[Chapter 5] 5.5 Interface Declarations

5.5 Interface Declarations

An interface declaration creates a reference type in Java. An interface declaration is similar to a class declaration, with the following two very important differences.

All of the methods in an interface are implicitly abstract. Every method declaration in an interface specifies the formal parameters and return type of the method, but it does not include an implementation of the method.
All of the variables in an interface are implicitly static and final.

Interfaces are most useful for declaring that an otherwise unrelated set of classes have a common set of methods. For example, if you want to store a variety of objects in a database, you might want all of those objects to have fetch and store methods. The fetch and store methods of each object require different implementations, so it makes sense to declare the fetch and store methods in an interface declaration. Then any class that needs fetch and store methods can implement the interface.

The formal definition for an interface declaration is:

[Graphic: Figure from the text]

While the above diagram may seem complicated, an interface declaration is really made up of five distinct things:

Optional modifiers that specify attributes of the class
The keyword interface
An identifier that names the interface
An optional extends clause that specifies the super interfaces of the declared interface
Any number of interface member declarations, which can include variables and methods

Here are some sample interface declarations:

interface Dyn {
    double squeeze();
}
interface Press extends Dyn {
    double squeeze(double theta);
}

Here is an example of a class that implements Press:

class Clamp implements Press {
      ...
    double squeeze() {
        return squeeze(0);
    }
    double squeeze(double theta) {
        return force*Math.cos(theta);
    }
      ...
}

Since the Press interface extends the Dyn interface, the Clamp class must implement the methods declared in both Dyn and Press.

References Class Declarations; ClassOrInterfaceName 4.1.6; Identifiers; Interfaces; Interface Members

Interface Modifiers

The keywords public and abstract can appear as modifiers at the beginning of an interface declaration. In this situation, these modifiers have the following meanings:

public

If an interface is declared public, it can be referenced by any class or interface. If the public modifier is not used, however, the interface can only be referenced by classes and interfaces in the same package. A single source file, or compilation unit, can only declare one public class or interface (see Compilation Units for an exception to this rule).

abstract

An interface is implicitly abstract; so all of the methods in an interface are implicitly abstract. Including the abstract modifier in an interface declaration is permitted, but it does not change the meaning of the interface declaration.

References Compilation Units; Inner interface modifiers; Interface method modifiers; Interface variable modifiers

Interface Name

The identifier that follows the keyword interface is the name of the interface. This identifier can be used as a reference type wherever the interface is accessible.

References Interface Types

Interface Inheritance

The extends clause specifies any super-interfaces of the interface being declared; the extends keyword can be followed by the names of one or more interfaces. If an interface has an extends clause, the clause can only name other interfaces.

Including an interface in the extends clause of another interface means that the declared interface inherits the variables and methods declared in the super-interface. A class that implements the declared interface must implement all of the methods in the declared interface, as well as all of the methods inherited from the super-interface.

If an interface declaration does not include an extends clause, the interface does not extend any other interfaces.

Interface Members

The members of an interface can be variables or methods; an interface cannot have constructors, static initializers, instance initializers, nested top-level classes or interfaces, or member classes:

[Graphic: Figure from the text]

References Interface Methods; Interface Variables

Interface Variables

Any field variables declared in an interface are implicitly static and final. In other words, field variables in an interface are named constants. Every field variable declaration in an interface must contain an initializer that sets the value of the named constant:

[Graphic: Figure from the text]

A variable declaration in an interface is made up of three distinct things:

Optional modifiers that specify attributes of the variable.
A type, which can be either a primitive type or a reference type.
Any number of identifiers that name variables. Each name must be followed by an initializer that sets the value of the constant.

References Variable initializers; Expression 4; Identifiers; Type 3

Interface variable modifiers

Variables in an interface are implicitly static and final. Including these modifiers in a variable declaration is permitted, but it is not necessary and it does not change the meaning of the variable declaration. Thus, by definition, all variables in an interface are named constants.

If an interface is declared public, a field variable declared in the interface is public, even if it is declared with the private or protected modifier. If an interface is not declared public, however, any field variables in the interface have the default accessibility, which means that they are only accessible in classes and interfaces in the same package.

It is an error to declare a field variable in an interface with the transient or volatile modifier.

