Interfaces

Introduction#

An interface is a reference type, similar to a class, which can be declared by using interface keyword.

Interfaces can contain only constants, method signatures, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods. Like abstract classes, Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

Interface is a common way to achieve full abstraction in Java.

Syntax#

public interface Foo { void foo(); /* any other methods */ }
public interface Foo1 extends Foo { void bar(); /* any other methods */ }
public class Foo2 implements Foo, Foo1 { /* implementation of Foo and Foo1 */ }

Declaring and Implementing an Interface

Declaration of an interface using the interface keyword:

public interface Animal {
    String getSound(); // Interface methods are public by default
}

Override Annotation

@Override
public String getSound() {
    // Code goes here...
}

This forces the compiler to check that we are overriding and prevents the program from defining a new method or messing up the method signature.

Interfaces are implemented using the implements keyword.

public class Cat implements Animal {

    @Override 
    public String getSound() {
        return "meow";
    }
}

public class Dog implements Animal {

    @Override
    public String getSound() {
        return "woof";
    }
}

In the example, classes Cat and Dog must define the getSound() method as methods of an interface are inherently abstract (with the exception of default methods).

Using the interfaces

Animal cat = new Cat();
Animal dog = new Dog();

System.out.println(cat.getSound()); // prints "meow"
System.out.println(dog.getSound()); // prints "woof"

Implementing multiple interfaces

A Java class can implement multiple interfaces.

public interface NoiseMaker {
    String noise = "Making Noise"; // interface variables are public static final by default

    String makeNoise(); //interface methods are public abstract by default
}

public interface FoodEater {
    void eat(Food food);
}

public class Cat implements NoiseMaker, FoodEater { 
    @Override
    public String makeNoise() {
        return "meow";
    }

    @Override
    public void eat(Food food) {
        System.out.println("meows appreciatively");
    }
}

Notice how the Cat class must implement the inherited abstract methods in both the interfaces. Furthermore, notice how a class can practically implement as many interfaces as needed (there is a limit of 65,535 due to JVM Limitation).

NoiseMaker noiseMaker = new Cat(); // Valid
FoodEater foodEater = new Cat(); // Valid
Cat cat = new Cat(); // valid

Cat invalid1 = new NoiseMaker(); // Invalid
Cat invalid2 = new FoodEater(); // Invalid

Note:

All variables declared in an interface are public static final
All methods declared in an interface methods are public abstract (This statement is valid only through Java 7. From Java 8, you are allowed to have methods in an interface, which need not be abstract; such methods are known as default methods)
Interfaces cannot be declared as final
If more than one interface declares a method that has identical signature, then effectively it is treated as only one method and you cannot distinguish from which interface method is implemented
A corresponding InterfaceName.class file would be generated for each interface, upon compilation

Extending an interface

An interface can extend another interface via the extends keyword.

public interface BasicResourceService {
    Resource getResource();
}

public interface ExtendedResourceService extends BasicResourceService {
    void updateResource(Resource resource);
}

Now a class implementing ExtendedResourceService will need to implement both getResource() and updateResource().

Extending multiple interfaces

Unlike classes, the extends keyword can be used to extend multiple interfaces (Separated by commas) allowing for combinations of interfaces into a new interface

public interface BasicResourceService {
    Resource getResource();
}

public interface AlternateResourceService {
    Resource getAlternateResource();
}

public interface ExtendedResourceService extends BasicResourceService, AlternateResourceService {
    Resource updateResource(Resource resource);
}

In this case a class implementing ExtendedResourceService will need to implement getResource(), getAlternateResource(), and updateResource().

Using Interfaces with Generics

Let’s say you want to define an interface that allows publishing / consuming data to and from different types of channels (e.g. AMQP, JMS, etc), but you want to be able to switch out the implementation details …

Let’s define a basic IO interface that can be re-used across multiple implementations:

public interface IO<IncomingType, OutgoingType> {

    void publish(OutgoingType data);
    IncomingType consume();
    IncomingType RPCSubmit(OutgoingType data);

}

Now I can instantiate that interface, but since we don’t have default implementations for those methods, it’ll need an implementation when we instantiate it:

    IO<String, String> mockIO = new IO<String, String>() {

        private String channel = "somechannel";

        @Override
        public void publish(String data) {
            System.out.println("Publishing " + data + " to " + channel);
        }

        @Override
        public String consume() {
            System.out.println("Consuming from " + channel);
            return "some useful data";
        }

        @Override
        public String RPCSubmit(String data) {
            return "received " + data + " just now ";
        }

    };

    mockIO.consume(); // prints: Consuming from somechannel
    mockIO.publish("TestData"); // Publishing TestData to somechannel
    System.out.println(mockIO.RPCSubmit("TestData")); // received TestData just now

