Prototype模式的重點在於clone()，它負責複製物件本身並傳回，但這個clone()本身在實作上存在一些困難，尤其是當物件本身又繼承另一個物件時，如何確保複製的物件完整無誤，在不同的程式語言中有不同的作法。

在Java中的作法是透過實作一個Cloneable介面，它只是一個聲明的介面，並無規定任何實作的方法，您的目的是改寫Object的clone ()方法，使其具備有複製物件的功能，這個方面建議您參考：How to avoid traps and correctly override methods from java.lang.Object。

用一個簡單的例子來實作上圖中的結構，這個例子利用了Java語言本身的clone特性：

• AbstractFurniture.java

public abstract class AbstractFurniture 
                             implements Cloneable {
    public abstract void draw();
    
    // 在Design Pattern上，以下的clone是抽象未實作的
    // 實際上在Java中class都繼承自Object
    // 所以在這邊我們直接重新定義clone() 
    // 這是為了符合Java現行的clone機制
    protected Object clone() throws CloneNotSupportedException { 
        return super.clone(); 
    }
}

• CircleTable.java

import java.awt.*;

public class CircleTable extends AbstractFurniture {
    protected Point center;    
    
    public void setCenter(Point center) {
        this.center = center;
    }
    
    protected Object clone () 
                     throws CloneNotSupportedException { 
        Object o = super.clone(); 
        if(this.center != null) {
            ((CircleTable) o).center = (Point) center.clone();
        }
        
        return o; 
    } 

    public void draw() { 
        System.out.println("\t圓桌\t中心：(" + center.getX() 
                            + ", " + center.getY()+ ")");
    } 
}

• SquareTable.java

import java.awt.*;

public class SquareTable extends AbstractFurniture {
    protected Rectangle rectangle;    
    
    public void setRectangle(Rectangle rectangle) {
        this.rectangle = rectangle;
    }
    
    protected Object clone () 
                      throws CloneNotSupportedException { 
        Object o = super.clone(); 
        if(this.rectangle != null) { 
            ((SquareTable) o).rectangle = (Rectangle) rectangle.clone();
        }
        
        return o; 
    } 

    public void draw() { 
        System.out.print("\t方桌\t位置：(" + rectangle.getX() 
                            + ", " + rectangle.getY()+ ")");
        System.out.println(" / 寬高：(" + 
                         rectangle.getWidth() 
                + ", " + rectangle.getHeight()+ ")");
    }
}

• House.java

import java.util.*;

public class House { 
    private Vector vector;

    public House() { 
        vector = new Vector(); 
    }

    public void addFurniture(AbstractFurniture furniture) { 
        vector.addElement(furniture); 
        
        System.out.println("現有家具....");
        
        Enumeration enumeration = vector.elements();
        while(enumeration.hasMoreElements()) { 
             AbstractFurniture f = 
                 (AbstractFurniture) enumeration.nextElement(); 
             f.draw(); 
        } 
        System.out.println(); 
    } 
}

• Application.java

import java.awt.*;

public class Application {
    private AbstractFurniture circleTablePrototype;
    
    public void setCircleTablePrototype(
                   AbstractFurniture circleTablePrototype) {
        this.circleTablePrototype = circleTablePrototype;
    }
    
    public void runAppExample() throws Exception {
        House house = new House(); 
        CircleTable circleTable = null;

        // 從工具列選擇一個家具加入房子中
        circleTable =
            (CircleTable) circleTablePrototype.clone();
        circleTable.setCenter(new Point(10, 10));
        house.addFurniture(circleTable); 
        
        // 從工具列選擇一個家具加入房子中
        circleTable = 
            (CircleTable) circleTablePrototype.clone();
        circleTable.setCenter(new Point(20, 30));
        house.addFurniture(circleTable); 
    }
    
    public static void main(String[] args) throws Exception {
        Application application = new Application();
        application.setCircleTablePrototype(
                            new CircleTable());
        application.runAppExample();
    }
}

在 Gof 的設計模式書中給出一個原型模式的應用：一個通用的圖型編輯器 Framework。在這個 Framework中有一個工具列，您可以在上面選擇音樂符號以加入樂譜中，並可以隨時調整音樂符號的位置等等。

