NullPointerException on Unit Test

Question

I'm testing a method on a class (a Presenter). The test used to work fine. Since then, I haven't changed anything on this part, neither in the gradle nor Android Studio.

My code is like that:

In the test

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

import java.util.List;

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.ArgumentMatchers.anyInt;


@RunWith(MockitoJUnitRunner.class)
public class MyPresenterTest {

    @Mock
    private CustomObj mCustomObj;

    @Mock
    private List<CustomObj> mCustomObjs;

    private MyPresenter mPresenter;

    @Before
    public void initTest() {
        // ...      

        Mockito.when(mCustomObjs.get(anyInt())).thenReturn(mCustomObj);
        mPresenter = Mockito.spy(new MyPresenter());
    }

    @Test
    public void testOnCustomObjSelected() throws Exception {

        CustomObj obj = mPresenter.onCustomObjSelected(0, mCustomObjs);

        // ... irrelevant code 
    }
}

In the Presenter

void onCustomObjSelected(int pos, final List<CustomObj> customObjs) {
    CustomObj obj = customObjs.get(pos); // The Exception happens here

    // ...
}

The Exception happens when I try to get the the CustomObj from the list, like if the list was null, when it's supposed to be mocked.

If I put a breakpoint on CustomObj obj = mPresenter.onCustomObjSelected(0, mCustomObjs); I can see that the list is correctly mocked.

Any idea why?

EDIT

Here is the error log

java.lang.NullPointerException
    at com.example.MyPresenter.onCustomObjSelected(MyPresenter.java:160)
    at com.example.MyPresenterTest.testOnCustomObjSelected(MyPresenterTest.java:82)
    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.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:78)
    at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:84)
    at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39)
    at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:161)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
    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 com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)


Process finished with exit code -1

Answer 1

The problem appears to be that you have mocked the behavior (using when) but not the creation of the object mCustomObjs. You need to call Mockito.mock to create the mock.

@Before
public void initTest() {

    // Add this line
    mCustomObjs = Mockito.mock(List.class);    

    Mockito.when(mCustomObjs.get(anyInt())).thenReturn(mCustomObj);
    mPresenter = Mockito.spy(new MyPresenter());
}