What is the difference between an interface and an abstract class?

An interface defines a contract for classes, specifying a set of methods that must be implemented. It can contain only method signatures (before Java 8) and constants. Multiple inheritance of interfaces is allowed.

An abstract class can contain abstract and concrete methods, fields, and constructors. It cannot be instantiated directly. It can have only one direct parent (restriction on multiple inheritance of classes).

Main differences:

Characteristic	Interface	Abstract class
Multiple inheritance	Yes (for implementation)	No (for extension)
Methods	Only abstract (before Java 8), can be default and static (from Java 8)	Abstract and concrete
Fields	Static and final constants	Any (static, final, primitive, reference)
Constructors	No	Yes
Implementation	Implemented (implements)	Extended (extends)