Java 方法，该方法返回导致NullPointerException的ArrayList
我试图使用MVC原则使用自定义类“Media”的ArrayList填充JTable，但是当视图类调用在模型中创建列表的方法时，我从视图类中得到了一个NullPointerException（该方法应该用数据库中的数据填充此列表，尽管我在实现DB之前使用了测试数据） 我尝试在不同的地方实例化我的ArrayList。如果我在模型内部执行该操作（应该如何执行）并将该ArrayList返回到我的控制器，我可以从控制器打印项目，但将其传递到我的视图会引发异常。如果我在控制器内创建ArrayList，我的视图可以很好地访问它。我还尝试将对模型的引用作为公共的（从面向对象的角度来看是不可取的），并尝试从我的角度直接调用该方法以获得相同的结果 模范班Java 方法，该方法返回导致NullPointerException的ArrayList,java,model-view-controller,arraylist,Java,Model View Controller,Arraylist,我试图使用MVC原则使用自定义类“Media”的ArrayList填充JTable，但是当视图类调用在模型中创建列表的方法时，我从视图类中得到了一个NullPointerException（该方法应该用数据库中的数据填充此列表，尽管我在实现DB之前使用了测试数据） 我尝试在不同的地方实例化我的ArrayList。如果我在模型内部执行该操作（应该如何执行）并将该ArrayList返回到我的控制器，我可以从控制器打印项目，但将其传递到我的视图会引发异常。如果我在控制器内创建ArrayList，我的视
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;
}
}
公共类SearchMediaModel{
//默认构造函数
公共SearchMediaModel（）{}
//返回媒体对象（测试数据）的ArrayList
公共ArrayList getTitlesFromDB（）{
ArrayList=新建ArrayList（）；
媒体测试=新电视盒（“权力游戏”）；
媒体测试2=新电视盒（“卡片屋”）；
媒体测试3=新电视盒（“女高音”）；
list.add（test3）；
添加（test2）；
列表。添加（测试）；
退货清单；
}
}
//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();
}
}
//将控制器添加到将列出数据库中所有标题的视图
公共类SearchMediaController{
私有搜索MediaView视图；
公共搜索媒体模型；
//构造函数实例化视图类和模型类，并将它们保存在
//类变量
公共搜索MediaController（）{
this.view=新的SearchMediaView（this）；
this.model=新的SearchMediaModel（）；
}
//从模型返回媒体的数组列表
//如果我试着在这里打印列表中的项目，它会起作用
公共阵列列表getMediaList（）{
返回model.getTitlesFromDB（）；
}
}
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();
公共类SearchMediaView扩展JFrame{
专用搜索媒体控制器；
公共SearchMediaView（SearchMediaController控制器）{
this.controller=控制器；
//此代码行中发生异常
ArrayList listForTheTable=controller.getMediaList（）；
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)
public SearchMediaController() {
this.view = new SearchMediaView(this);
this.model = new SearchMediaModel();
}
public SearchMediaController() {
this.model = new SearchMediaModel();
this.view = new SearchMediaView(this);
}