Mock new object creation with Mockito

Question

I need to mock a class object whenever a new object was created for that class. I am not using any java framework to do dependency injections. Below is my code.

Addition.java

public int sum(int a, int b) {
    return a + b;
}

Application.java

public int doOperation(int a, int b) {
    Addition addition = new Addition();
    return addition.sum(a, b);
}

ApplicationTest.java

@RunWith(PowerMockRunner.class)
@PrepareForTest(Addition.class)

        Addition addition = PowerMockito.mock(Addition.class);
        PowerMockito.when(addition.sum(ArgumentMatchers.anyInt(), ArgumentMatchers.anyInt())).thenReturn(15);

        PowerMockito.whenNew(Addition.class).withNoArguments().thenReturn(addition);

        Application applicationMock = PowerMockito.mock(Application.class);
        int res = applicationMock.doOperation(1, 5);

        assertEquals(res, 15);

In Application.java I am creating Addition class instance. I need to return a mock object instead of a new object. When I run my application I am getting 0 as res.

java.lang.AssertionError: 
Expected :0
Actual   :15
<Click to see difference>


    at org.junit.Assert.fail(Assert.java:88)
    at org.junit.Assert.failNotEquals(Assert.java:834)
    at org.junit.Assert.assertEquals(Assert.java:645)
    at org.junit.Assert.assertEquals(Assert.java:631)
    at com.dbs.poc.PowerMockAdditionTest.sumWithMocks(PowerMockAdditionTest.java:38)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:68)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:326)
    at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:89)
    at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:97)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:310)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.executeTestInSuper(PowerMockJUnit47RunnerDelegateImpl.java:131)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.access$100(PowerMockJUnit47RunnerDelegateImpl.java:59)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner$TestExecutorStatement.evaluate(PowerMockJUnit47RunnerDelegateImpl.java:147)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.evaluateStatement(PowerMockJUnit47RunnerDelegateImpl.java:107)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.executeTest(PowerMockJUnit47RunnerDelegateImpl.java:82)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:298)
    at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:87)
    at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:50)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:218)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:160)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:134)
    at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:34)
    at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:44)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:136)
    at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:117)
    at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:57)
    at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:59)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Answer 1

Here's the problem/missing link:

Application applicationMock = PowerMockito.mock(Application.class);
int res = applicationMock.doOperation(1, 5);

What does applicationMock.doOperation(x, y)? - Nothing and returns 0. (you can configure (Power)mock(ito) to throw an exception instead -> "strict mock")

What you want is:

a) To test a "real Application" (no mock):

Application testApp = new Applicaiton();
int res = testApp.doOperation(1, 5);
// ...

Or b) "program" your mock, to fake/copy the real implementation (e.g. as outlined by @pvpkiran):

Application applicationMock = PowerMockito.mock(Application.class);
PowerMockito.when(applicationMock.doOperation(anyInt(), anyInt())).then...

Answer 2

You are missing one line. Try this.

 Application applicationMock = PowerMockito.mock(Application.class);
 PowerMockito.when(applicationMock.doOperation(anyInt(), anyInt())).thenCallRealMethod();
 int res = applicationMock.doOperation(1, 5);

You have declared application as a Mock. and not defined the behaviour for doOperation method. Hence the control doesn't step into the method and as a consequence nothing works.

Here is the complete code,

@RunWith(PowerMockRunner.class)
@PrepareForTest({Application.class})
public class ApplicationTest {

    @Test
    public void test() throws Exception {
        Addition addition = PowerMockito.mock(Addition.class);
        PowerMockito.when(addition.sum(ArgumentMatchers.anyInt(), ArgumentMatchers.anyInt())).thenReturn(15);

    PowerMockito.whenNew(Addition.class).withNoArguments().thenReturn(addition);

        Application applicationMock = PowerMockito.mock(Application.class);
        when(applicationMock.doOperation(anyInt(), anyInt())).thenCallRealMethod();
        int res = applicationMock.doOperation(1, 5);

        assertEquals(res, 15);
    }
}

If you wish to test only the addition method. You do not need to mock Application class. Instead create a new instance of the Application class. With that you do not need the fix, your current code should suffice.

use

Application application = new Application();

instead of

Application applicationMock = PowerMockito.mock(Application.class);

If you have a valid reason to use mock instead of a real instance, then this is the solution.