gistfile1.txt

interface Answer<T> {}

enum Answers implements Answer<Object> {
    CALLS_REAL_METHODS(new Answer<>() {}),
    RETURNS_SELF(new Answer<>() {}),
    CUSTOM(null),
    ;

    private Answer<Object> implementation;

    Answers(Answer<Object> implementation) {
        this.implementation = implementation;
    }
    
    void setCustomImplementation(Answer<Object> customImplementation) {
        if (this != CUSTOM) {
            throw new UnsupportedOperationException();
        }
        implementation = customImplementation;
    }
}