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

Key differences

• Inheritance: one abstract class can be extended, many interfaces can be implemented.
• State: abstract classes hold instance fields, interfaces hold constants only.
• Constructors: abstract classes have them, interfaces do not.
• Methods: since Java 8 interfaces can have default and static method bodies.

interface Flyer {
    default void takeOff() { System.out.println("lifting off"); }
}
abstract class Bird {
    abstract void sing();
}