Calculator in Java / Android

You are allowed to change this code and play with it. Have fun!

package com.zaigham.calculator;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.regex.Pattern;

public class MainActivity extends AppCompatActivity {

    private TextView txt1;
    private String display= "";
    private String currentOperator="";
    private String result ="";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        txt1 = (TextView)findViewById(R.id.textview);
        txt1.setText(display) ;
    }

    private void updateScreen()
    {
        txt1.setText(display);
    }

    public void onClickNumber (View v)
    {
        if (result!=""){
            clear();
            updateScreen();
        }
        Button b =(Button) v;
        display += b.getText();
        updateScreen();
    }

    public boolean isOperator(char op)
    {
        switch (op)
        {
            case'+':
            case'-':
            case'*':
            case'/': return true;
            default: return false;
        }
    }

    public void onClickOperator (View v)
    {
        if (display=="") return;
        Button b =(Button) v;
        if (result!="")
        {
            String _display=result;
            clear();
            display = _display;
        }
        if (currentOperator!="")
        {
            Log.d("Calc",""+display.charAt(display.length()-1));
           if(isOperator(display.charAt(display.length()-1)))
           {
               display = display.replace
                       (display.charAt(display.length()-1),b.getText().charAt(0));
               updateScreen();
               return;
           }
           else{
               getResult();
               display=result;
               result="";
           }
            currentOperator = b.getText().toString();
        }
        display += b.getText();
        currentOperator = b.getText().toString();
        updateScreen();
    }

    private void clear()
    {
        display="";
        currentOperator="";
        result="";
    }

    public void onClickClear(View v)
    {
        clear();
        updateScreen();
        result="";
    }

    public double operate(String a,String b, String op)
    {
        switch(op)
        {
            case"+": return Double.valueOf(a) + Double.valueOf(b);
            case"-": return Double.valueOf(a) - Double.valueOf(b);
            case"*": return Double.valueOf(a) * Double.valueOf(b);
            case"/":try {
                return Double.valueOf(a) / Double.valueOf(b);
            }catch(Exception e) {
                Log.d("Calc", e.getMessage());
            }
                default:return-1;
        }
    }

    private boolean getResult()
    {
        if(currentOperator=="") return false;
        String[] operation = display.split(Pattern.quote(currentOperator));
        if (operation.length<2) return false;
        result= String.valueOf(operate(operation[0],operation[1],currentOperator)) ;
        return true;
    }

    public void onClickEqual(View v)
    {
        if (display=="") return;
        if (!getResult())return;
        txt1.setText(display+"\n"+String.valueOf(result));
    }
}