title: Mockito on Android, Context.getString(id) and NullPointerException
body: <p>I've just started learning a Mockito testing framework, I followed official tutorial from: <a href="https://developer.android.com/training/testing/unit-testing/local-unit-tests.html" rel="noreferrer">developer.android.com</a></p>
  
  <p>The code is:</p>
  
  <pre><code>private static final String FAKE_STRING = "HELLO WORLD";
  
  @Mock
  Context mMockContext;
  
  @Test
  public void readStringFromContext_LocalizedString() {
      // Given a mocked Context injected into the object under test...
      when(mMockContext.getString(R.string.hello_world))
              .thenReturn(FAKE_STRING);
      ClassUnderTest myObjectUnderTest = new ClassUnderTest(mMockContext);
  
      // ...when the string is returned from the object under test...
      String result = myObjectUnderTest.getHelloWorldString();
  
      // ...then the result should be the expected one.
      assertThat(result, is(FAKE_STRING));
  }
  </code></pre>
  
  <p>I wrote following ClassUnderTest:</p>
  
  <pre><code>public class ClassUnderTest {
  
  private Context context;
  
  public ClassUnderTest(Context context)
  {
      this.context=context;
  }
  
  public String getHelloWorldString()
  {
      return context.getString(R.string.hello_world);
  }
  </code></pre>
  
  <p>}</p>
  
  <p>and after running I'm getting NullPointerException:</p>
  
  <pre><code>    java.lang.NullPointerException
  at android.content.Context.getHelloWorldString(Context.java:293)
  at com.darekk.draggablelistview.unit_tests.MockitoTest.readStringFromContext_LocalizedString(MockitoTest.java:46)
  at java.lang.reflect.Method.invokeNative(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:511)
  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.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.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
  at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
  at org.junit.runners.Suite.runChild(Suite.java:128)
  at org.junit.runners.Suite.runChild(Suite.java:27)
  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$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
  at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:59)
  at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:262)
  at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1619)
  </code></pre>
  
  <p>When I replace android Context with my own class it works.</p>
  
  <p>Platform: IntelliJ IDEA Community Edition 15.0.6</p>
  
  <p>I really appreciate any help.</p>