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An interface declaration introduces a new reference type whose members are classes, interfaces, constants, and methods. This type has no instance variables, and typically declares one or more abstract methods; otherwise unrelated classes can implement the interface by providing implementations for its abstract methods.
Interfaces may not be directly instantiated. A nested interface is any interface whose declaration occurs within the body of another class or interface. A top level interface is an interface that is not a nested interface. We distinguish between two kinds of interfaces – normal interfaces and annotation types. Details that are specific to particular kinds of interfaces are discussed in the sections dedicated to these constructs.
Programs can use interfaces to make it unnecessary for related classes to share a common abstract superclass or to add methods to Object. An interface may be declared to be a direct extension of one or more other interfaces, meaning that it inherits all the member types, instance methods, and constants of the interfaces it extends, except for any members that it may override or hide.
A class may be declared to directly implement one or more interfaces, meaning that any instance of the class implements all the abstract methods specified by the interface or interfaces. A class necessarily implements all the interfaces that its direct superclasses and direct superinterfaces do.
This multiple interface inheritance allows objects to support multiple common behaviors without sharing a superclass. A variable whose declared type is an interface type may have as its value a reference to any instance of a class which implements the specified interface. It is not sufficient that the class happen to implement all the abstract methods of the interface; the class or one of its superclasses must actually be declared to implement the interface, or else the class is not considered to implement the interface.
An interface declaration specifies a new named reference type. The Identifier in an interface declaration specifies the name of the interface. It is a compile-time error if an interface has the same simple name as any of its enclosing classes or interfaces. An interface declaration may include interface modifiers. It is a compile-time error if the same keyword appears more than once as a modifier for an interface declaration.
If two or more distinct interface modifiers appear in an interface declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for InterfaceModifier. Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs. This implies that all methods declared in the interface, and all nested types declared in the interface, are implicitly strictfp.
These type variables are known as the type parameters of the interface. The type parameter section follows the interface name and is delimited by angle brackets.
In an interface’s type parameter section, a type variable T directly depends on a type variable S if S is the bound of Twhile T depends on S if either T directly depends on S or T directly depends on a type variable U that depends on S using this definition recursively. It is a compile-time error if a type variable in a interface’s type parameter section depends on itself.
It is a compile-time error to refer to a type parameter of an interface I anywhere in the declaration of a field or type member of I. All of these parameterized types share the same interface at run time. If an extends clause is provided, then the interface being declared extends each of the other named interfaces and therefore inherits the member types, methods, and constants of each of the other named interfaces.
These other named interfaces are the direct superinterfaces of the interface being declared. Any class that implements the declared interface is also considered to implement all the interfaces that this interface extends.
The superinterface relationship is the transitive closure of the direct superinterface relationship. An interface K is a superinterface of interface I if either of the following is true:. K is a direct superinterface of I.
There exists an interface J such that K is a superinterface of Jand J is a superinterface of Iapplying this definition recursively. Interface I is said to be a subinterface of interface K whenever K is a superinterface of I. While every class is an extension of class Objectthere is no single interface of which all interfaces are extensions. An interface I directly depends on a type T if T is mentioned in the extends clause of I either as a superinterface or as a qualifier in the fully qualified form of a superinterface name.
An interface I depends on a reference type T if any of the following is true:.
Chapter 2: Introduction
I directly depends on T. I directly depends on an interface J that depends on T using this definition recursively. It is a compile-time error if an interface depends on itself. The members of an interface type are:. If an interface has no direct superinterfaces, then annohations interface implicitly declares a public abstract member method m with signature sreturn type rand throws clause annotatiohs corresponding to each public instance method m with signature sreturn 9.40 rand throws clause t declared in Objectunless an abstract method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.
It is a compile-time error if the interface explicitly declares such a method m in the case where m is declared to be final in Object. However, this is discouraged as a matter of style. Every field declaration in the body of an interface is implicitly publicstaticand final.
It is permitted to redundantly specify any or all of these modifiers for such annotagions. It is a compile-time error if the same keyword appears more than once as a modifier for a field declaration. If two or more distinct field modifiers appear in a field declaration, it is customary, though not required, that annottaions appear in the order consistent with that shown above in the production for ConstantModifier.
