GenericArgumentMatcher.java

@FunctionalInterface
public static interface GenericArgumentMatcher<T>{
    boolean matches(T argument);
}

GenericMatchers.java

public static class GenericMatchers {
    public static class GenericArgumentMatcherWrapper<T> extends ArgumentMatcher<T> {
        private final GenericArgumentMatcher<T> delegate;

        public GenericArgumentMatcherWrapper(GenericArgumentMatcher<T> delegate) {
            this.delegate = delegate;
        }

        @SuppressWarnings("unchecked")
        @Override
        public boolean matches(Object argument) {
            try {
                return delegate.matches((T) argument);
            } catch (ClassCastException e) {
                return false;
            }
        }
    }

    public static <T> T argThat(GenericArgumentMatcher<T> matcher) {
        return Matchers.argThat(new GenericArgumentMatcherWrapper<T>(matcher));
    }
}

SampleJava8Test.java

public class SampleJava8Test {
	public void testTest() {
		final MyClass m = mock(MyClass.class);
		when(m.getSomething(GenericMatchers.argThat(a -> a.isValid()))).thenReturn(true);
	}
}