圖型編輯器Framework是通用的，然而它並不知道這些音樂符號的型態，有人或許會想到繼承圖型編輯器Framework來為每個音樂符號設計一個框架子類別，但由於音樂符號的種類很多，這會產生相當多的子類別，為了避免這種情況，可以透過Prototype模式來減少子類別的數目，可以設計出以下的結構：

• MazeDirector.java

public class MazeDirector {
    private int[][] maze;
    private IMazeBuilder mazeBuilder;

    public void setMaze(int[][] maze) {
        this.maze = maze;
    }
    
    public void setMazeBuilder(IMazeBuilder mazeBuilder) {
        this.mazeBuilder = mazeBuilder;
    }
    
    public void buildMaze() {
        for(int i = 0; i < maze.length; i++) {
            for(int j = 0; j < maze[i].length; j++) {
                // 由於mazeBuilder是IMazeBuilder型態
                // 所以無論Builder實例為何，這邊的程式都無需變動
                switch (maze[i][j]) {
                    case 0:
                        mazeBuilder.createRoadBlock();
                        break;
                    case 1:
                        mazeBuilder.createWallBlock();
                        break;
                    case 2:
                        mazeBuilder.createTreasureBlock();
                        break;
                    default:
                        System.out.println("undefined");
                }
            }
            mazeBuilder.nextRow();    
        }
    }
} 

• IMazeBuilder.java

public interface IMazeBuilder {
    public void createRoadBlock();
    public void createWallBlock();
    public void createTreasureBlock();
    public void nextRow();
} 

• SoliderMazeBuilder.java

public class SolidMazeBuilder implements IMazeBuilder {
    public void createWallBlock() {
        System.out.print("█");
    }
    
    public void createRoadBlock() {
        System.out.print("　");
    }
    
    public void createTreasureBlock() {
        System.out.print("$ ");
    }
    
    public void nextRow() {
        System.out.println();
    }
} 

• DiamondMazeBuilder.java

public class DiamondMazeBuilder implements IMazeBuilder {
    public void createWallBlock() {
        System.out.print("◇");
    }
    
    public void createRoadBlock() {
        System.out.print("　");
    }
    
    public void createTreasureBlock() {
        System.out.print("* ");
    }
    
    public void nextRow() {
        System.out.println();
    }    
}  

public class Main {
    public static void main(String[] args) {
        int[][] maze = {{1, 1, 1, 1, 1, 1, 1}, 
                        {1, 0, 0, 0, 0, 2, 1}, 
                        {1, 0, 1, 0, 1, 0, 1}, 
                        {1, 0, 2, 1, 0, 1, 1}, 
                        {1, 1, 0, 1, 0, 1, 1}, 
                        {1, 0, 0, 2, 0, 0, 1}, 
                        {1, 1, 1, 1, 1, 1, 1}};
        
        MazeDirector mazeDirector = new MazeDirector();
        mazeDirector.setMaze(maze);
        
        System.out.println("Build SolidMaze....");
        mazeDirector.setMazeBuilder(new SolidMazeBuilder());
        mazeDirector.buildMaze();
        
        System.out.println("Build DiamondMaze....");
        
        mazeDirector.setMazeBuilder(
                       new DiamondMazeBuilder());
        mazeDirector.buildMaze();
    }
} 

• AbstractEditor.java

public abstract class AbstractEditor {
    private IDocument document;
    public abstract IDocument createDocument();

    public void newDocument() { 
        document = createDocument(); 
        document.open();
    }

    public void saveDocument() { 
        if(document != null) 
            document.save(); 
    }

    public void closeDocument() { 
        if(document != null) 
            document.close(); 
    }
} 

• IDocument.java

public interface IDocument {
    public void open();
    public void save();
    public void close();
} 

• RTFEditor.java

public class RTFEditor extends AbstractEditor { 
    public IDocument createDocument() { 
        return new RTFDocument(); 
    } 
}                

