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

An interface declares a contract: a set of method signatures with no state that any type can implement. A class can implement many interfaces, which models capability. An abstract class is a partially implemented base class that can hold fields, constructors and concrete methods; a class can extend only one.

interface Flyer { void fly(); }
abstract class Bird {
    protected String name;
    Bird(String name) { this.name = name; }
    abstract void sing();
    void sleep() { System.out.println("sleeping"); }
}

Use an interface when unrelated types must share a capability and when you want multiple inheritance of type. Use an abstract class when related types share code and state, and you want to force subclasses to fill in specific steps. Many designs use both: an abstract class implements a primary interface and provides default behaviour, while callers depend only on the interface. Prefer interfaces at module boundaries and abstract classes internally.