We can also do something more useful with that interface, let’s say we want to use it to wrap some basic RabbitMQ functions:

public class RabbitMQ implements IO<String, String> {

    private String exchange;
    private String queue;

    public RabbitMQ(String exchange, String queue){
        this.exchange = exchange;
        this.queue = queue;
    }

    @Override
    public void publish(String data) {
        rabbit.basicPublish(exchange, queue, data.getBytes());
    }

    @Override
    public String consume() {
        return rabbit.basicConsume(exchange, queue);
    }

    @Override
    public String RPCSubmit(String data) {
        return rabbit.rpcPublish(exchange, queue, data);
    }

}

Let’s say I want to use this IO interface now as a way to count visits to my website since my last system restart and then be able to display the total number of visits - you can do something like this:

import java.util.concurrent.atomic.AtomicLong;

public class VisitCounter implements IO<Long, Integer> {

    private static AtomicLong websiteCounter = new AtomicLong(0);
    
    @Override
    public void publish(Integer count) {
        websiteCounter.addAndGet(count);
    }

    @Override
    public Long consume() {
        return websiteCounter.get();
    }

    @Override
    public Long RPCSubmit(Integer count) {
        return websiteCounter.addAndGet(count);
    }
    
}

Now let’s use the VisitCounter:

    VisitCounter counter = new VisitCounter();

    // just had 4 visits, yay
    counter.publish(4);
    // just had another visit, yay
    counter.publish(1);

    // get data for stats counter
    System.out.println(counter.consume()); // prints 5

    // show data for stats counter page, but include that as a page view
    System.out.println(counter.RPCSubmit(1)); // prints 6

When implementing multiple interfaces, you can’t implement the same interface twice. That also applies to generic interfaces. Thus, the following code is invalid, and will result in a compile error:

interface Printer<T> {
    void print(T value);
}

// Invalid!
class SystemPrinter implements Printer<Double>, Printer<Integer> {
    @Override public void print(Double d){ System.out.println("Decimal: " + d); }
    @Override public void print(Integer i){ System.out.println("Discrete: " + i); }
}

Usefulness of interfaces

Interfaces can be extremely helpful in many cases. For example, say you had a list of animals and you wanted to loop through the list, each printing the sound they make.

{cat, dog, bird}

One way to do this would be to use interfaces. This would allow for the same method to be called on all of the classes

public interface Animal {
    public String getSound();
}

Any class that implements Animal also must have a getSound() method in them, yet they can all have different implementations

public class Dog implements Animal {
    public String getSound() {
        return "Woof";
    }
}

public class Cat implements Animal {
    public String getSound() {
        return "Meow";
    }
}

public class Bird implements Animal{
    public String getSound() {
        return "Chirp";
    }
}

We now have three different classes, each of which has a getSound() method. Because all of these classes implement the Animal interface, which declares the getSound() method, any instance of an Animal can have getSound() called on it

Animal dog = new Dog();
Animal cat = new Cat();
Animal bird = new Bird();

dog.getSound(); // "Woof"
cat.getSound(); // "Meow"
bird.getSound(); // "Chirp"

Because each of these is an Animal, we could even put the animals in a list, loop through them, and print out their sounds

Animal[] animals = { new Dog(), new Cat(), new Bird() };
for (Animal animal : animals) {
    System.out.println(animal.getSound());
}

Because the order of the array is Dog, Cat, and then Bird, “Woof Meow Chirp” will be printed to the console.

Interfaces can also be used as the return value for functions. For example, returning a Dog if the input is “dog”, Cat if the input is “cat”, and Bird if it is “bird”, and then printing the sound of that animal could be done using

public Animal getAnimalByName(String name) {
    switch(name.toLowerCase()) {
        case "dog":
            return new Dog();
        case "cat":
            return new Cat();
        case "bird":
            return new Bird();
        default:
            return null;
    }
}

public String getAnimalSoundByName(String name){
    Animal animal = getAnimalByName(name);
    if (animal == null) {
        return null;
    } else { 
        return animal.getSound();
    }
}

String dogSound = getAnimalSoundByName("dog"); // "Woof"
String catSound = getAnimalSoundByName("cat"); // "Meow"
String birdSound = getAnimalSoundByName("bird"); // "Chirp"
String lightbulbSound = getAnimalSoundByName("lightbulb"); // null

Interfaces are also useful for extensibility, because if you want to add a new type of Animal, you wouldn’t need to change anything with the operations you perform on them.

Implementing interfaces in an abstract class

A method defined in an interface is by default public abstract. When an abstract class implements an interface, any methods which are defined in the interface do not have to be implemented by the abstract class. This is because a class that is declared abstract can contain abstract method declarations. It is therefore the responsibility of the first concrete sub-class to implement any abstract methods inherited from any interfaces and/or the abstract class.

public interface NoiseMaker {
    void makeNoise();
}

public abstract class Animal implements NoiseMaker {
    //Does not need to declare or implement makeNoise()
    public abstract void eat();
}

//Because Dog is concrete, it must define both makeNoise() and eat()
public class Dog extends Animal {
    @Override
    public void makeNoise() {
        System.out.println("Borf borf");
    }

    @Override
    public void eat() {
        System.out.println("Dog eats some kibble.");
    }
}

From Java 8 onward it is possible for an interface to declare default implementations of methods which means the method won’t be abstract, therefore any concrete sub-classes will not be forced to implement the method but will inherit the default implementation unless overridden.

