Defining interfaces (interface)

An interface in Java is a contract that defines what a class can do without specifying how. It contains abstract methods (until Java 8 added defaults and statics). Interfaces enable abstraction, multiple inheritance of type, and polymorphism.

Definition of interface

An interface is defined using the interface keyword. It can contain:

• Abstract methods (implicitly public abstract)
• Constant fields (implicitly public static final)
• Default methods (Java 8+)
• Static methods (Java 8+)
• Private methods (Java 9+)

Syntax:

interface Drawable {
// constant
int COLOR = 0xFF0000; // public static final

    // abstract method (no body)
    void draw();

    // default method (Java 8)
    default void display() {
        System.out.println("Displaying");
    }

    // static method (Java 8)
    static void info() {
        System.out.println("Drawable interface");
    }

}

Characteristics:

• All fields are implicitly public static final.
• All abstract methods are implicitly public abstract (no need to write these modifiers).
• Interfaces cannot be instantiated directly.
• An interface can extend multiple interfaces.

Example with only abstract methods (traditional):

interface Animal {
    void eat();
    void sleep();
}