java - NullPointerException 势不可挡

Question

我目前正在使用 vaadin 开发一个小型地址表，只是为了稍微了解一下 vaadin。我遇到了一个问题，我想手动对表中的条目进行排序。问题是，当我尝试执行此操作时，我收到 NullPointerException:

item.getItemProperty(Columns.NAME).getValue()

我在这里尝试了一些事情。我检查了 item 且 item 不为 null，item.getItemProperty(Columns.NAME) 也不为 null。因此，当我尝试创建 if 时，我仍然得到 NullPointerException

if(item.getItemProperty(Columns.NAME).getValue() == null){}

该值为空，我明白这一点，但我想将此值赋予其他项目，但我不知道如何做。

SEVERE: Servlet.service() for servlet [com.example.addresstable.AddresstableUI$Servlet] in context with path [/Addresstable] threw exception [com.vaadin.server.ServiceException: java.lang.NullPointerException] with root cause
java.lang.NullPointerException
  at com.vaadin.data.util.IndexedContainer$IndexedContainerProperty.getValue(IndexedContainer.java:898)
  at com.example.addresstable.AddressTable.addAddressToTable(AddressTable.java:114)
  at com.example.addresstable.AddresstableUI.init(AddresstableUI.java:26)
  at com.vaadin.ui.UI.doInit(UI.java:639)
  at com.vaadin.server.communication.UIInitHandler.getBrowserDetailsUI(UIInitHandler.java:222)
  at com.vaadin.server.communication.UIInitHandler.synchronizedHandleRequest(UIInitHandler.java:74)
  at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
  at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1402)
  at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:305)
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
  at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
  at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
  at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
  at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
  at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
  at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
  at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
  at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
  at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
  at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
  at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
  at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
  at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
  at java.lang.Thread.run(Unknown Source)

Answer 1

IndexedContainer 的 Item 是 IndexedContainerItem。
当调用它的 getItemProperty 时，结果永远不会为 null:

@Override
public Property getItemProperty(Object id) {
    return new IndexedContainerProperty(itemId, id);
}

但是当你调用Property的getValue时，如果没有itemId或propertyId，它可能为null:

@Override
public T getValue() {
    return (T) items.get(itemId).get(propertyId);
}

可能是 Vaadin 错误:link

编辑

如果您想检查容器中是否包含该项目，您可以执行以下操作:

public class MyIndexedContainer extends IndexedContainer {
    @Override
    public Item getUnfilteredItem(Object itemId) { // make it public
      return super.getUnfilteredItem(itemId);
    }
}

然后

Table table = new Table();
MyIndexedContainer mic = new MyIndexedContainer();
table.setContainerDataSource(mic);
//...
// check that the container contains the item
if (mic.getUnfilteredItem(item) != null) {
    //...
}