Abstract Class vs Interface · Interfaces in Java | Java

Comparison

	Abstract class	Interface
Keyword	`extends`	`implements`
How many per class	one	many
Fields	any (instance state allowed)	only constants
Constructors	yes	no
Method access	any	public (private helpers allowed)
Use when	classes share state and code (IS-A)	unrelated classes share a capability (CAN-DO)
Examples	`AbstractList`, `HttpServlet`	`Comparable`, `Runnable`, `List`

Rule of Thumb

Tip: Prefer interfaces for defining types (PaymentGateway, Repository). Add an abstract base class only when several implementations share real fields and code. Spring and most modern code depend on interfaces, which makes testing with mocks easy.

Functional Interfaces

An interface with exactly one abstract method is a functional interface (e.g. Runnable, Comparator). It can be implemented with a lambda. Lambdas are covered in the Advanced level.

Example

public class Main {
    public static void main(String[] args) {
        Greeter classic = new Greeter() {          // anonymous class
            public String greet(String name) { return "Hello, " + name; }
        };
        Greeter lambda = name -> "Namaste, " + name;   // lambda
        System.out.println(classic.greet("Asha"));
        System.out.println(lambda.greet("Ravi"));
    }
}

@FunctionalInterface
interface Greeter {
    String greet(String name);
}

Output

Hello, Asha
Namaste, Ravi