@Singleton
public final class MyClass {
       
    @Inject
    @Preference(nodePath="/instance/certainlocation")
    private IEclipsePreferences eclipsePrefs;
...
    public Double theMethodToTest() {
...
    eclipsePrefs.getInt("foo", Integer.valueOf(1)); // if foo doesn't exist use number 1
...
   }
}

public class MyClassTest {
    @Mock
    protected IEclipsePreferences preferences;
    MyClass myClass;
    @Before
    public void setUp() throws Exception {
	MockitoAnnotations.initMocks(this);
     Mockito.when(preferences.getInt("foo", Integer.valueOf(4)));
	context = EclipseContextFactory.create("mockedContext");
	context.set(IEclipsePreferences.class, preferences);
	ContextInjectionFactory.setDefault(context);
          myClass= ContextInjectionFactory.make(MyClass.class, context);
 }

@Test
public void somethingToTest() {
    myClass.theMethodToTest();
}

Mockito.when(preferences.getInt("foo", Mockito.anyInt())).doReturn(Integer.valueOf(4)));

    @Inject
    @Preference(nodePath="/instance/certainlocation")
    @VisibleForTests /* guava annotation */
    void setEclipsePreferences(IEclipsePreferences eclipsePrefs) {
       someValue = eclipsePrefs.getInt("foo", Integer.valueOf(1));
    }