創建在編譯時

Question

不知道我有一個抽象類的類的實例：

public abstract class Room { 

} 

和繼承的類未在編譯時已知的，如：

public class MagicRoom extends Room { 

    public MagicRoom(){ 
     System.out.println("Creating a MagicRoom."); 
    } 

    public String magic = ""; 
} 

或：

public class Dungeon extends Room { 

    public Dungeon(){ 
     System.out.println("Creating a Dungeon"); 
    } 

    public String keeper = ""; 
} 

我有一個類，我會從創建這些類的實例：

import java.lang.reflect.Constructor; 
import java.lang.reflect.InvocationTargetException; 

public class MazeGame { 

    public static Room makeRoom(Class roomClass) 
     throws IllegalArgumentException, InstantiationException, 
      IllegalAccessException, InvocationTargetException, 
      SecurityException, NoSuchMethodException{ 

     Constructor c = roomClass.getConstructor(); 
     return c.newInstance(); 

    } 

} 

makeRoom是我嘗試創建一個類，我不知道在編譯時間，但我不知道該怎麼把它作爲它的返回類型而不是Room。由於makeRoom返回房間我得到一個異常，如果我嘗試使用屬於繼承的類中的字段：

import java.lang.reflect.InvocationTargetException; 

public class FactoryTest { 

    public static void main(String[] args) 
     throws IllegalArgumentException, SecurityException, 
      InstantiationException, IllegalAccessException, 
      InvocationTargetException, NoSuchMethodException{ 

     MazeGame game = new MazeGame(); 

     Room magicRoom = MazeGame.makeRoom(MagicRoom.class); 

     /* 
     * Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
     * magic cannot be resolved or is not a field 
     */ 

     magicRoom.magic = "a"; 

    } 
} 

Answer 1

作出這樣的方法一般：

public static <T extends Room> T makeRoom(Class<T> roomClass) 
    throws IllegalArgumentException, InstantiationException, 
     IllegalAccessException, InvocationTargetException, 
     SecurityException, NoSuchMethodException{ 

    // This is enough, if you have 0-arg constructor in all your subclasses 
    return roomClass.newInstance(); 
} 

，然後調用它像：

MagicRoom magicRoom = MazeGame.makeRoom(MagicRoom.class); 

Answer 2

你必須投客房對象MagicRoom。

MagicRoom magicRoom = (MagicRoom) MazeGame.makeRoom(MagicRoom.class); 

另外，我知道這只是一個例子，但你應該使這些屬性爲私有的，並使用訪問器/增變器方法。

例如

public class MagicRoom extends Room { 

    public MagicRoom(){ 
    System.out.println("Creating a MagicRoom."); 
    } 

    private String magic = ""; 

    public String getMagic() { 
    return this.magic; 
    } 

    public void setMagic(String magic) { 
    this.magic = magic; 
    } 

}