Question

It’s a calculator app, when i click foreg. 9, i get null pointer exceptions at the lines with stars. What could be going wrong? I’m just a beginner.

nine.setOnClickListener(new OnClickListener() {
                public void onClick(View arg0) {
                    String con= input.getText().toString();             
                    handlenumber("9",con);   **
                }
private void handlenumber(String number,String com)  **
    {
        number=new String();
        com=new String();

        if(com.equals("0") || com.equals("") || r!=0){

            input.setText(number);  **                            
            r=0;
        }
        else
            {
            input.setText(com.concat(number));                          
            }   
    }

Answer 1

From what you showed it seems like input is null, make sure you initialize it / assign to it.

Also, you have two parameters passed to handlenumber, don’t reassign to them use them. e.g. remove the lines:

number=new String();
com=new String();