intervaltimer.MainActivity.onCreate(MainActivity.java:32)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
09-16 17:22:35.203: E/AndroidRuntime(275):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
09-16 17:22:35.203: E/AndroidRuntime(275):  ... 11 more

Any assistance would be greatly appreciated. Thanks.

Answer 1

Given the information you give, the obvious possibility is: myButton might be null (which will throw a NullPointerException), that is, the element corresponding to the id does not exists on the view (yet), or it is not of the type Button (which will throw a ClassCastException).

Make sure that you call setContentView on the activity before your code above, and be sure that it really is a Button.

A sample from the output from your phone would help more. Find out how to use the adb logcat command, it will show the exact exception that you get.