number do not delete backspace button android

Question

I am using an implementation for a backspace button for a calculator, but when I erase two or more numbers and click again another number it gave me the ones I erase with the new ones, for example, I put 2569 and when I push backspace to 25 and push the button 7 its gave me 25697, any idea, here is the code for the backspace button.

btnBackSpace.setOnClickListener( new OnClickListener(){ 
    public void onClick(View arg0){

        String str=editX.getText().toString();
            if (str.length() >1 ) { 
                str = str.substring(0, str.length() - 1);
            editX.setText(str);
        }
            else if (str.length() <=1 ) {
            editX.setText("0");
            }
     }
});