Java Interface

Interface

An interface class is a class that is 100% abstract (no concrete members).

There’s special keyword for it: interface

• Like abstract methods in an abstract class, interface method signatures form a contract with the classes that implement that interface (another keyword: implements)

Interface

• Non-instantiable (cannot use new)
• Contain method signatures (and nothing else)
• Use implements keyword to implement
• A class may implement multiple interfaces
• Each interface signature must be implemented
• Interfaces specify what, not how

Interface

When a class implements an interface, it has an “is-a” relationship with the interface

Why Have Interfaces?

• Code development and abstraction: Define “what” before the “how” is implemented
• Faster design
• Program maintenance: Easier maintenance and resilience to change
• Flexibility
• ensure compatibility across changes

Comparable Interface

“This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class’s natural ordering, and the class’s compareTo method is referred to as its natural comparison method.”

Here’s what the Comparable interface looks like:

public interface Comparable<T> {
    int compareTo(T obj);
}

Example of compareTo use

import java.util.Arrays;

public class Example {
    public static void main(String[] args) {
        String a = "A";
        String b = "B";
        System.out.println(a.compareTo(b));
    }
}