Adding methods or not adding methods to interface?

Question

In Java 8, we can have default implementations for methods in interfaces, in addition to declarations which need to be implemented in the concrete classes.

Is it a good design or best practice to have default methods in an interface, or did Java 8 come-up with that only to provide more support on older APIs? Should we start with using default methods in new Java 8 projects?

Please help me to understand what is good design here, in detail.

Answer 1

Prior java8, you were looking towards versioned capabilities when talking about "reasonable" ways of extending interfaces:

You have something like:

interface Capability ...

interface AppleDealer {
  List<Apples> getApples();
}

and in order to retrieve an AppleDealer, there is some central service like

public <T> T getCapability (Class<T> type);

So your client code would be doing:

AppleDealer dealer = service.getCapability(AppleDealer.class);

When the need for another method comes up, you go:

interface AppleDealerV2 extends AppleDealer { ...

And clients that want V2, just do a getCapability(AppleDealerV2.class) call. Those that don't care don't have to modify their code!

Please note: of course, this only works for extending interfaces. You can't use this approach neither to change signatures nor to remove methods in existing interfaces.

Thus: just adding a new method to an interface; and having default to implement that method right there; without breaking any existing client code is a huge step forward!

Meaning: why wouldn't you use default methods on existing interfaces? Existing code will not care. It doesn't know about the new defaulted methods.