從Java中的JList獲取選定的值

Question

我如何從Jlist獲取選定的值？ 我嘗試了以下代碼，但是所有變量都顯示為空。 為什么索引變量為null？

 public class GetSelectedValueFromJList extends JFrame implements ActionListener {
    private JList list;
    private JButton checkButton;

    public GetSelectedValueFromJList() {



        String[] nameList = { "Value 1", "Value 2", "Value 3", "Value 4", "Value 5"};
        list = new JList(data);
        checkButton = new Button("Check");
        button.addActionListener(this);

        //add list to frame
        add(list);
        add(checkButton);

    }

    public void actionPerformed(ActionEvent e)
    {
        if(e.getCommand().equals("Check"))
        {
            int index = list.getSelectedIndex();
            System.out.println("Index Selected: " + index);
            String s = (String) list.getSelectedValue();
            System.out.println("Value Selected: " + s);
        }
    }

Answer 1

最初，在JList沒有選擇任何元素，因此，如果不從列表中選擇一個元素，則返回的索引將為-1，返回值將為null。 嘗試以下代碼，然后從列表中選擇和元素，然后測試其是否有效：

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;

public class Frame extends JFrame implements ActionListener
{
    private JList list;
    private JButton checkButton;

    public Frame()
    {
        setBounds(100,100,300,300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        String[] nameList = { "Value 1", "Value 2", "Value 3", "Value 4", "Value 5" };
        list = new JList(nameList);
        checkButton = new JButton("Check");
        checkButton.addActionListener(this);

        // add list to frame
        JPanel panel = new JPanel();
        panel.add(list);
        panel.add(checkButton);
        add(panel);
        setVisible(true);
    }

    public static void main(String[] args)
    {
        new Frame();
    }

    public void actionPerformed(ActionEvent e)
    {
        if (e.getActionCommand().equals("Check"))
        {
            int index = list.getSelectedIndex();
            System.out.println("Index Selected: " + index);
            String s = (String) list.getSelectedValue();
            System.out.println("Value Selected: " + s);
        }
    }
}

Answer 2

這不是答案，因為它不能真正解決所陳述的問題。 因此，我必須盡快將其刪除。 我將其發布，以顯示該代碼的輕微變體（作為MCVE ），以演示如果選擇了列表中的項目，則在輸出中沒有看到null 。 好吧，這是鼓勵您發布實際上顯示了所述問題的代碼的MCVE 。

import java.awt.BorderLayout;
import java.awt.event.*;
import javax.swing.*;

public class GetSelectedValueFromJList
        extends JFrame implements ActionListener {

    private JList list;
    private JButton button;

    public GetSelectedValueFromJList() {
        String[] nameList = {
            "Value 1", "Value 2", "Value 3", "Value 4", "Value 5"
        };
        list = new JList(nameList);
        list.setSelectedIndex(2);

        button = new JButton("Check");
        button.addActionListener(this);

        add(list);
        add(button, BorderLayout.PAGE_END);

        pack();
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("Check")) {
            int index = list.getSelectedIndex();
            System.out.println("Index Selected: " + index);
            String s = (String) list.getSelectedValue();
            System.out.println("Value Selected: " + s);
        }
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                new GetSelectedValueFromJList().setVisible(true);
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

Answer 3

此代碼將獲得多個選擇的值...

 int[] selectedIndices = jList1.getSelectedIndices();
String[] myArray = new String[50];
for (int i = 0; i < selectedIndices.length; i++) {
               myArray[i] =  String.valueOf(jList1.getModel().getElementAt(selectedIndices[i]));
        }

此代碼將獲得一個值...

String myString = String.valueOf(jList1.getModel().getElementAt(jList1.getSelectedIndex());

要么

 String myString = String.valueOf(jList1.getSelectedValue());