值未从 class 转移到另一个 JAVA

Question

我有 idS 方面的问题。在 sellContainer 中 idS 为 1，在 sellCtr 中始终为 0，因此 addItemToSell 方法不起作用。总是给空指针异常。

public class SellCtr{
    private SellContainer sellContainer;
    private OrderLineSell ols;

    private int idS;
    private int idOls;

    public SellCtr(){
       sellContainer = SellContainer.getInstance();
    }

    public void createSell(String date, Customer c, Employee e){
       Sell sell = new Sell(date,c,e);
       sellContainer.addSell(sell);
       idS = sell.getId();
    } 

    public void addItemToSell(OrderLineSell orderLineSell){
       sellContainer.findSell(idS).addOrderLineSell(orderLineSell);
    }

    public int getId(){
       return idS;
    }

    public int getIdOls(){
       return idOls;
    }

    public Sell findSell(int idS){
      return sellContainer.findSell(idS);
    }
}

另一个class是：

public class SellContainer{
    private ArrayList<Sell> sales;
    private static int idS=0;
    private static SellContainer instance=null;

    public SellContainer(){
        sales = new ArrayList<>();
    }

    public static SellContainer getInstance(){
        if(instance == null){
            instance = new SellContainer();
        }
        return instance;
    } 

    public void addSell(Sell s){
        idS++;
        s.setId(idS);
        sales.add(s);


    }

    public Sell findSell(int idS){
        for(Sell sell : sales){
            if(sell.getId()==idS){
                return sell;
            }
        }
        return null;
    }
}

这是错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at controlLayer.SellCtr.addItemToSell(SellCtr.java:24)
at tuiLayer.SaleAddItemToSaleGUI.actionPerformed(SaleAddItemToSaleGUI.java:93)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
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.access0(Unknown Source)
at java.awt.EventQueue.run(Unknown Source)
at java.awt.EventQueue.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.run(Unknown Source)
at java.awt.EventQueue.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain.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)

我已经重建了项目并且异常发生了变化。

编辑 3：我已将 idS 设为静态，现在可以使用了。非常感谢你们！

Answer 1

我建议你

确保您正在编译所有类。像 maven 或 gradle 这样的构建工具会有所帮助。
你对单例使用 enum。这些是线程安全且更简单的。
禁用 eclipse 中允许您运行无法编译的代码的功能。这只会延迟发现错误，使它们变得更糟。

我会替换

public class SellContainer{
    private ArrayList<Sell> sales;
    private static int idS=0;
    private static SellContainer instance=null;

    public SellContainer(){
        sales = new ArrayList<>();
    }

    public static SellContainer getInstance(){
        if(instance == null){
            instance = new SellContainer();
        }
        return instance;
    }

与

public enum SellContainer {
    INSTANCE;
    private final List<Sell> sales = new ArrayList<>();
    private int idS = 0;

Answer 2

在return对象返回被调用方法之前初始化对象。在 SellContainer 中，如果条件失败导致将 Sell Object 返回为 NULL。你需要修改这个逻辑。

return 空；

值未从 class 转移到另一个 JAVA

Value is not transfered from a class to another JAVA

java

nullpointerexception