References Interface Modifiers; Variable modifiers

Interface variable type

If the interface variable declaration uses a primitive type, the variable contains a constant value of the specified primitive type. If the declaration uses a reference type, the variable contains a constant reference to the specified type of object. The presence of square brackets in a variable declaration, after either the type or variable name, indicates that the variable contains a reference to an array.

References Array Types; Primitive Types; Reference Types

Interface variable name

The identifier that follows the variable type is the name of the variable. This identifier can be used anywhere that the variable is accessible.

It is an error to declare two field variables with the same name in the same interface. It is also an error to declare a field variable with the same name as a method declared in the same interface or any of its super-interfaces.

An interface that extends another interface inherits all of the variables in its super-interface. Any class that implements an interface has access to all of the variables defined in that interface, as well as the variables inherited from super-interfaces.

If a field variable is declared with the same name as a variable declared in a super-interface, the variable in the super-interface is considered to be shadowed. If a variable is shadowed in an interface, it cannot be accessed as a field of that interface. However, a shadowed variable can be accessed by casting a reference to an object that implements the interface to a reference to the appropriate super-interface in which the variable is not shadowed. For example:

interface A {
    int x = 4;
}
interface B extends A {
    int x = 7;
}
class Z implements B {
    Z() {
        int i = x;              // i gets the value of B's x
        int h = ((A)this).x;    // h gets the value of A's x
    }
}

The variable x in interface A is shadowed by the variable x in interface B. Class Z implements interface B, so a reference to x produces the value 7, as defined in interface B. However, it is possible to access the shadowed variable by casting this to a reference to interface A.

In some situations, an interface may inherit multiple field variables with the same name. This leads to a single, ambiguous variable name. For example:

interface A {
    int x = 4;
}
interface B {
    int x = 43;
}
interface C extends A, B {
    int y = 22;
}
class Z implements C {
    public static void main (String[] argv) {
        System.out.println(x);        // Ambiguous
    }
}

In this example, the interface C inherits two variables named x. This is fine, as long as C does not refer to the variable x by its simple name in any of its declarations. If C needs to use x, it must qualify the name with the appropriate interface name (e.g., A.x). Class Z implements interface C, so it also has access to two variables named x. As a result, the use of x in main() is ambiguous. This problem can be resolved by qualifying the variable name with the appropriate interface name (e.g., B.x).

A class that implements multiple interfaces can also inherit multiple field variables with the same name. Again, this leads to a single, ambiguous variable name:

interface A {
    int x = 4;
}
interface B {
    int x = 43;
}
class Z implements A, B {
    public static void main (String[] argv) {
        System.out.println(x);        // Ambiguous
    }
}

The class Z implements both interface A and interface B, so it inherits two variables named x. As a result, the use of x in main() is ambiguous. This problem can again be resolved by qualifying the variable name with the appropriate interface name (e.g., B.x).

References Field Expressions; Identifiers; Interface method name

Interface variable initializers

Every variable declaration in an interface must include an initializer that sets the value of the constant. The initializer does not, however, have to be a constant expression. If the variable is of a non-array type, the expression in the initializer is evaluated and the variable is set to the result of the expression, as long as the result is assignment-compatible with the variable. If the variable is of an array type, the initializer must be an array initializer.

The initializer for a variable in an interface cannot refer to any variables that are declared after its own declaration.

References Variable initializers; Array Types; Assignment Operators; Constant Expressions; Expression 4

Interface Methods

Any methods declared in an interface are implicitly abstract. In other words, methods in an interface do not have a specified implementation:

[Graphic: Figure from the text]

A method declaration in an interface is made up of six distinct things:

Optional modifiers that specify attributes of the method
A type that specifies the type of value returned by the method
An identifier that names the method
A list of formal parameters that specifies the values that are passed to the method
An optional throws clause that specifies any exceptions that can be thrown by the method
A semicolon, since the method declaration does not include an implementation

References ClassOrInterfaceName 4.1.6; Exception Handling 9; Method formal parameters; Identifiers; Type 3

Interface method modifiers

Methods in an interface are implicitly abstract. Including this modifier in a method declaration is permitted, but it is not necessary and it does not change the meaning of the method declaration. Thus, by definition, none of the methods in an interface has a specified implementation.

If an interface is declared public, a method declared in the interface is public, even if it is declared with the private or protected modifier. If the interface is not declared public, however, any methods in the interface have the default accessibility, which means that they are only accessible in classes and interfaces in the same package.

