So it's a basics question I know, but is the main difference between abstract classes and interfaces is just that you can use non-abstract methods in abstract classes but not in interfaces ??

you can use non-abstract methods in interfaces too :) Since java7 I think. But it's rarely a good idea to do that.

Interfaces are there do define a type of the class/object and define what it can do.
Abstract classes on top of that define some default behaviour

Interfaces: all the contents are static.
Abstract classes: not necessarily (i.e. you can have instance-speciffic state)

In Java you can implement multiple interfaces, but you can only extend one class.

Interfaces can be proxied and loaded dynamically w/ ServiceLoader easily. It's not the case for Abstract classes.

Usually abstract classes are used when you have the base behaviour defined in your class but you still need a few things for that clockwork to run. It's an "almost working mechanism". Derivates of an abstract class supply those missing implementation-speciffic properties/implementations.

Interfaces are usually used to define an abstract set of properties of the class: type, methods' signatures. That's all.

Wow, thanks man 😃

@ahtolba Think of it this way (if you are familiar with SOAP):

interface is a WSDL file

abstract class is a set of stubs generated from that wsdl file: they know what structures to send, what responses to receive, how to map/process them, but they still require you to implement a few methods, like getConnection() or so.