Explain the differences between an interface and an abstract class.

An abstract class can contain both abstract methods (without implementation) and concrete methods (with implementation). It supports various access modifiers (private, protected, public) and can have constructors. A class can only extend one abstract class (single inheritance). Abstract classes are primarily used for sharing common behaviors among related classes.

An interface, on the other hand, is a contract that defines a set of methods that a class must implement. Traditionally, interfaces only contained abstract methods, but Java 8 and later versions allow them to include static and default methods. All members of an interface are implicitly public, and variables are implicitly public, static, and final. A class can implement multiple interfaces.