Android Fake Calculator

package com.example.calculator;

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.media.Image;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.util.TypedValue;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

    Button buttonEquals, buttonSqrt;
    EditText firstOperand, secondOperand, operation, result, sqrtNumber;
    ImageView imgView;

    String resultToBePrinted;
    int firstNumber = 0;
    int secondNumber = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        buttonEquals = (Button) findViewById(R.id.btn_result);
        buttonSqrt = (Button) findViewById(R.id.btn_sqrt);

        firstOperand = (EditText) findViewById(R.id.tv_firstOperand);
        secondOperand = (EditText) findViewById(R.id.tv_secondOperand);
        operation = (EditText) findViewById(R.id.tv_operation);
        result = (EditText) findViewById(R.id.tv_result);
        sqrtNumber = (EditText) findViewById(R.id.tv_sqrt);
        imgView = (ImageView) findViewById(R.id.imageView);

        buttonEquals.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                firstNumber = Integer.parseInt(String.valueOf(firstOperand.getText()));
                secondNumber = Integer.parseInt(String.valueOf(secondOperand.getText()));
                String sign = String.valueOf(operation.getText());

                switch (sign) {
                    case "-":
                        resultToBePrinted = String.valueOf(firstNumber - secondNumber);
                        break;
                    case "+":
                        resultToBePrinted = String.valueOf(firstNumber + secondNumber);
                        break;
                    case"/":
                        resultToBePrinted = String.valueOf(firstNumber / secondNumber);
                        break;
                    case"*":
                        resultToBePrinted = String.valueOf(firstNumber * secondNumber);
                        break;
                    default:
                        resultToBePrinted = "error";
                        break;

                }
                setResult();
                drawRectangle(firstNumber, secondNumber);
            }
        });

        buttonSqrt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int number = Integer.parseInt(String.valueOf(sqrtNumber.getText()));
                double result = Math.sqrt(number);
                sqrtNumber.setText(String.valueOf(result));
            }
        });
    }


    protected void setResult(){
        result.setText(resultToBePrinted);
    }

    protected void drawRectangle(int firstNumber, int secondNumber){

        imgView.setVisibility(View.VISIBLE);
        Bitmap bitmap = Bitmap.createBitmap((int) getWindowManager()
                .getDefaultDisplay().getWidth(), (int) getWindowManager()
                .getDefaultDisplay().getHeight(), Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        imgView.setImageBitmap(bitmap);
        Paint paint = new Paint();
        float left = imgView.getWidth()/2;
        float top = imgView.getHeight()/2;
        int a = firstNumber * 10;
        int b = secondNumber * 10;
        paint.setStrokeWidth(1);
        paint.setColor(Color.BLUE);

        Resources r = getResources();
        float aPx = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, firstNumber, r.getDisplayMetrics());
        float bPx = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, secondNumber, r.getDisplayMetrics());


        aPx *=10;
        bPx *=10;
        canvas.drawRect(10,10,90,90,paint);
    }
}