How to use JMockit MockUp for default interface method - jmockit

Question

Trying to apply a MockUp on a Java 8 default interface method, and JMockit tells me that method cannot be found. This has been tried with JMockit 1.15, 1.19, and 1.25. Here's a very simple use case:
#RunWith(JMockit.class)
public class TestTest {
public interface MyInterface {
default void foo(int f) {
bar(String.valueOf(f));
}
void bar(String s);
}
public class MyClass implements MyInterface {
public void bar(String s) {
System.out.println(s);
}
}
#Test
public void testtest() throws Exception {
new MockUp<MyClass>() {
#Mock
void foo(int i) {
System.out.println("MOCKMOCK " + (i*2));
}
#Mock
void bar(String s) {
System.out.println("MOCK " + s);
}
};
MyClass baz = new MyClass();
baz.foo(5);
baz.bar("Hello world");
}
}
This gets me the error
java.lang.IllegalArgumentException: Matching real methods not found for the following mocks:
com.example.dcsohl.TestTest$1#foo(int)
at com.example.dcsohl.TestTest$1.<init>(TestTest.java:29)
at com.example.dcsohl.TestTest.testtest(TestTest.java:29)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...
How can we #Mock this method?

Answer 1

Slightly modifying your use case to return strings instead of printing to standard out the following solution will work.
import static org.junit.Assert.assertEquals;
import org.junit.Test;
import mockit.Expectations;
public class TestTest {
public interface MyInterface {
default String foo(int f) {
return bar(String.valueOf(f));
}
String bar(String s);
}
public class MyClass implements MyInterface {
public String bar(String s) {
return s;
}
}
#Test
public void testtest() throws Exception {
MyClass baz = new MyClass();
new Expectations(MyClass.class) {{
baz.foo(anyInt); result = "FOOMOCK";
baz.bar(anyString); result = "BARMOCK";
}};
assertEquals(baz.foo(5), "FOOMOCK");
assertEquals(baz.bar("Hello world"), "BARMOCK");
}
}
There are many useful examples of how to mock out interfaces with method bodies (ie default or static methods) outlined in the examples section on the jmockit github repository.

Answer 2

Use #Mocked instead of a MockUp, it supports default methods.