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Using Java operators
Building a Java program
Multidimensional arrays
Shift operators
Your first Java program
Constructors and polymorphism - Order of constructor calls
Name hiding
Returning an array
AWT Image Processing
Applets And Graphics


The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists and return types, but no method bodies. An interface can also contain data members of primitive types, but these are implicitly static and final. An interface provides only a form, but no implementation.

An interface says: “This is what all classes that implement this particular interface will look like.” Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes. (Some object-oriented programming languages have a keyword called protocol to do the same thing.)

To create an interface, use the interface keyword instead of the class keyword. Like a class, you can add the public keyword before the interface keyword (but only if that interface is defined in a file of the same name) or leave it off to give “friendly” status.

To make a class that conforms to a particular interface (or group of interfaces) use the implements keyword. You’re saying “The interface is what it looks like and here’s how it works.” Other than that, it bears a strong resemblance to inheritance. The diagram for the instrument example shows this:

Once you’ve implemented an interface, that implementation becomes an ordinary class that can be extended in the regular way.

You can choose to explicitly declare the method declarations in an interface as public. But they are public even if you don’t say it. So when you implement an interface, the methods from the interface must be defined as public. Otherwise they would default to “friendly” and you’d be restricting the accessibility of a method during inheritance, which is not allowed by the Java compiler.

You can see this in the modified version of the Instrument example. Note that every method in the interface is strictly a declaration, which is the only thing the compiler allows. In addition, none of the methods in Instrument5 are declared as public, but they’re automatically public anyway:


// Interfaces

import java.util.*;

interface Instrument5

class Wind5 implements Instrument5

public String what()

public void adjust()


class Percussion5 implements Instrument5

public String what()

public void adjust()


class Stringed5 implements Instrument5

public String what()

public void adjust()


class Brass5 extends Wind5

public void adjust()


class Woodwind5 extends Wind5

public String what()


public class Music5

static void tuneAll(Instrument5[] e)

public static void main(String[] args)

} ///:~

The rest of the code works the same. It doesn’t matter if you are upcasting to a “regular” class called Instrument5, an abstract class called Instrument5, or to an interface called Instrument5. The behavior is the same. In fact, you can see in the tune( ) method that there isn’t any evidence about whether Instrument5 is a “regular” class, an abstract class or an interface. This is the intent: Each approach gives the programmer different control over the way objects are created and used.

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