Java Mock object without dependency injection

Question

I am interested in Mocking objects in the JUnit test suite, however I only came across mocking frameworks that use dependency injection to inject mock objects. However, I would like to be able to mock classes / functions without having to enter this mock object along the lines of @patch () in python.

Trivial example:

//dependency injection public String bar(Foo foo) { return foo.foo(); //just pass in mock Foo object } //.... onto test code Foo mockedFoo = <Mocked Foo object>; String response = bar(mockedFoo); assertEqual(response, <mockedFoo return value>); //case I am looking for public String bar() { Foo foo = new Foo(); //how to create mock object here? return foo.foo(); //or simply how to mock a single function? } //... onto test code <force the Foo class or foo method to be mocked from here without touching bar() source code> String response = bar(); assertEqual(response, <mocked response>); 

Answer 1

You can use Powermock to test the test class to return the layout when calling new .

Powermock Design Layout Tutorial

your code will look like this:

 RunWith(PowerMockRunner.class) @PrepareForTest( Bar.class ) public class BarTest { @Test public void test(){ Foo mockedFoo = createMock(Foo.class); //set up mockedFoo here ... //This will make a call to new Foo() inside Bar.class //return your mock instead of a real new one expectNew(Foo.class).andReturn(mockedFoo); ... replay(mockedFoo, File.class); Bar bar = new Bar(); String response = bar.bar(); assertEqual(response, <mocked response>); verify(mockedFoo, File.class); } } 

Answer 2

Simply put, you could mock foo like this

 public String bar() { Foo foo = Mockito.mock(Foo.class); return foo.foo(); } 

The problem with this is that foo.foo() essentially not do anything, since you have not determined that #foo() should return when we call the mocked version. Using a more complete example, you can do something like this:

 class MyTest { Foo mockedFoo = Mockito.mock(Foo.class); @Before public void setUp() throws Exception { Mockito.when(mockedFoo.foo()).thenReturn("This is mocked!"); } @Test public void testMock() { String returnedFoo = mockedFoo.foo(); Assert.assertEquals("This is mocked!", returnedFoo); } }