Abstract Classes and Interfaces

In simple words#

Learning objectives#

• Write abstract classes
• Write and implement interfaces
• Use default methods
• Choose between them

Explanation#

abstract class: may have fields, constructors, concrete and abstract methods. A class extends only one.

interface: methods are abstract by default; can have default and static methods; constants only. A class implements many.

Use an interface for a capability (Comparable, Payable, Notifier). Use an abstract class for a shared base with state.

Most Java frameworks (Spring) are built around interfaces.

Code example: Notifier interface + abstract base#

1interface Notifier {
2    void send(String to, String message);
3    default void sendAll(java.util.List<String> recipients, String message) {
4        recipients.forEach(r -> send(r, message));
5    }
6}
7

8abstract class LoggingNotifier implements Notifier {
9    private int sent;
10    protected void log(String to) { sent++; System.out.println("[" + sent + "] sent to " + to); }
11}
12

13class SmsNotifier extends LoggingNotifier {
14    public void send(String to, String message) {
15        // call SMS provider here
16        log(to);
17    }
18}
19

20public class App {
21    public static void main(String[] args) {
22        Notifier n = new SmsNotifier();
23        n.sendAll(java.util.List.of("+91 98xxxxxx01", "+91 98xxxxxx02"), "Your order has shipped");
24    }
25}

[1] sent to +91 98xxxxxx01
[2] sent to +91 98xxxxxx02

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The interface defines send and a default sendAll.
2. The abstract class adds shared logging state.
3. SmsNotifier only implements send.
4. Callers depend on the Notifier interface.

Real-world use case#

Common mistakes#

Best practices#

• Program to interfaces
• Small, focused interfaces
• Abstract classes for shared state
• Default methods for convenience only

Remember this#

Practice exercise#

1class EmailNotifier implements Notifier {
2    public void send(String to, String message) {
3        System.out.println("Email to " + to + ": " + message);
4    }
5}

Mini quiz#

Want more? Practise all Java MCQs · Java interview questions

Related topics#

My notes#

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