Java 简明教程

Java - Interfaces

Java Interfaces

Java 接口是抽象方法的集合。该接口用于实现 abstraction，在其中您可以定义方法，但没有其实现（没有方法的主体）。接口是一种引用类型，类似于 class。

Java interface is a collection of abstract methods. The interface is used to achieve abstraction in which you can define methods without their implementations (without having the body of the methods). An interface is a reference type and is similar to the class.

除了抽象方法，接口还可能包含常数、默认 methods、静态方法和嵌套类型。只有对默认方法和静态方法存在方法主体。

Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

编写接口类似于编写类。但是，类描述对象的 attributes 和行为。接口包含类实现的行为。除非实现该接口的类是抽象类，否则在类中需要定义接口的所有方法。

Writing an interface is similar to writing a class. However, a class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements. Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

Java Interfaces and Classes: Similarities and Differences

Similarities

接口在以下方面类似于类 -

An interface is similar to a class in the following ways −

  1. An interface can contain any number of methods.

  2. An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

  3. The byte code of an interface appears in a .class file.

  4. Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

Differences

然而，接口在以下几个方面与类不同，包括 -

However, an interface is different from a class in several ways, including −

  1. You cannot instantiate an interface.

  2. An interface does not contain any constructors.

  3. All of the methods in an interface are abstract.

  4. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

  5. An interface is not extended by a class; it is implemented by a class.

  6. An interface can extend multiple interfaces.

Declaring an Interface in Java

interface 关键字用于声明接口。以下是声明接口的一个简单示例 −

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example to Declare a Java Interface

以下是接口示例 −

Following is an example of an interface −

/* File name : NameOfInterface.java */
import java.lang.*;
// Any number of import statements

public interface NameOfInterface {
   // Any number of final, static fields
   // Any number of abstract method declarations\
}

Propeties of Java Interface

接口具有以下属性 −

Interfaces have the following properties −

  1. An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.

  2. Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.

  3. Methods in an interface are implicitly public.

Example

/* File name : Animal.java */
interface Animal {
   public void eat();
   public void travel();
}

Implementing Interfaces in Java

当一个类实现一个接口时，可以将该类视为在签署一份合约，同意执行接口的特定行为。如果一个类没有执行接口的所有行为，则该类必须声明自身为抽象的。

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

类使用 implements 关键字来实现接口。implements 关键字出现在类声明中，紧跟声明的 extends 部分。

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Example: Implement an Interface in Java

/* File name : MammalInt.java */
public class MammalInt implements Animal {

   public void eat() {
      System.out.println("Mammal eats");
   }

   public void travel() {
      System.out.println("Mammal travels");
   }

   public int noOfLegs() {
      return 0;
   }

   public static void main(String args[]) {
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
}
interface Animal {
   public void eat();
   public void travel();
}
Mammal eats
Mammal travels

Rules for defining overriding methods in Java Interfaces

当在接口中定义 overriding methods 时，需要遵循以下几条规则 −

When overriding methods defined in interfaces, there are several rules to be followed −

  1. Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

  2. The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

  3. An implementation class itself can be abstract and if so, interface methods need not be implemented.

Rules for implementing Java Interfaces

实现接口时，有以下几个规则 −

When implementation interfaces, there are several rules −

  1. A class can implement more than one interface at a time.

  2. A class can extend only one class, but implement many interfaces.

  3. An interface can extend another interface, in a similar way as a class can extend another class.

Extending Java Interfaces

接口可以扩展另一个接口，就像一个类可以扩展另一个类一样。*extends*关键字用于扩展接口，子接口继承父接口的方法。

An interface can extend another interface in the same way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

Example: How to Extend Java Interface

以下的 Sports 接口被 Hockey 和 Football 接口扩展。

The following Sports interface is extended by Hockey and Football interfaces.

// Filename: Sports.java
public interface Sports {
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
}

// Filename: Football.java
public interface Football extends Sports {
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
}

// Filename: Hockey.java
public interface Hockey extends Sports {
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

Hockey 接口有四个方法，但它从 Sports 继承了两个方法；因此，实现 Hockey 的类需要实现所有六个方法。类似地，实现 Football 的类需要定义 Football 中的三个方法和 Sports 中的两个方法。

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Example

interface Sports {
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
}

interface Football extends Sports {
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
}

interface Hockey extends Sports {
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

public class HockeyDemo implements Hockey {

   public void setHomeTeam(String name) {
      System.out.println("Home team: " + name);
   }

   public void setVisitingTeam(String name) {}

   public void homeGoalScored() {}

   public void visitingGoalScored() {}

   public void endOfPeriod(int period) {}

   public void overtimePeriod(int ot) {}

   public static void main(String[] args) {
      Hockey hockeyDemo = new HockeyDemo();
      hockeyDemo.setHomeTeam("India");
   }
}
Home team: India

Extending Multiple Java Interfaces

Java 类只能扩展一个父类。不允许多重继承。但是，接口不是类，并且一个接口可以扩展多个父接口。

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

extends 关键字只用一次，父接口在一个逗号分隔的列表中声明。

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

例如，如果 Hockey 接口同时扩展 Sports 和 Event，它将被声明为 −

For example, if the Hockey interface extended both Sports and Event, it would be declared as −

public interface Hockey extends Sports, Event

Example: How to Extend Multiple Java Interface

interface Sports {
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
}

interface Football extends Sports {
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
}

interface Hockey extends Sports {
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

interface Event {
   public void organize();
}
public class HockeyDemo implements Hockey, Event {

   public void setHomeTeam(String name) {
      System.out.println("Home team: " + name);
   }

   public void setVisitingTeam(String name) {}

   public void homeGoalScored() {}

   public void visitingGoalScored() {}

   public void endOfPeriod(int period) {}

   public void overtimePeriod(int ot) {}

   public static void main(String[] args) {
      HockeyDemo hockeyDemo = new HockeyDemo();
      hockeyDemo.setHomeTeam("India");
      hockeyDemo.organize();
   }

   public void organize() {
      System.out.println("Match organized. ");
   }
}
Home team: India
Match organized.

Tagging Java Interfaces

扩展接口最常见的用法是当父接口不包含任何方法时。例如，java.awt.event 包中的 MouseListenter 接口扩展了 java.util.EventListener，它被定义为 −

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as −

Example

package java.util;
public interface EventListener
{}

没有方法的接口被称为*tagging*接口。标记接口有两个基本设计目的 −

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces −

Creates a common parent − 与 EventListener 接口一样（Java API 中的其他数十个接口对其进行了扩展），您可以使用标记接口在接口组之间创建公共父接口。例如，当接口扩展 EventListener 时，JVM 知道该特定接口将在事件委托场景中使用。

Creates a common parent − As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class − 这是术语“标记”的由来。实现标记接口的类不需要定义任何方法（因为该接口没有任何方法），但该类通过多态性成为接口类型。

Adds a data type to a class − This situation is where the term, tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.