interface - Java 8: virtual extension methods vs abstract class -

i'm looking @ new virtual extension methods in java 8 interfaces:

public interface myinterface {    default string mymethod() {        return "myimplementation";     } } 

i purpose in allowing interface evolve on time, , multiple inheritance bit, awfully abstract class me.

if you're doing new work abstract classes prefered on extension methods provide implementation "interface" or these 2 approaches conceptually equivalent?

one primary purpose of such constructs preserve backwards compatibility. addition of closures java language quite major alteration, , things need updated take advantage of this. example, collection in java 8 have methods such foreach() work in conjunction lambdas. adding such methods pre-existing collection interface not feasible, since break backwards compatibility. class wrote in java 7 implementing collection no longer compile since lack these methods. consequently, these methods introduced "default" implementation. if know scala, can see java interfaces becoming more scala traits.

as interfaces vs abstract classes, 2 still different in java 8; still can't have constructor in interface, example. hence, 2 approaches not "conceptually equivalent" per se. abstract classes more structured , can have state associated them, whereas interfaces can not. should use whichever makes more sense in context of program, in java 7 , below.