Interface vs implementation

Concept

• Interface: the what, the public face of a class or component. It declares the set of operations (names, parameters, return types and contract) that other objects can use. It says nothing about how they work. In UML it is a classifier with <<interface>>; in Java it is the interface construct.
• Implementation: the how, the actual code and data (private fields, algorithms) that carry out the operations. It is hidden from the clients.

Separating them is the basis of encapsulation, abstraction and loose coupling: clients depend only on the interface, so the implementation can be changed or replaced without changing the clients.

   Client ----uses----> <<interface>>
                         Shape
                         + area()
                          ^     ^
                          :     :  (realization)
                      Circle   Rectangle
                      (implementations)

Difference

Example in Java

interface Shape {                    // interface
    double area();
}
class Circle implements Shape {      // implementation 1
    private double r;
    Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
}
class Rectangle implements Shape {   // implementation 2
    private double w, h;
    Rectangle(double w, double h) { this.w = w; this.h = h; }
    public double area() { return w * h; }
}
// Client depends only on the interface
Shape s = new Circle(2);
System.out.println(s.area());

In C++ the same is done with an abstract class that has pure virtual functions (virtual double area() = 0;), and in C# with the interface keyword.