[英]Implementing the observer pattern in Java, same observer across multiple classes
因此,我正在修改一個現有的開源項目,創建了一個新的監視器類,該類用於使用觀察者模式觀察現有類中的更改,該類本身正在由其他一些監視類進行監視。
在原始系統的主類中,我可以使用它,但是,當程序切換到其他類時,我試圖通過新類的構造函數(由包含發送的接口Manager表示)傳遞管理器。 ()函數,程序將使用該函數更新監視器。但是,當我嘗試從另一個類調用send時,它不起作用(它在名為Main的第一類中起作用)。
當我在此新類中聲明的“ MouseHandler”類中調用send()時,出現以下錯誤:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at kabalpackage.GameArea$MouseHandler.mouseReleased(GameArea.java:565)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
我嘗試在此MouseHandler類中聲明另一個Manager變量,並將其傳遞給構造函數,但遇到了相同的錯誤。
這是它工作的類,為簡單起見,我省略了很多代碼。
/*added necessary imports */
import java.util.*;
import monitors.Manager;
import monitors.MonManager;
public class Main
extends JFrame
{
// added manager variable
public static Manager manager;
public Main(Manager m)
{
// this is where I set the manager provided in the constructor
this.manager = m;
// ..... lots of code skipped
// I try and pass the manager in to the new class here
private GameArea gameArea = new GameArea(manager);
private class MenuBar
extends JMenuBar
{
//.....lots of code skipped
}
private class MenuListener
implements ActionListener
{
private MenuListener() {}
public void actionPerformed(ActionEvent e)
{
String event = e.getActionCommand();
if (event.equals("New Game"))
{
// here is where i call send on the monitor
System.out.println("Sending win 0");
manager.send("win 0");
Main.this.gameArea.newGame();
System.out.println("New game");
}
}
}
}
}
}
如您所見,在該類中聲明了一個新類。 這是從上面的類調用的GameArea類,您可以看到我修改了構造函數以采用一個管理器(接口)對象,因此可以將SAME監視器管理器傳遞給下一個類。 我再次從此類中省略了很多代碼,包括一些導入(它仍然具有import java.util。*;我認為這是發送事件所必需的)。
import monitors.MonManager;
import monitors.Manager;
public class GameArea
extends JPanel
{
public static Manager manager;
// loads of variables omitted //
// heres the constructor for the class, a manager is passed in and set to variable Manager
public GameArea(Manager m)
{
setLayout(null);
setBackground(this.BACKGROUND_COLOR);
setCursor(new Cursor(12));
this.manager = m;
loadImages();
}
// more code omitted //
public void newGame()
{
removeAll();
// this creates the mouse handler class where i try to call manager.send
MouseHandler mh = new MouseHandler();
}
// more code omitted //
public class MouseHandler
extends MouseInputAdapter
{
// variables omitted
private MouseHandler() {
}
public void mouseMoved(MouseEvent e)
{
//
}
public void mousePressed(MouseEvent e)
{
// omitted code //
}
public void mouseDragged(MouseEvent e)
{
// omitted code //
}
public void mouseClicked(MouseEvent e)
{
// omitted code
}
public void mouseReleased(MouseEvent e)
{
// more omitted code
if ((this.SRC_STACK instanceof DealtCardsStack)) {
GameArea.this.moves.clear();
// a call of manager
System.out.println("Sending moves");
manager.send("moves");
} else {
GameArea.this.moves.add(new Move(this.SRC_STACK, this.DST_STACK, this.TMP_LIST));
// Here is the call for the manager, which creates an error.
System.out.println("Sending moves");
manager.send("moves");
}
}
else
{
this.SRC_STACK.showCards(this.TMP_LIST);
}
// skipped some code//
}
}
最后,這是我的管理員界面中由監視器管理器實現的代碼:
package monitors;
public interface Manager {
public void send(String message);
}
如果問題不清楚,我們很抱歉。 有很多代碼可以省略,因此看起來很凌亂,但是我認為遺漏的任何內容都不會對錯誤產生影響。 因此,基本上在Main類中,我的消息被發送到實現Manager的監視器,但是在另一類中,它們不是,並且我也不知道為什么。
非常感謝有耐心閱讀的任何人,如果您能說出我的問題所在,也特別感謝!
字段初始化是在實際構造函數執行之前完成的,因此該行
private GameArea gameArea = new GameArea(manager);
正在傳遞未初始化的管理器,從而導致NullPointerException。
只需將初始化放入構造函數中即可: gameArea = new GameArea(manager);
解決了問題。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.