Add Mockito tutorial for void methods (#2869)

mockito/src/test/java/org/baeldung/mockito/MockitoVoidMethodsTest.java

@@ -0,0 +1,62 @@
+package org.baeldung.mockito;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import static org.junit.Assert.assertEquals;
+
+import static org.mockito.Mockito.*;
+import org.mockito.ArgumentCaptor;
+import static org.mockito.Matchers.any;
+import org.mockito.stubbing.Answer;
+import org.mockito.runners.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class MockitoVoidMethodsTest {
+
+    @Test
+    public void whenAddCalledVerified() {
+        MyList mockVoid = mock(MyList.class);
+        mockVoid.add(0, "");
+        verify(mockVoid, times(1)).add(0, "");
+    }
+
+    @Test(expected = Exception.class)
+    public void givenNull_addThrows() {
+        MyList mockVoid = mock(MyList.class);
+        doThrow().when(mockVoid).add(isA(Integer.class), isNull());
+        mockVoid.add(0, null);
+    }
+
+    @Test
+    public void whenAddCalledValueCaptured() {
+        MyList mockVoid = mock(MyList.class);
+        ArgumentCaptor<String> valueCapture = ArgumentCaptor.forClass(String.class);
+        doNothing().when(mockVoid).add(any(Integer.class), valueCapture.capture());
+        mockVoid.add(0, "captured");
+        assertEquals("captured", valueCapture.getValue());
+    }
+
+    @Test
+    public void whenAddCalledAnswered() {
+        MyList mockVoid = mock(MyList.class);
+        doAnswer((Answer<Void>) invocation -> {
+            Object arg0 = invocation.getArgument(0);
+            Object arg1 = invocation.getArgument(1);
+
+            //do something with the arguments here
+            assertEquals(3, arg0);
+            assertEquals("answer me", arg1);
+
+            return null;
+        }).when(mockVoid).add(any(Integer.class), any(String.class));
+        mockVoid.add(3, "answer me");
+    }
+
+    @Test
+    public void whenAddCalledRealMethodCalled() {
+        MyList mockVoid = mock(MyList.class);
+        doCallRealMethod().when(mockVoid).add(any(Integer.class), any(String.class));
+        mockVoid.add(1, "real");
+        verify(mockVoid, times(1)).add(1, "real");
+    }
+}

mockito/src/test/java/org/baeldung/mockito/MyList.java

@@ -14,4 +14,9 @@ class MyList extends AbstractList<String> {
         return 1;
     }
 
+    @Override
+    public void add(int index, String element) {
+        // no-op
+    }
+
 }