JComboBox错误

Question

我试图从“Java编程第9版简介”(第9版，由梁编写)中编译以下程序。关于JComboBox，下面的示例出错了：

import javax.swing.*;

public class GUIComponents
{
    public static void main (String[] args)
    {
        JButton jbtOK = new JButton ("OK");                                             // Creates a button with test OK
        JButton jbtCancel = new JButton ("Cancel");                                 // Creats a cancel button
        JLabel jlblName = new JLabel ("Enter your name: ");                 //  Creates a label with the respective text
        JTextField jtfName = new JTextField ("Type Name Here");     // Creates a text field with the respective text
        JCheckBox jchkBold = new JCheckBox ("Bold");                        // Creates a check boc wth the text bold
        JCheckBox jchkItalic = new JCheckBox ("Italic");
        JRadioButton jrbYellow = new JRadioButton ("Yellow");               // Creates a radio button with text Yellow
        JRadioButton jrbRed = new JRadioButton  ("Red");                        // Creates a radio Button with text Red
        **JComboBox jcboColor = new JComboBox (new String[] {"Freshman", "Sophomore", "Junior", "Senior"});**
        JPanel panel = new JPanel ();                                                           // Creates a panel to group components
        panel.add (jbtOK);                                                                          // Add the OK button to the panel
        panel.add (jbtCancel);                                                                      // Add the Cancel button to the panel
        panel.add (jlblName);                                                                       // Add the lable to the panel
        panel.add (jtfName);
        panel.add (jchkBold);
        panel.add (jchkItalic);
        panel.add (jrbRed);
        panel.add (jrbYellow);
        panel.add (jcboColor);

        JFrame frame = new JFrame ();
        frame.add (panel);
        frame.setTitle ("Show GUI Components");
        frame.setSize (450,100);
        frame.setLocation (200, 100);
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        frame.setVisible (true);
     }
}

正在产生的错误是：

warning: [unchecked] unchecked call to JComboBox(E[]) as  a member of the raw type JComboBox
    JcomboBox jcboColor = new JComboBox(new String[] {"Freshman", "Sophomore", "Junior", "Senior"});

Where E is a time-variable: 
    E extends Object Declared in class JComboBox

Answer 1

这是警告而不是错误。您缺少了JComboBox所期望的泛型类型，这是Java1.7中引入的。没有它，每次从ComboBoxModel检索值时都需要强制转换，将String类型添加到声明中，以匹配模型数据。

JComboBox<String> jcboColor = new JComboBox<>(new String[] { ... });

从Generics中阅读这篇有趣的文章什么是“未经检查”的警告？

Answer 2

到目前为止，Oracle在其Swing教程中提供了以下内容：

https://docs.oracle.com/javase/tutorial/uiswing/examples/layout/CardLayoutDemoProject/src/layout/CardLayoutDemo.java

运行此操作将导致以下行抛出此警告：

JComboBox cb = new JComboBox(comboBoxItems);

可能是因为代码自2008年以来就没有更新过？这个解决方案帮助我清理了警告并顺利编译，所以很有价值。

Answer 3

这是一种警告，而不是imageComboBox = new JComboBox( names );造成的错误。

再运行一次程序，它就能正常工作了。

下面是运行的映像