How to delete the last letter from EditText with button?

Using substring() method , you can do it,

String str = myEditText.getText().toString();
str = str.substring ( 0, str.length() - 1 );
// Now set this Text to your edit text
myEditText.setText ( str );

you need to write above lines in onClick() method.

You can retrieve the text of EditText and then get the sub-string of that text and set again that text to EditText as below...

String text = editText.getText().toString();
editText.setText(text.substring(0, text.length() - 1));

You can also use following procedure....it will be more efficient.

int length = editText.getText().length();
if (length > 0) {
    editText.getText().delete(length - 1, length);
}

You should use switch-case as below...and handle your nuttondel onclick() as follows...

public class MainActivity extends Activity implements OnClickListener {

    Button buttona, buttonb;
    Button buttonDel;
    EditText editText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editText = (EditText) findViewById(R.id.editText);

        addListenerOnButton();

    }


   public void addListenerOnButton() {

    buttona = (Button) findViewById(R.id.buttona);
    buttona.setOnClickListener(this);

    buttonb = (Button) findViewById(R.id.buttonb);
    buttonb.setOnClickListener(this);

    buttonDel = (Button) findViewById(R.id.buttondel);
    buttonDel.setOnClickListener(this);


   }

    public void onClick(View v) {

       switch(v.getId()) {

          case R.id.buttona:
                      editText.setText(editText.getText().toString()+buttona.getText().toString());
              break;

          case R.id.buttonb:
                      editText.setText(editText.getText().toString()+buttonb.getText().toString());
              break;

          case R.id.buttondel:

              int length = editText.getText().length();
              if (length > 0) {
                  editText.getText().delete(length - 1, length);
              }
              break;
       }

    }

}