Question

单击我的提交按钮时，它会根据使用的字段验证输入字段，然后调用writeItemToFile（）方法。

private void btnSubmit_actionPerformed(ActionEvent e)
{
    if (validateInput()){
        if (enabledFields == "music")
            createMusicItem();
        else
            if (enabledFields == "product")
                createProductItem();}
    writeItemToFile();
}//btnSubmit_actionPerformed

writeItemToFile（）方法很小，坦率地说可以跳过，但是我的作业需要它。它调用Inventory类中的write（Item项）方法。它沿着代码顶部的项目声明发送。

 Item item;

问题是Item类是抽象的，所以当它被传递时它会给我一个NullPointerException。下面是我的写（项目项）方法，只是为了衡量。我的问题是，有没有办法实例化抽象类？我该如何避免这种方法。

public void write(Item item)
{
    invFilename = item.getFileName();
    File inventoryFile = new File(invFilename);

    try
    {
        invWriter = new FileWriter(inventoryFile, true);
    } // try

    catch (IOException e)
    {
        System.out.println("ERROR: File " + invFilename + "could not opened: "
                + e.getMessage());
    }//catch

    try
    {
        invWriter.write(item.getFileRecord());
    }//try

    catch (IOException e)
    {
        System.out.println("ERROR: Product " + item.getFileRecord()
                + "could not be written to file " + invFilename + ": "
                + e.getMessage());
    }//catch
}//write(Item item)

请保持温和。

错误讯息：

 Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Inventory.write(Inventory.java:13)
at AddItemFrame.writeItemToFile(AddItemFrame.java:575)
at AddItemFrame.btnSubmit_actionPerformed(AddItemFrame.java:500)
at AddItemFrame.access$1(AddItemFrame.java:492)
at AddItemFrame$3.actionPerformed(AddItemFrame.java:217)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

由NullPointerException方法触发的write可能是由null Item类型的item.getFileName();参数引起的，具体在：

null - ＆gt;调用Object new File(invFilename)的方法。

也可能是由于：

造成的

File - ＆gt;使用null参数初始化新item（如果null不是item.getFileName()，但Item是。）

......等等。

与String是抽象类的事实无关。

注意

正如评论中其他人所述，equals内容比较由==完成，而不是{{1}}运算符。

Answer 2

Item类是抽象的

这意味着你不能拥有这个类的对象。您需要扩展抽象类并传递新类的对象。

检查here：抽象类无法实例化，但可以进行子类化

Answer 3

“问题是Item类是抽象的，所以当它被传递时它会给我一个NullPointerException。”

摘要并不意味着null。抽象类不能直接实例化，但它的子类可以。因此，拥有诸如

之类的功能是完全合法的

public String getAbstrClassesToString(AnAbstractClass abstract_class)  {
   try  {
      return  abstract_class.toString();
   }  catch(NullPointerException npx)  {
      if(abstract_class == null)  {
         throw  new NullPointerException("abstract_class");
      }

      //Just in case the toString() throws an NPX
      throw  npx;
   }
}

现在你可以用

来调用它

getAbstrClassesToString(null);

这显然会产生NullPointerException，或，你可以用

来调用它

getAbstrClassesToString(new ConcreteSubClassOfAnAbstractClass());

这是完全合法的，显然不 null。

换句话说，您的NullPointerException问题与抽象无关。

更多信息：https://www.google.com/search?q=abstract+class+java+tutorials

Answer 4

必须实例化要传递的Item类型的对象。

正如其他人所指出的，你不能简单地通过以下方式创建它：

Item item = new Item(); // this does not work!

但是，您可以extend该类并创建所需的所有抽象方法，然后实例化新类。

class ItemB extends Item
{
  ...
}

然后你可以使用新类

ItemB item = new ItemB();

NullPointerException尝试调用Abstract类时 - Java

4 个答案: