Creating Mocks and need of mocking

Introduction

Mocking is a technique that allows testing of a class’s functionality in total isolation. Additionally, Mocking does not require property file read, file server read, or even a database connection to test the functionality.
Mockito makes creating mock objects straightforward as it uses Java Reflection to create mock objects for an interface.
This blog focuses on helping you understand how to create a mock. 

Using Mockito.mock()

The Mockito.mock() method enables us to create a mock object of a class or an interface.
We can then use the mock to return values for its methods and verify if they were called.
Mock objects. mock() creates mocks without bothering about the order of the method calls that mock is going to make while in action.

Step 1 − Create an interface called Calculator to provide mathematical functions

File: Calculator.java

public interface Calculator {
   public double add(double inp1, double inp2);
   public double subtract(double inp1, double inp2);
   public double multiply(double inp1, double inp2);
   public double divide(double inp1, double inp2);
}

Step 2 − Create a JAVA class to represent mathApp

File: mathApp.java

public class mathApp {
   private Calculator calc;


   public void setCalculator(Calculator calc){
      this.calc = calc;
   }
   
   public double add(double inp1, double inp2){
      return calc.add(inp1, inp2); 
   }
   
   public double subtract(double inp1, double inp2){
      return calc.subtract(inp1, inp2);
   }
   
   public double multiply(double inp1, double inp2){
      return calc.multiply(inp1, inp2);
   }
   
   public double divide(double inp1, double inp2){
      return calc.divide(inp1, inp2);
   }
}

Step 3 − Test the mathApp class.

Let's test the mathApp class by injecting a mock of Calculator in it. Mockito will create a mock.
Here we have appended two mock method calls, add() and subtract(), to the mock object via when(). However, we've called subtract() during testing before we call add(). When a mock object is made using create(), the order of execution does not matter.

File: MathsAppSection .java

package com.tutorialspoint.mock;

import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.runners.MockitoJUnitRunner;

// @RunWith will attach a runner with the test class to initialize the test data
@RunWith(MockitoJUnitRunner.class)
public class MathsAppSection  {
   private mathApp mathApp;
   private Calculator calc;

   @Before
   public void setUp(){
      mathApp = new mathApp();
      calc = mock(Calculator.class);
      mathApp.setCalculator(calc);
   }

   @Test
   public void testAddAndSubtract(){

      //adding the behavior to add numbers
      when(calc.add(20.0,10.0)).thenReturn(30.0);

      //subtracting the behavior to subtract numbers
      when(calc.subtract(20.0,10.0)).thenReturn(10.0);

      //testing the subtract functionality
      Assert.assertEquals(mathApp.subtract(20.0, 10.0),10.0,0);

      //testing the add functionality
      Assert.assertEquals(mathApp.add(20.0, 10.0),30.0,0);

      //verifying call to calc is made or not
      verify(calc).add(20.0,10.0);
      verify(calc).subtract(20.0,10.0);
   }
}

Step 4 − Execute test cases

Next we create a java class file called RunTest in C:\> Mockito_WORKSPACE to execute Test case(s).

File: RunTest.java

import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class RunTest {
   public static void main(String[] args) {
      Result result = JUnitCore.runClasses(MathsAppSection .class);
      
      for (Failure failure : result.getFailures()) {
         System.out.println(failure.toString());
      }
      
      System.out.println(result.wasSuccessful());
   }
} 

Step 5 − Verify the Result

Compile the classes using the javac compiler.

C:\Mockito_WORKSPACE>javac Calculator.java mathApp.java MathsAppSection .java RunTest.java

Now run the Test Runner to get the result.

C:\Mockito_WORKSPACE>java RunTest

Verify the output.

true