![]() What should method doSomething of class Z do? In multiple inheritance many such conflicts do exist. Why multiple inheritance is not more popular? Take the example: class X There is no corresponding rule that can be applied for classes. If you add multiple inheritance of classes, this simple rule which is the most straightforward way to resolve conflicts becomes vastly more complicated. The thing to keep in mind with default implementations is that in the spec for polymorphic method binding for Java, any class in the inheritance hierarchy takes precedence over any default method even if that class doesn't implement the interface that defined it. It's a pragmatic addition, not a conceptual one. The addition of default is not meant to change the point of interfaces. In the past whenever you wanted to implement an iterator, you had to specify a remove() method which (in my experience) is almost always to throw an unsupported operation exception. I think a good example of it's use is the remove() method on Iterator. The point of a default method is not meant to be a place to add non-trivial implementations. ![]() What's good for the goose is good for the gander, right? Well, interfaces are more like ducks. By adding a default implementation, interfaces are more free to evolve. The reason is that once an interface was released, it was impossible to add anything to it without breaking existing dependencies on that interface. Prior to Java 8, there were no default interfaces so this issue did not exist in Java at all and this was completely intentional.Ī different but related question would be: "if the diamond inheritance problem is so bad, why was it introduced in Java 8?" To understand that you need to understand why the default keyword was introduced to allow method implementations on interfaces. Multiple interfaces just add multiple requirements for the developer.Īs mentioned in other answers, the main reason is the diamond inheritance problem. Supporting an interface doesn't inherit anything, it adds a requirement for the developer to support certain methods. Interfaces are often used for delegates, and it is entirely normal for an instance to be a delegate for multiple classes, and therefore to support multiple interfaces. Interfaces are an entirely different matter. The correct way is to inherit from button, and have two instance variables of type string and color. Multiple inheritance would inherit from button, string, and color. And in C++, most actual uses come from someone confusing "is a" and "has a" and would be handled better in a different way.įor example, I implement a button that displays some text in some color. Two highly popular languages, Objective-C and Swift, don't have multiple inheritance and nobody is complaining (and one of them is brand new, so multiple inheritance isn't missing for historical reasons, but because someone said "it's not worth the bother"). Most C++ code doesn't use multiple inheritance. ![]() Answer: Multiple inheritance is a pain in the arse. It's not "why can't Java", it's "why doesn't Java". In the eyes of java designers, multiple inheritence simply is not useful enough to warrant the additional complexity. However, it would end up introducing a fair amount of additional complexity. Of course, you could come up with some semantics for multiple inheritance to make fields work properly. This is different from methods, because for methods you can simply override the method with the desired behavior. But this would mean that there were certain pairs of classes that couldn't be inherited together just because they happened to use the same field name. What happens? You propose that the compiler simply reject this. Consider the case that you want to inherit from two superclasses each of which have a foobar field. Any code that depends on the single-inheritance nature of Java would break.įields are more problematic for multiple inheritance than methods. For example, the Class.getSuperclass() method would no longer make sense, as a class might have multiple superclasses. It would be much more difficult to add multiple inheritance now. Is there a way to make this work without specifying a generic type without type parameters and using unchecked operations? I could also not find an example of this on the internet.Partially, this is historical. ![]() So my code looks like this: List> objectList = null īasically my problem lies in the first line - I want to specify two "constraints" for the type because I need the first one to make sure I can print the ID in the loop and the second one to make sure I can use Collections.sort() on the list. What I'd like to do is declare a list of such objects, fill it, then sort it and call getId() on them. Moreover, they also all implement Comparable with themselves as type parameter, so they are all comparable to themselves. I have a bunch of classes in java that all implement an interface called IdObject (specifying a getId() method).
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