🔌 Interfaces & Abstraction · Intermediate

Implementing multiple interfaces in Java

A class can implement many interfaces.

🧩 The mysteryOne person can hold a driver's licence, a library card and a passport at once. Can a Java class collect contracts the same way?

One parent, many contracts

A class can extend only one class, but it can implement any number of interfaces, separated by commas. Its objects then count as every one of those types and can be passed wherever any of them is expected.

interface Flyer { void fly(); }
interface Swimmer { void swim(); }
class Duck implements Flyer, Swimmer {
    public void fly() { }
    public void swim() { }
}

The word order is fixed

When a class has both, **extends comes first, then implements. Interfaces can also combine: an interface may extend several interfaces** at once.

class Robot extends Machine
        implements Walker, Talker { }
 
interface Amphibian
        extends Flyer, Swimmer { }

🔮 Predict it

How many types is a duck?

What does this print?

interface Flyer { }
interface Swimmer { }
class Duck implements Flyer, Swimmer { }
void main() {
    Object o = new Duck();
    System.out.println(o instanceof Swimmer);
    System.out.println(o instanceof Comparable);
}

true false
false false
true true

Show the answer

A Duck is a Flyer and a Swimmer — every interface its class implements, in any listed position. It never signed Comparable, so that check is false.

⚠️ The trap

Backwards declaration

Writing implements before extends is a compile error, even though both parts are fine on their own. The legal order is class Kid extends Thread implements Runnable (legal, if redundant — Thread already implements Runnable).

class Kid implements Runnable
        extends Thread { }   // error

🤔 Think first

Same method, two contracts

Door and Window both declare the abstract method void close();. Class Car implements both. Does Car need two close() methods?

Think about it, then reveal the answer

No — **one public close() satisfies both**. Identical abstract signatures are the same requirement, and neither interface supplies a body, so nothing competes. (When two interfaces *do* bring bodies, things get spicier — soon!)

💼 In the real world

Real classes wear many badges

ArrayList implements List, RandomAccess, Cloneable and Serializable all at once. Your own domain classes do the same: an Order can be Comparable<Order> for sorting and Serializable for caching, while still extending your BaseEntity.

Key takeaways

class Duck implements Flyer, Swimmer { ... }
extends comes first: class A extends B implements C, D
An interface may extend several interfaces: interface C extends A, B
Same abstract method in two interfaces? One implementation satisfies both

💡 One person can hold a driver's licence, a library card and a passport at the same time.

🤯 Did you know?

Java's own String implements five interfaces: Serializable, Comparable, CharSequence, plus Constable and ConstantDesc, which were added in Java 12.

Practice questions

Robot already has a superclass. Which keyword fills the blank?

class Robot extends Machine ___ Walker, Talker { }

implements
extends
uses
inherits

Check your answer

implements. Classes are extended once; interfaces are implemented, and you list them after implements separated by commas.

What does this print?

interface A { }
interface B { }
class C implements A, B { }
void main() {
    Object o = new C();
    System.out.println(o instanceof A);
    System.out.println(o instanceof B);
    System.out.println(o instanceof Runnable);
}

true true false
true false false
false false false
Compile error

Check your answer

true true false. A C object is an A and a B because its class implements both. It isn't a Runnable, so that check is false.

Next: put a field in an interface and Java silently adds three keywords to it. Which three?