What elements or methods can be declared in an interface in object-oriented programming?

In object-oriented programming (using Java and Android as examples), interfaces can declare:

• Abstract methods (without implementation), which must be implemented by classes implementing the interface.
• Constants (fields public static final), i.e., static immutable values.

Starting from Java 8, interfaces can also declare:

• Methods with default implementation (default methods).
• Static methods with implementation.

Example of an interface:

public interface MyInterface {
    void abstractMethod();  // abstract method

    default void defaultMethod() {
        System.out.println("Default implementation");
    }

    static void staticMethod() {
        System.out.println("Interface static method");
    }

    int CONSTANT = 10;  // constant
}