What is an interface in Java?

Question

Just as a counterpoint to this question: what is an interface in Java?

Answer 1

An interface is a special form of an abstract class which does not implement any methods. In Java, you create an interface like this:

interface Interface
{
    void interfaceMethod();
}

Since the interface can't implement any methods, it's implied that the entire thing, including all the methods, are both public and abstract (abstract in Java terms means "not implemented by this class"). So the interface above is identical to the interface below:

public interface Interface
{
    abstract public void interfaceMethod();
}

To use this interface, you simply need to implement the interface. Many classes can implement an interface, and a class can implement many interfaces:

interface InterfaceA
{
     void interfaceMethodA();
}

interface InterfaceB
{
    void interfaceMethodB();
}

public class ImplementingClassA
    implements InterfaceA, InterfaceB
{
    public void interfaceMethodA()
    {
        System.out.println("interfaceA, interfaceMethodA, implementation A");
    }

    public void interfaceMethodB()
    {
        System.out.println("interfaceB, interfaceMethodB, implementation A");
    }
}

public class ImplementingClassB
    implements InterfaceA, InterfaceB
{
    public void interfaceMethodA()
    {
         System.out.println("interfaceA, interfaceMethodA, implementation B");
    }

    public void interfaceMethodB()
    {
        System.out.println("interfaceB, interfaceMethodB, implementation B");
    }
}

Now if you wanted you could write a method like this:

public void testInterfaces()
{
    ImplementingClassA u = new ImplementingClassA();
    ImplementingClassB v = new ImplementingClassB();
    InterfaceA w = new ImplementingClassA();
    InterfaceA x = new ImplementingClassB();
    InterfaceB y = new ImplementingClassA();
    InterfaceB z = new ImplementingClassB();

    u.interfaceMethodA();
    // prints "interfaceA, interfaceMethodA, implementation A"
    u.interfaceMethodB();
    // prints "interfaceB, interfaceMethodB, implementation A"
    v.interfaceMethodA();
    // prints "interfaceA, interfaceMethodA, implementation B"
    v.interfaceMethodB();
    // prints "interfaceB, interfaceMethodB, implementation B"
    w.interfaceMethodA();
    // prints "interfaceA, interfaceMethodA, implementation A"
    x.interfaceMethodA();
    // prints "interfaceA, interfaceMethodA, implementation B"
    y.interfaceMethodB();
    // prints "interfaceB, interfaceMethodB, implementation A"
    z.interfaceMethodB();
    // prints "interfaceB, interfaceMethodB, implementation B"
}

However, you could never do the following:

public void testInterfaces()
{
    InterfaceA y = new ImplementingClassA();
    InterfaceB z = new ImplementingClassB();

    y.interfaceMethodB(); // ERROR!
    z.interfaceMethodA(); // ERROR!
}

The reason you can't do this is that y is of type interfaceA, and there is no interfaceMethodB() in interfaceA. Likewise, z is of type interfaceB and there is no interfaceMethodA() in interfaceB.

I mentioned earlier that interfaces are just a special form of an abstract class. To illustrate that point, look at the following code.

interface Interface
{
    void abstractMethod();
}

abstract public class AbstractClass
{
    abstract public void abstractMethod();
}

You would inherit from these classes almost exactly the same way:

public class InheritsFromInterface
    implements Interface
{
    public void abstractMethod() { System.out.println("abstractMethod()"); }
}

public class InteritsFromAbstractClass
    extends AbstractClass
{
    public void abstractMethod() { System.out.println("abstractMethod()"); }
}

In fact, you could even change the interface and the abstract class like this:

interface Interface
{
    void abstractMethod();
}

abstract public class AbstractClass
    implements Interface
{
    abstract public void abstractMethod();
}

public class InheritsFromInterfaceAndAbstractClass
    extends AbstractClass implements Interface
{
    public void abstractMethod() { System.out.println("abstractMethod()"); }
}

However, there are two differences between interfaces and abstract classes.

The first difference is that interfaces cannot implement methods.

interface Interface
{
    public void implementedMethod()
    {
        System.out.println("implementedMethod()");
    }
}

The interface above generates a compiler error because it has an implementation for implementedMethod(). If you wanted to implement the method but not be able to instantiate the class, you would have to do it like this:

abstract public class AbstractClass
{
    public void implementedMethod()
    {
        System.out.println("implementedMethod()");
    }
}

That's not much of an abstract class because none of its members are abstract, but it is legal Java.

The other difference between interfaces and abstract classes is that a class can inherit from multiple interfaces, but can only inherit from one abstract class.

abstract public class AbstractClassA { }
abstract public class AbstractClassB { }
public class InheritsFromTwoAbstractClasses
    extends AbstractClassA, AbstractClassB
{ }

The code above generates a compiler error, not because the classes are all empty, but because InheritsFromTwoAbstractClasses is trying to inherit from two abstract classes, which is illegal. The following is perfectly legal.

interface InterfaceA { }
interface InterfaceB { }
public class InheritsFromTwoInterfaces
    implements InterfaceA, InterfaceB
{ }

The first difference between interfaces and abstract classes is the reason for the second difference. Take a look at the following code.

interface InterfaceA
{
    void method();
}

interface InterfaceB
{
    void method();
}

public class InheritsFromTwoInterfaces
    implements InterfaceA, InterfaceB
{
    void method() { System.out.println("method()"); }
}

There's no problem with the code above because InterfaceA and InterfaceB don't have anything to hide. It's easy to tell that a call to method will print "method()".

Now look at the following code:

abstract public class AbstractClassA
{
    void method() { System.out.println("Hello"); }
}

abstract public class AbstractClassB
{
    void method() { System.out.println("Goodbye"); }
}

public class InheritsFromTwoAbstractClasses
    extends AbstractClassA, AbstractClassB
{ }

This is exactly the same as our other example, except that because we're allowed to implement methods in abstract classes, we did, and because we don't have to implement already-implemented methods in an inheriting class, we didn't. But you may have noticed, there's a problem. What happens when we call new InheritsFromTwoAbstractClasses().method()? Does it print "Hello" or "Goodbye"? You probably don't know, and neither does the Java compiler. Another language, C++ allowed this kind of inheritance and they resolved these issues in ways that were often very complicated. To avoid this kind of trouble, Java decided to make this "multiple inheritance" illegal.

The downside to Java's solution that the following can't be done:

abstract public class AbstractClassA
{
    void hi() { System.out.println("Hello"); }
}

abstract public class AbstractClassB
{
    void bye() { System.out.println("Goodbye"); }
}

public class InheritsFromTwoAbstractClasses
    extends AbstractClassA, AbstractClassB
{ }

AbstractClassA and AbstractClassB are "mixins" or classes that aren't intended to be instantiated but add functionality to the classes that they are "mixed into" through inheritance. There's obviously no problem figuring out what happens if you call new InheritsFromTwoAbstractClasses().hi() or new InheritsFromTwoAbstractClasses().bye(), but you can't do that because Java doesn't allow it.

(I know this is a long post, so if there are any mistakes in it please let me know and I will correct them.)

Answer 2

Here's a more concise answer on Wikipedia.