Question

我正在尝试使用MVC原理使用自定义类“ Media”的ArrayList填充JTable，但是当我调用其在模型中创建列表的方法时，我正在从View类中获取NullPointerException。应该使用数据库中的数据填充此列表，尽管我在实现DB之前正在使用测试数据。

我尝试在不同的地方实例化ArrayList。如果我在模型中执行此操作（应该如何完成）并将该ArrayList返回给控制器，则可以从控制器打印项目，但将其传递给View会引发异常。如果我在控制器内部创建ArrayList，则我的View可以正常访问它。我还尝试将对模型的引用保留为公共（未从面向对象的视角进行修改），并尝试直接从视图中调用该方法以得到相同的结果。

模型类

public class SearchMediaModel {

    //Default constructor
    public SearchMediaModel() { }

    //returns an ArrayList of Media Objects (test data)
    public ArrayList<Media> getTitlesFromDB() {
        ArrayList<Media> list = new ArrayList<>();
        Media test = new TvBox("Game of Thrones");
        Media test2 = new TvBox("House of Cards");
        Media test3 = new TvBox("The Sopranos");
        list.add(test3);
        list.add(test2);
        list.add(test);
        return list;

    }

}

控制器类

//controller to the view that will list all the titles in the DB
public class SearchMediaController {

    private SearchMediaView view;
    public SearchMediaModel model;

    //Constructor instantiates both View and Model classes and saves them in the
    //class variables
    public SearchMediaController() {
        this.view = new SearchMediaView(this);
        this.model = new SearchMediaModel();
    }


    //returns an Array List of Medias from the model
    //If I try printing the items from the list here, it works
    public ArrayList<Media> getMediaList() {
        return model.getTitlesFromDB();
    }

}

查看课程

public class SearchMediaView extends JFrame{

    private SearchMediaController controller;

    public SearchMediaView(SearchMediaController controller) {
        this.controller = controller;

//exception happening in this line of code
        ArrayList<Media> listForTheTable= controller.getMediaList();

堆栈跟踪

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at searchcustomer.SearchMediaController.getMediaList(SearchMediaController.java:29)
    at searchcustomer.SearchMediaView.<init>(SearchMediaView.java:22)
    at searchcustomer.SearchMediaController.<init>(SearchMediaController.java:21)
    at frontPage.FrontPageController.actionPerformed(FrontPageController.java:95)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6632)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6397)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5008)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4840)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2772)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4840)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

我希望能够将在我的Model上实例化的ArrayList传递给Controller，然后传递给View，而不必将其另存为类变量到Controller上并为其创建一个getter（这是唯一可能的方法）我能想到的解决方案）。

Answer 1

您正在向SearchMediaController构造函数传递一个SearchMediaController实例，该实例尚未完全初始化为SearchMediaView构造函数，该构造函数试图在实例变量media之前被访问初始化。

更改

public SearchMediaController() {
    this.view = new SearchMediaView(this);
    this.model = new SearchMediaModel();
}

到

public SearchMediaController() {
    this.model = new SearchMediaModel();
    this.view = new SearchMediaView(this);
}

返回导致NullPointerException的ArrayList的方法

1 个答案: