Why does Java need interfaces and Smalltalk does not?

Question

I have been programming in Smalltalk for some time, but I never really needed interfaces to implement anything. Then why can't languages such as Java get rid of interfaces? Is it only Smalltalk or is there another language which doesn't need interfaces?

Answer 1

Because Java is statically typed while Smalltalk is not. Interfaces don't serve any purpose when you don't declare types and your variables aren't going to be typechecked. But in a statically typed language like Java, they're extremely handy, because they let you have a variable whose type is defined by the methods the object implements instead of its class. It brings you a lot closer to the dynamic typing Smalltalk has natively without giving up the benefits of typechecking.

Answer 2

It is a polymorphism issue: in Java you have static types and therefore you need to know which messages can your object answer... in Smalltalk (and other non-static languages) you just need to implement right methods to have polymorphism.

For instance:

• In Java you need to implement Cloneable, who defines method Cloneable.clone to have cloneble objects. Then, the compiler knows your object will understand that method (otherwise it will throw an error)
• In smalltalk, you just need to implement method #clone. Compiler never knows/don't care about which messages understands your objects until it is called.

That also means you can have polymorphic objects without being part of same hierarchy... multi inheritance, mixins and other approachs (traits are present on Pharo) are just reuse technics, not a design constraint.

This way of do things is often called "duck typing"... see: http://en.wikipedia.org/wiki/Duck_typing