java - Calculator in Android Studio -

i have been trying build simple calculator in android studio.i beginner , have basic knowledge of java.i trying accept values in single text field instead of using 2 different text field.i have used on click listener on operator buttons , inside on equals button.but application closes click on operator buttons.how can use single text view 2 accept 2 numbers or more , apply mathematical operator on them?

edit:as have stated have little knowledge of java.i did google search before posting question.

        add.setonclicklistener(new view.onclicklistener() {          @override         public void onclick(view view) {             n.settext("");             final float = float.parsefloat(n.gettext().tostring());             n.settext("");             equals.setonclicklistener(new view.onclicklistener() {                 @override                 public void onclick(view view) {                     float b = float.parsefloat(n.gettext().tostring());                     float c = + b;                     ans.settext("" + c);                     n.settext("");                 }             });         }     }); 

on first line do:

n.settext(""); 

only after read text with

n.gettext().tostring() 

which returns empty string passed parsefloat(). parsefloat() not expect empty string throws exception , application closes.

so, not settext("") before read value need.

more generally, consider saving current state each time there click. can, example, enable equal button after number typed, operator button clicked , number typed.