我的代码中的java.lang.NullPointerException - 包装类

时间:2013-09-26 00:47:02

标签: java nullpointerexception

我有4节课。我们来说类:Graphics,Manager,DataInputOutput和Files。 在Graphics中我有一个方法调用Manager中的另一个方法,该方法将调用DataInputOutput中的一个方法,该方法将调用Files中的方法。

但是我总是在方法getCars()上得到java.lang.NullPointerException异常,我得到它(异常)类DataInputOutPut,Manager和Graphics我知道某些东西没有初始化或指向null但我只是不要到达目的地。有人可以帮助我吗?

呼叫流程如下:Graphics --->经理---> DataOutputInput --->文件

以下是与错误相关的代码:

Classe Graphics:

public class Graphics extends JFrame {

private Manager manager;

public Graphics() {
manager = new Manager();


(..)

tablePanel.setData((manager.getCars()));

}

Classe Manager;

public class Manager {

DataInputOutput io = new DataInputOutput();

    public List<Car> getCars()
    {
        return io.getCars();
    }
}

Classe DataInputOutput:

public class DataInputOutput{

private Database database;
private Files files;

public DataInputOutput()
{
    database = Database.getInstance();
    Files files = new Files();
}

     public List<Car> getCars()
     {
         return files.getCars();
     }
}

班级档案:

public class Files {

private List<Car> carList;

public Files(){
    carList = new ArrayList<Car>();
}


public void addCar(Car sta) {
        carList.add(sta);
}

public List<Car> getCars() {
        return Collections.unmodifiableList(carList);
}

这是堆栈打印:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at controller.DataInputOutput.getCars(DataInputOutput.java:59)
at controller.Manager.getCars(Manager.java:41)
at view.Graphics.<init>(Graphics.java:125)
at view.App$1.run(App.java:27)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

1 个答案:

答案 0 :(得分:4)

public DataInputOutput()
{
    database = Database.getInstance();
    Files files = new Files();
}

Files files = new Files();

未设置this.files

它定义了一个局部变量,this.files的值为null