Geoff L. Geoff L. - 1 year ago 173
Java Question

How to JUNIT a java.util.Function using Mockito

I would like to perform a junit test using Mockito on the toEntity function.

public class MyEntityTransform {
public Function<MyDTO , MyEntity> toEntity = new Function<MyDTO , MyEntity >() {
public MyEntity apply(MyDTO record) {
return new MyEntity();

Unfortunately the toEntity is NULL when I mock the class and I don't know how I can test it correctly.

public class MyTest {

private MyService _classUnderTest;

private MyEntityTransform myEntityTransform

public void setUp() {

When I RUN the JUNIT test, Mockito give me the error :

Misplaced argument matcher detected here:

-> at com.example.MyTest.setUp(

You cannot use argument matchers outside of verification or stubbing.
Examples of correct usage of argument matchers:
doThrow(new RuntimeException()).when(mock).someVoidMethod(anyObject());

Also, this error might show up because you use argument matchers with
methods that cannot be mocked. Following methods cannot be
stubbed/verified: final/private/equals()/hashCode(). Mocking methods
declared on non-public parent classes is not supported.

Do you have suggestions?

Answer Source

You're using public fields, which is not a good idea. But anyway, what you want to mock is the function, not the instance of MyEntityTransform. So you would need something like

private MyService _classUnderTest;

@Mock // or @Spy
private MyEntityTransform myEntityTransform;

public void prepare() {
    myEntityTransform.toEntity = mock(Function.class);

But quite frankly, I wouldn't use a public field of type Function. Instead, I would use a public method:

public class MyEntityTransform {
    public MyEntity toEntity(MyDTO record) {
        return new MyEntity();   

Then you can mock MyEntityTransform and make its toEntity method return what you want. And if you need to pass a Function doing what the method does, use a method reference:
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