Default methods

Introduced in Java 8, default methods are a way of specifying an implementation inside an interface. This could be used to avoid the typical “Base” or “Abstract” class by providing a partial implementation of an interface, and restricting the subclasses hierarchy.

Observer pattern implementation

For example, it’s possible to implement the Observer-Listener pattern directly into the interface, providing more flexibility to the implementing classes.

interface Observer {
    void onAction(String a);
}

interface Observable{
    public abstract List<Observer> getObservers();

    public default void addObserver(Observer o){
        getObservers().add(o);
    }

    public default void notify(String something ){
        for( Observer l : getObservers() ){
            l.onAction(something);
        }
    }
}

Now, any class can be made “Observable” just by implementing the Observable interface, while being free to be part of a different class hierarchy.

abstract class Worker{
    public abstract void work();
}

public class MyWorker extends Worker implements Observable {

    private List<Observer> myObservers = new ArrayList<Observer>();
    
    @Override
    public List<Observer> getObservers() {
        return myObservers;
    }

    @Override
    public void work(){
        notify("Started work");

        // Code goes here...

        notify("Completed work");
    }
    
    public static void main(String[] args) {    
        MyWorker w = new MyWorker();
       
        w.addListener(new Observer() {
            @Override
            public void onAction(String a) {
                System.out.println(a + " (" + new Date() + ")");
            }
        });
        
        w.work();
    }
}

Diamond problem

The compiler in Java 8 is aware of the diamond problem which is caused when a class is implementing interfaces containing a method with the same signature.

In order to solve it, an implementing class must override the shared method and provide its own implementation.

interface InterfaceA {
    public default String getName(){
        return "a";
    }
}

interface InterfaceB {
    public default String getName(){
        return "b";
    }
}

public class ImpClass implements InterfaceA, InterfaceB {

    @Override
    public String getName() {    
        //Must provide its own implementation
        return InterfaceA.super.getName() + InterfaceB.super.getName();
    }
    
    public static void main(String[] args) {    
        ImpClass c = new ImpClass();
        
        System.out.println( c.getName() );                   // Prints "ab"
        System.out.println( ((InterfaceA)c).getName() );     // Prints "ab"
        System.out.println( ((InterfaceB)c).getName() );     // Prints "ab"
    }
}

There’s still the issue of having methods with the same name and parameters with different return types, which will not compile.

Use default methods to resolve compatibility issues

The default method implementations come in very handy if a method is added to an interface in an existing system where the interfaces is used by several classes.

To avoid breaking up the entire system, you can provide a default method implementation when you add a method to an interface. This way, the system will still compile and the actual implementations can be done step by step.

For more information, see the Default Methods topic.

Modifiers in Interfaces

The Oracle Java Style Guide states:

Modifiers should not be written out when they are implicit.

(See Modifiers in Oracle Official Code Standard for the context and a link to the actual Oracle document.)

This style guidance applies particularly to interfaces. Let’s consider the following code snippet:

interface I {
    public static final int VARIABLE = 0;

    public abstract void method();

    public static void staticMethod() { ... }
    public default void defaultMethod() { ... }
}

Variables

All interface variables are implicitly constants with implicit public (accessible for all), static (are accessible by interface name) and final (must be initialized during declaration) modifiers:

public static final int VARIABLE = 0;

Methods

All methods which don’t provide implementation are implicitly public and abstract.

public abstract void method();

All methods with static or default modifier must provide implementation and are implicitly public.

public static void staticMethod() { ... }

After all of the above changes have been applied, we will get the following:

interface I {
    int VARIABLE = 0;
    
    void method();

    static void staticMethod() { ... }
    default void defaultMethod() { ... }
}

Strengthen bounded type parameters

Bounded type parameters allow you to set restrictions on generic type arguments:

class SomeClass {

}

class Demo<T extends SomeClass> {

}

But a type parameter can only bind to a single class type.

An interface type can be bound to a type that already had a binding. This is achieved using the & symbol:

interface SomeInterface {

}

class GenericClass<T extends SomeClass & SomeInterface> {

}

This strengthens the bind, potentially requiring type arguments to derive from multiple types.

Multiple interface types can be bound to a type parameter:

class Demo<T extends SomeClass & FirstInterface & SecondInterface> {

}

But should be used with caution. Multiple interface bindings is usually a sign of a code smell, suggesting that a new type should be created which acts as an adapter for the other types:

interface NewInterface extends FirstInterface, SecondInterface {

}

class Demo<T extends SomeClass & NewInterface> {

}