An interface in Java is a reference type that declares abstract methods (and constants) without providing a full implementation. A class that implements the interface must define all its methods.

interface Shape {
    double area();          // implicitly public abstract
}

class Circle implements Shape {
    double r;
    Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
}

Key points:

• Achieves 100% abstraction (before Java 8) and supports multiple inheritance of type, since a class can implement many interfaces.
• Since Java 8 an interface can also contain default and static methods.
• Fields are implicitly public static final.