It is an error to declare a method in an interface with the static, final, native, or synchronized modifier. These modifiers are not allowed because defining a method in an interface is not meant to imply anything about the nature of the implementation, other than the return type of the method and the types of the formal parameters. A class that implements the interface has control over the implementation of the methods and can use any of these modifiers when they are appropriate for the implementation.

References Interface Modifiers; Method modifiers

Interface method return type

A method declaration in an interface must specify the type of value returned by the method. The return value can be of a primitive type or of a reference type. The presence of square brackets in a method declaration, after either the return type or the formal parameters, indicates that the method returns a reference to the specified type of array. If the method does not return a value, the declaration uses void to indicate that. The return type comes before the name of the method in the method declaration.

References Array Types; Method return type; Primitive Types; Reference Types

Interface method name

The identifier that follows the return type is the name of the method. This identifier can be used anywhere that the method is accessible.

It is an error to declare two methods that have the same name, the same number of parameters, and the same type for each corresponding parameter in the same interface. It is also an error to declare a method with the same name as a variable declared in the same interface or any of its super-interfaces.

An interface that extends another interface inherits all of the methods in its super-interface. Any class that implements an interface must provide an implementation for each of the methods defined in that interface, as well as each of the methods inherited from super-interfaces.

If an interface inherits methods from multiple super-interfaces that have the same name, formal parameters, and return type, there is no problem. The various super-interfaces are in agreement about the method. The interface can also override the inherited methods by declaring a method with the same name, formal parameters, and return type. In any case, a class that implements the interface has to provide a single implementation for the method.

However, if an interface inherits methods from multiple super-interfaces that have the same name and same formal parameters, but different return types, a compile-time error results. By the same token, if the interface attempts to override an inherited method with a method that has the same name and same formal parameters, but a different return type, a compile-time error results.

If an interface inherits methods from multiple super-interfaces that have the same name but different formal parameters, there is no problem. The methods are simply considered overloaded in the interface. The interface can even declare additional methods that have the same name but different formal parameters. A class that implements the interface simply has to provide an implementation for each of the overloaded methods.

References Identifiers; Interface variable name; Method Call Expression

Interface method formal parameters

The formal parameters in a method declaration specify a list of variables to which values are assigned when the method is called. If a method has no formal parameters, the parentheses must still appear in the method declaration. The presence of square brackets in a formal parameter declaration, either as part of a reference type or after the name of a formal parameter, indicates that the formal parameter is an array type.

References Array Types; Method formal parameters; Method formal parameters; Type 3

Interface method throws clause

If a method is expected to throw any exceptions, the method declaration must declare that fact in a throws clause. If the declaration of a method in an interface contains a throws clause, any method in a sub-interface that overrides that method cannot include any classes in its throws clause that are not declared in the overridden method.

References Exception Handling 9; Method throws clause

Nested Top-Level Interfaces

Nested top-level interfaces are interfaces that are declared inside of another class. Just as with a top-level interface declaration, the declaration of a nested top-level interface creates a reference type in Java. Here's the formal definition of a nested top-level interface:

[Graphic: Figure from the text]

A nested top-level interface can be accessed outside of its enclosing class by qualifying its name with the name of its enclosing class, as follows:

EnclosingClass.InnerInterface

The syntax for declaring nested top-level interfaces is not supported prior to Java 1.1.

References Nested top-level classes and interfaces; SimpleInterfaceDeclaration 5.5

Inner interface modifiers

The keywords public, abstract, and static can be used in the declaration of a nested top-level interface. In this situation, these modifiers have the following meanings:

public

If a nested top-level interface is declared public, it is accessible from any class or interface that can access the enclosing class. If the public modifier is not used, however, the nested top-level interface can only be referenced by classes and interfaces in the same package as the enclosing class.

abstract

A nested top-level interface is implicitly abstract; thus, all of the methods in the interface are implicitly abstract. Including the abstract modifier in a nested top-level interface declaration is permitted, but it does not change the meaning of the interface declaration.

static

A nested top-level interface is implicitly static. Including the static modifier in a nested top-level interface declaration is permitted, but it does not change the meaning of the interface declaration.

References Interface Modifiers

Inner interface members

The remainder of a nested top-level interface declaration is the same as that for a top-level interface declaration, which is described in Interface Declarations.

References Interface Declarations; Interface Methods; Interface Variables