It is a compile-time error for the body of an interface declaration to declare two fields with the same name. If the interface declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superinterfaces of the interface. It is possible for an interface to inherit more than one field with the same name.
Such a situation does not in itself cause a compile-time error. However, any attempt within the body of the interface to refer to any such field by its simple name will result in a annottions error, because such a reference is ambiguous. There might be several paths by which the same field declaration might be inherited from an interface. In such a situation, the field is considered to be inherited only once, and it may be referred to by its simple name without ambiguity.
If two fields with the same name are inherited by an interface because, for example, two of its direct superinterfaces declare fields with that name, then a single ambiguous member results. Any use of this ambiguous member will result in a compile-time error.
Such a reference could occur within a variable initializer for a field. If a single field is inherited multiple times from the same 9.40 because, for example, both this interface and one of this interface’s annogations superinterfaces extend the interface that declares the field, then only a single member annotatiobs.
This situation does not in itself cause a compile-time error. Every declarator in a field declaration of an interface must have a variable initializer, or a compile-time error occurs. It is a compile-time error if the initializer of an interface annottations uses the simple name of annotationa same field or another field whose declaration occurs textually later in the same interface. This program causes two compile-time errors, because j is referred to in the initialization of f before j is declared, and because the annotayions of k refers to k itself.
It is permitted, but discouraged as a matter of style, to redundantly specify the public modifier for a method declaration in an interface.
A default method is a method that is declared in an interface with the default modifier; its body is always represented by a block. It provides a default implementation for any class that implements the interface without overriding the method. Annotatiosn interface can declare static methods, which are invoked without reference to a particular object.
It is a compile-time error to use the name of a annotstions parameter of any surrounding declaration in the header or body of a static method of an interface.
An interface method lacking a default modifier or a static modifier is implicitly abstractso its body is represented by a semicolon, not a block. It is permitted, but discouraged as a matter of style, to redundantly specify the abstract modifier for annotationz a method declaration. It is a compile-time error if the same keyword appears more than once as a modifier for a method declaration in an interface.
It is a compile-time error if a method is declared with more than one cc the modifiers abstractdefaultor static. It is a compile-time error if an abstract method declaration contains the keyword strictfp.
A method in an interface may be generic. An interface I inherits from its direct superinterfaces all abstract and default methods m for which all of the following are true:. There exists no method m ‘ that is a member of a direct superinterface, J ‘, of I m distinct from m ‘, J distinct from J ‘such that m ‘ overrides from J ‘ the declaration of the method m.
Note that methods are overridden on a signature-by-signature basis. The third clause above prevents a subinterface from re-inheriting a method that has already been overridden by another of its superinterfaces.
For example, in this program:. Right inherits name from Topbut Bottom inherits name from Leftnot Right. This is because name from Left overrides the declaration of name in Top. An interface does not inherit static methods from its superinterfaces. If an interface I declares a static method mand the signature of m is a subsignature of an instance method m ‘ in a superinterface of Iand m ‘ would otherwise be accessible to code in Ithen a compile-time error occurs.
In essence, a static method in an interface cannot “hide” an instance method in a superinterface. An instance method m 1declared in or inherited by an interface Ioverrides from I another instance method, m 2declared in interface Jiff both of the following are true:. I is a subinterface of J.
Chapter 9. Interfaces
The presence or absence of the strictfp modifier has absolutely no effect on the rules for overriding methods. For example, it is permitted for a method that is not FP-strict to override an FP-strict method and it is permitted for an FP-strict method to 9.4. a method that is not FP-strict. It is a compile-time error if a default method is override-equivalent with a non- private method of the class Objectbecause any class implementing the interface will inherit its own implementation of the method.
The prohibition against declaring one of the Annotatjons methods as a default method may be surprising. There are, after all, cases like java.
List in which the behavior of annotztions and equals are precisely defined. The motivation becomes clearer, however, when some broader design decisions are understood:. So, every implementing class would automatically override an interface’s toString default.
This is longstanding behavior in the Java programming language. It is not something we wish to change with the annotztions of default methods, because that would conflict with the goal of allowing interfaces to unobtrusively evolve, only providing default behavior when a class doesn’t already have it through the class hierarchy.