方法，该方法返回导致ArrayList的NullPointerException

Question

我试图使用使用MVC原则的自定义类"Media“的ArrayList来填充一个ArrayList，但是当它调用在模型中创建列表的方法时，它会从我的View类中得到一个NullPointerException (虽然在实现DB之前我正在使用测试数据)来填充这个列表。

我尝试在不同的地方实例化我的ArrayList。如果我在模型中这样做(应该如何完成)并将该ArrayList返回给我的控制器，我可以从控制器中打印条目，但是将它传递给我的视图会抛出异常。如果我在控制器内创建ArrayList，我的视图就可以访问它了。我还尝试将对模型的引用保留为公共的(不建议使用和面向对象的透视图)，并试图从我的视图直接调用该方法以获得相同的结果。

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();
    }

}

视图类

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)

我本来希望能够将这个在我的模型上实例化的ArrayList传递给我的控制器，然后传递给我的视图，而不必将它保存为我的Controller上的类变量，并为它创建一个getter (这是我能想到的唯一其他可能的解决方案)。

Answer 1

您正在SearchMediaController构造函数中传递一个SearchMediaController实例，该实例未被完全初始化为SearchMediaView构造函数，后者试图在初始化media实例变量之前访问它。

变化

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

至

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