Question

I know about C++ pure virtual classes, but Java went one step further and created a first-class (no pun intended) concept for multiple-interface (not implementation) inheritance, the interface. It’s now a staple of major statically-typed languages. Did Java invent the interface concept? Or did it appear in older languages also as a first-class concept?

Answer 1

Here is abstract from the article The Java Language Environment.

An interface in the Java language is simply a specification of methods that an object declares it implements. An interface does not include instance variables or implementation code–only declarations of constants and methods. The concept of an interface in the Java language was borrowed from the Objective-C concept of a protocol.

http://java.sun.com/docs/white/langenv/Object.doc2.html#6185