java.lang.ClassCastException: android.view.View cannot be cast to android.view.ViewGroup

Question

I am now implementing a drag and drop operation . In this operation drag from a relative layout to anther one is allowed . If there's a child in a layout and another image is drag to it the images must exchange position . Here's my problem when i cast a view to view group , Class cast exception occurs . I really don't know how to solve it as I'm a new to android . The error line at log cat points at this ((ViewGroup)parent).addView(nextChild). Please give me advice . Sorry if my question bother you .

This is my Log cat output :

01-09 01:57:27.829: E/AndroidRuntime(2021): FATAL EXCEPTION: main
01-09 01:57:27.829: E/AndroidRuntime(2021): Process: com.example.barnyar, PID: 2021
01-09 01:57:27.829: E/AndroidRuntime(2021): java.lang.ClassCastException: android.view.View cannot be cast to android.view.ViewGroup
01-09 01:57:27.829: E/AndroidRuntime(2021):     at com.example.barnyar.MainActivity$MyDragListener.onDrag(MainActivity.java:751)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.View.dispatchDragEvent(View.java:17371)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1300)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1286)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1286)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1286)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1286)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1286)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewRootImpl.handleDragEvent(ViewRootImpl.java:5026)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewRootImpl.access$800(ViewRootImpl.java:96)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:3213)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.os.Looper.loop(Looper.java:136)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at android.app.ActivityThread.main(ActivityThread.java:5017)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at java.lang.reflect.Method.invokeNative(Native Method)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at java.lang.reflect.Method.invoke(Method.java:515)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-09 01:57:27.829: E/AndroidRuntime(2021):     at dalvik.system.NativeStart.main(Native Method)

This is my code :

@Override 
public boolean onDrag(View v, DragEvent event) {

    View parent = new View(MainActivity.this);
        switch (event.getAction()) {

        case DragEvent.ACTION_DRAG_STARTED:
                Toast.makeText(getApplicationContext(), "Start Drag ", Toast.LENGTH_LONG).show();
                parent = v;
                Log.i("class",parent+"");
            break;

        case DragEvent.ACTION_DRAG_ENTERED:
            v.setBackground(normalShape);   //change the shape of the view
            break;


        case DragEvent.ACTION_DRAG_EXITED:
            v.setBackground(normalShape);   //change the shape of the view back to normal
            break;

        case DragEvent.ACTION_DROP:

            if(v.getClass().toString().equals("class android.widget.RelativeLayout")){
                Log.i("class","Relative");



                if(((ViewGroup)v).getChildCount()!=0){

                View nextChild = ((ViewGroup)v).getChildAt(0);
                Log.i("child",((ViewGroup)v).getChildCount()+"");   
                      ((ViewGroup)parent).addView(nextChild);//the error line
                 View view = (View) event.getLocalState();
                  ViewGroup viewgroup = (ViewGroup) view.getParent();
                  viewgroup.removeView(view);
                  RelativeLayout containView = (RelativeLayout) v;
                  containView.addView(view);
                  view.setVisibility(View.VISIBLE);

                }


                else {
                  View view = (View) event.getLocalState();
                  ViewGroup viewgroup = (ViewGroup) view.getParent();
                  viewgroup.removeView(view);
                  RelativeLayout containView = (RelativeLayout) v;
                  containView.addView(view);
                  view.setVisibility(View.VISIBLE);
                }
                }

            else {

                  Log.i("CLass", v.getClass()+"Cant drop");
                  View view = (View) event.getLocalState();
                  view.setVisibility(View.VISIBLE);

                  break;
               }
              break;

        case DragEvent.ACTION_DRAG_ENDED:
            v.setBackground(normalShape);   //go back to normal shape

        default:
            break;
        }
        return true;
    }
}

Answer 1

ViewGroup vs View casting

ViewGroup is also a View (extends View ) so instead of casting to ViewGroup keep them as View objects and only cast when really needed to (that is when accessing methods that aren't available in View , only in ViewGroup ).

// childCount is for ViewGroup only, if problem still occurs you can
// check first if v object really is a ViewGroup instance with the
// below instanceof check
if( v instanceof ViewGroup && parent instanceof ViewGroup && ((ViewGroup)v).getChildCount()!=0 ){

     // perform the cast only once, also now we know it's safe thanks to the instanceof
     ViewGroup vg = (ViewGroup)v

     View nextChild = vg.getChildAt(0);
     Log.i("child", vg.getChildCount()+"");   

     // no guarantee here whatever that parent is a more specific ViewGroup
     // so if we need this to be a ViewGroup we'll have to cast but check
    // with instanceof first
    ((ViewGroup)parent).addView(nextChild); //this was the error line

     View view = (View) event.getLocalState();
     ViewGroup viewgroup = (ViewGroup) view.getParent();
     viewgroup.removeView(view);
     RelativeLayout containView = (RelativeLayout) v;
     containView.addView(view);
     view.setVisibility(View.VISIBLE);

} else {
     // PERFORM SOME LOGGING HERE
     // so you know what views got dropped in here that didn't match your requirements in the first place
     // TODO

}

instanceof

Also avoid checking the name of a Class with a String.equals(String):boolean like you did in the snippet below.

In your case this would even fail since the "class " in front will not be part of the result.

if(v.getClass().toString().equals("class android.widget.RelativeLayout")){

The proper way to check the type of an Object in Java is to use isntanceof like I did in the code below.

if( v instanceof android.widget.RelativeLayout ){

More information about instanceof can be found on this StackOverflow question .

Answer 2

Do less casting and use instanceof to check the type.

if(v instanceof ViewGroup){
    ViewGroup vg = (ViewGroup)v;

    // TODO your code that needs a ViewGroup.
}

Answer 3

problem is here if(((ViewGroup)v).getChildCount()!=0) and View nextChild = ((ViewGroup)v).getChildAt(0);

remove casting and try this way if(v.getChildCount()!=0)

Answer 4

View is the parent of ViewGroup and so you cannot type cast ViewGroup, a child to its parent which is View. Thats the reason why it is throwing ClassCastException.