mockito adds a functionality to a mock object using the methods when(). take a look at the following code snippet.

//add the behavior of calc service to add two numbers
when(calcservice.add(10.0,20.0)).thenreturn(30.00);

here we've instructed mockito to give a behavior of adding 10 and 20 to the add method of calcservice and as a result, to return the value of 30.00.

at this point of time, mock recorded the behavior and is a working mock object.

//add the behavior of calc service to add two numbers
when(calcservice.add(10.0,20.0)).thenreturn(30.00);

example

step 1 − create an interface called calculatorservice to provide mathematical functions

file: calculatorservice.java

public interface calculatorservice {
   public double add(double input1, double input2);
   public double subtract(double input1, double input2);
   public double multiply(double input1, double input2);
   public double divide(double input1, double input2);
}

step 2 − create a java class to represent mathapplication

file: mathapplication.java

public class mathapplication {
   private calculatorservice calcservice;

   public void setcalculatorservice(calculatorservice calcservice){
      this.calcservice = calcservice;
   }
   
   public double add(double input1, double input2){
      return calcservice.add(input1, input2);
   }
   
   public double subtract(double input1, double input2){
      return calcservice.subtract(input1, input2);
   }
   
   public double multiply(double input1, double input2){
      return calcservice.multiply(input1, input2);
   }
   
   public double divide(double input1, double input2){
      return calcservice.divide(input1, input2);
   }
}

step 3 − test the mathapplication class

let's test the mathapplication class, by injecting in it a mock of calculatorservice. mock will be created by mockito.

file: mathapplicationtester.java

import static org.mockito.mockito.when;

import org.junit.assert;
import org.junit.test;
import org.junit.runner.runwith;
import org.mockito.injectmocks;
import org.mockito.mock;
import org.mockito.runners.mockitojunitrunner;

// @runwith attaches a runner with the test class to initialize the test data
@runwith(mockitojunitrunner.class)
public class mathapplicationtester {
	
   //@injectmocks annotation is used to create and inject the mock object
   @injectmocks 
   mathapplication mathapplication = new mathapplication();

   //@mock annotation is used to create the mock object to be injected
   @mock
   calculatorservice calcservice;

   @test
   public void testadd(){
      //add the behavior of calc service to add two numbers
      when(calcservice.add(10.0,20.0)).thenreturn(30.00);
		
      //test the add functionality
      assert.assertequals(mathapplication.add(10.0, 20.0),30.0,0);
   }
}

step 4 − execute test cases

create a java class file named testrunner in c:\>mockito_workspace to execute the test case(s).

file: testrunner.java

import org.junit.runner.junitcore;
import org.junit.runner.result;
import org.junit.runner.notification.failure;

public class testrunner {
   public static void main(string[] args) {
      result result = junitcore.runclasses(mathapplicationtester.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 javac compiler as follows −

c:\mockito_workspace>javac calculatorservice.java mathapplication.
   java mathapplicationtester.java testrunner.java

now run the test runner to see the result −

c:\mockito_workspace>java testrunner

verify the output.

true