Android Studio "Unfortunately 'the program' has stopped" :     public void onClick(View v)
    {
        if (v == buttonOne)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("1");
        }

        else if (v == buttonTwo)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("2");
        }

        else if (v == buttonThree)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("3");
        }

        else if (v == buttonFour)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("4");
        }

        else if (v == buttonFive)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("5");
        }

        else if (v == buttonSix)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("6");
        }

        else if (v == buttonSeven)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("7");
        }

        else if (v == buttonEight)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("8");
        }

        else if (v == buttonNine)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("9");
        }

        else if (v == buttonZero)
        {
            TextView output = (TextView)findViewById(R.id.output);
            output.append("0");
        }

        else if(v == buttonEnter)
        {

            TextView output = (TextView)findViewById(R.id.output);
            temp = Integer.parseInt(output.getText().toString());
            compareNumber(temp);
            output.setText("");
        }
    }


Hi, I am trying to compare number using button. For example, if I press buttonOne it append 1 to stack. if buttonTwo is pressed, it append 2 to stack. 
and For example if I press buttonOne and buttonTwo, it should store number 12(twelve). I tried this using parseInt as you can see in buttonEnter statement but when I run it the application stops.

Please help!