• RTFDocument.java

public class RTFDocument implements IDocument {
    public RTFDocument() { 
        System.out.println("建立RTF文件"); 
    }

    public void open() { 
        System.out.println("開啟文件"); 
    }

    public void save() { 
        System.out.println("儲存文件"); 
    }

    public void close() { 
        System.out.println("關閉文件"); 
    }
}

• CustomDialog.java

public class CustomDialog {
    private IButton button;
    private ITextField textField;
    
    public void setButton(IButton button) {
        this.button = button;    
    }
    
    public void setTextField(ITextField textField) {
        this.textField = textField;
    }

    public void layoutAllComponents() {
        // ....
    }
    
    public void showDialog() {
        this.paintDialog();
        button.paintButton();
        textField.paintTextField();
    }
  
    public void paintDialog() {
        System.out.println("custom dialog paints....");
    }
}

• CustomDialog.java

public class CustomDialog {
    private IButton button;
    private ITextField textField;
    
    public CustomDialog(IWidgetFactory widgetFactory) {
        setWidgetFactory(widgetFactory);
    }
    
    // 由於客戶端只依賴於抽象的工廠，工廠如何實作並無關客戶端的事
    // 要抽換工廠並不需要改動客戶端的程式
    public void setWidgetFactory(IWidgetFactory widgetFactory) {
        setButton(widgetFactory.getButton());
        setTextField(widgetFactory.getTextField());
     
    }

    public void layoutAllComponents() {
        // layout all components
    }
    
    // 這邊也是依賴抽象，實際改變了元件實例
    // 客戶端代碼也不用更改
    public void setButton(IButton button) {
        this.button = button;    
    }
    
    public void setTextField(ITextField textField) {
        this.textField = textField;
    }
    
    public void showDialog() {
        this.paintDialog();
        button.paintButton();
        textField.paintTextField();
    }
  
    public void paintDialog() {
        System.out.println("custom dialog paints....");
    }
} 

• IButton.java

public interface IButton {
    public void paintButton();
} 

• ITextField.java

public interface ITextField {
    public void paintTextField();
} 

• IWidgetFactory.java

public interface IWidgetFactory {
    public IButton getButton();
    public ITextField getTextField();
} 

• MacButton.java

public class MacButton implements IButton {
    public void paintButton() {
        System.out.println("Mac button paints....");
    }
} 

• WindowsButton.java

public class WindowsButton implements IButton {
    public void paintButton() {
        System.out.println("Windows button paints....");
    }
} 

• MacTextField.java

public class MacTextField implements ITextField {
    public void paintTextField() {
        System.out.println("Mac textField paints....");
    }
} 

• WindowsTextField.java

public class WindowsTextField implements ITextField {
    public void paintTextField() {
        System.out.println("Windows textField paints....");
    }
} 

• MacWidgetFactory.java

public class MacWidgetFactory implements IWidgetFactory {
    public IButton getButton() {
        return new MacButton();
    }
    
    public ITextField getTextField() {
        return new MacTextField();
    }
} 

• WindowsWidgetFactory.java

public class WindowsWidgetFactory 
                          implements IWidgetFactory {
    public IButton getButton() {
        return new WindowsButton();
    }
    
    public ITextField getTextField() {
        return new WindowsTextField();
    }
}

• IMusicBox.java

public interface IMusicBox {
    public void play();
}

• PianoBox.java

public class PianoBox implements IMusicBox {
    public void play() {
        System.out.println("撥放鋼琴音樂:)");
    }
}

• ViolinBox.java

public class ViolinBox implements IMusicBox {
    public void play() {
        System.out.println("撥放小提琴音樂^_^");
    }
}

• MusicBoxFactory.java

public class MusicBoxFactory {
    public static IMusicBox createMusicBox(String name) 
                               throws InstantiationException, 
                                      IllegalAccessException, 
                                      ClassNotFoundException {
        // 這邊使用的是Java的Reflection機制來產生實例
        // 不過客戶端不用管啦
        // 以後就算改變了這邊的程式，客戶端程式是不用更改的
        return (IMusicBox) Class.forName(name).newInstance();
    }
}

• MusicBoxDemo.java

public class MusicBoxDemo {
    public static void main(String[] args) throws Exception {
        playMusicBox(MusicBoxFactory.createMusicBox("PianoBox"));
        playMusicBox(MusicBoxFactory.createMusicBox("ViolinBox"));
    }
    
    public static void playMusicBox(IMusicBox musicBox) {
        musicBox.play();
    }
}