How to perform a method of another class when I click the Accept Button of my Dialog which is located in another class?

Question

This snippet here is in my MainActivity.java which calls the "newdrawing()" method of my class named tools.java.

public void onClick(View v) {
    String color = null;

    switch (v.getId()){

        case R.id.newdraw_a:
            tools buttons = new tools(this);
            buttons.newdrawing();
            break;

The "newdrawing()" method is a dialog which asks the user to add another drawing or cancel. When the user clicks "Accept", I want to call a method from another class named "canvas_class.java".

    public class tools extends View{

    public canvas_class drawing;

    public tools(Context context) {
            super(context);
        }

    public void newdrawing(){
        final AlertDialog.Builder newDialog = new AlertDialog.Builder(this.getContext());
        newDialog.setTitle("New Drawing?");
        newDialog.setMessage("You will overwrite all your current drawings. Are you sure you want to add another drawing?");
        newDialog.setPositiveButton("Accept", new DialogInterface.OnClickListener(){
            public void onClick(DialogInterface dialog, int which){
                dialog.dismiss();
                drawing.newdrawing();
            }
        });
        newDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
                dialog.cancel();
            }
        });
        newDialog.show();

    }

}

Now, I want to ask what is wrong with my code in tools.java that it force closes when I click Accept. Thank you.

My canvas_class looks like this

public class canvas_class extends View {

private Canvas drawCanvas;

public canvas_class(Context context, AttributeSet attrs) {
    super(context, attrs);
    setupDrawing();
}

public void newdrawing(){
    drawCanvas.drawColor(0, PorterDuff.Mode.CLEAR);
    invalidate();

}

Use interface or broadcast listener, check this http://stackoverflow.com/questions/19026515/how-to-use-interface-to-communicate-between-two-activities but search more before asking questions, since this type of ques have lot of answers — Brendon, Jan 27 '16 at 11:12
Thank you Brendon. But, can you please explain further? Newbie here ^_^ — biancii, Jan 27 '16 at 11:16
If you have a null pointer exception this is because 'drawing' member is not initialized. You need to pass an instance in the constructor, or do a new inside the constructor. — Jose Luis, Jan 27 '16 at 11:17

score 0 · Answer 1 · answered Jan 27 '16 at 11:34

0

You are most probably getting a NullPointerException as

drawing is null so it throws a exception, please initialize the

canvas_class drawing variable so that it doesnt throw a exception

answered Jan 27 '16 at 11:34

JAAD

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score 0 · Answer 2 · answered Jan 27 '16 at 13:47

I don't know if 'this.attrs' is legal, perhaps someone else could help you, but your constructor for the 'tools' class could be something like this:

public tools(Context context) {
   super(context);
   drawing = new canvas_class(context, this.attrs);
}

And is better to name classes with uppercase:

class Tools
class CanvasClass

I hope this helps.

How to perform a method of another class when I click the Accept Button of my Dialog which is located in another class?

2 Answers2