Decorator模式

最新推荐文章于 2025-08-15 14:42:50 发布

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最新推荐文章于 2025-08-15 14:42:50 发布

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分类专栏： Java 设计模式文章标签： decorator string video class list

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本文介绍了装饰模式，通过Component给出抽象接口，Concrete Component定义接收附加责任的类，Decorator持有构件实例，Concrete Decorator添加附加责任。并以图书馆的图书与录像带添加“可借阅”装饰为例进行演示，还给出了Component类设计及类结构简化的建议。

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Component 给出一个抽象接口，以规范准备接收附加责任的对象。
Concrete Component 定义一个将要接收附加责任的类。
Decorator 持有一个构件（Component）对象的实例，并定义一个与抽象构件接口一致的接口。
Concrete Decorator 负责给构件对象"贴上"附加的责任。

该例子演示了通过装饰模式为图书馆的图书与录像带添加"可借阅"装饰。

// Component
abstract class LibraryItem {
private int numCopies;

public int getNumCopies() {
return numCopies;
}

public void setNumCopies(int numCopies) {
this.numCopies = numCopies;
}

public abstract void Display();
}

// ConcreteComponent
class Book extends LibraryItem {
private string author;
private string title;

public Book(string author,string title,int numCopies) {
    this.author = author;
    this.title = title;
    this.numCopies = numCopies;
}

public void Display() {
    System.out.println( " Book ------ " );
    System.out.println( " Author: ", author );
    System.out.println( " Title: " + title );
    System.out.println( " # Copies: " + NumCopies );
}
}

// ConcreteComponent
class Video extends LibraryItem {
private string director;
private string title;
private int playTime;

public Video( string director, string title,
    int numCopies, int playTime ) {
    this.director = director;
    this.title = title;
    this.NumCopies = numCopies;
    this.playTime = playTime;
}

public void Display() {
    System.out.println( " Video ----- " );
    System.out.println( " Director: " + director );
    System.out.println( " Title: " + title );
    System.out.println( " # Copies: " + NumCopies );
    System.out.println( " Playtime: " + playTime );
}
}

// Decorator
abstract class Decorator extends LibraryItem {
protected LibraryItem libraryItem;

public Decorator ( LibraryItem libraryItem ) {
this.libraryItem = libraryItem;
}

public void Display() {
libraryItem.Display();
}
}

// ConcreteDecorator
class Borrowable extends Decorator {
protected ArrayList<String> borrowers = new ArrayList><String>();

public Borrowable( LibraryItem libraryItem ) {
super( libraryItem );
}

public void BorrowItem( string name ) {
borrowers.Add( name );
libraryItem.numCopies--;
}

public void ReturnItem( string name ) {
borrowers.Remove( name );
libraryItem.NumCopies++;
}

public void Display() {
    super.Display();
    for( string borrower in borrowers )
      System.out.println( " borrower: " + borrower );
}
}

public class DecoratorApp {
public static void Main( string[] args ) {
    // Create book and video and display
    Book book = new Book( "Schnell", "My Home", 10 );
    Video video = new Video( "Spielberg", "Schindler's list", 23, 60 );
    book.Display();
    video.Display();

    // Make video borrowable, then borrow and display
    System.out.println( " Video made borrowable:" );
    Borrowable borrowvideo = new Borrowable( video );
    borrowvideo.BorrowItem( "Cindy Lopez" );
    borrowvideo.BorrowItem( "Samuel King" );

borrowvideo.Display();
}
}

尽量保持Component作为一个"轻"类，不要把太多的逻辑和状态放在Component类里。

如果只有一个ConcreteComponent类而没有抽象的Component类（接口），那么Decorator类经常可以是ConcreteComponent的一个子类。

如果只有一个ConcreteDecorator类，那么就没有必要建立一个单独的Decorator类，而可以把Decorator和ConcreteDecorator的责任合并成一个类。