Question

最近我在挥杆方面遇到了一些问题。我正在创建一个项目，该项目需要非常频繁地在JFrame和JPanel中编辑内容（在本例中为显示在JButton上的字符串），并且想掌握如何做到这一点。

我搜索了很长时间，发现的主要答案是，可能需要在调用.revalidate（）之后调用.repaint（）。但是，我一直无法使我的代码正常运行。

现在，框架将按原样绘制，但是按下按钮时，框架中的文本不会改变-实际上，它会产生较大的错误日志，可在此处查看：https://pastebin.com/7P85cB8h

下面是我的代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Actions extends JFrame implements ActionListener
{
    JButton Beans;
    String String1;
    JPanel things;

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

    public Actions()
    {
        JPanel things = new JPanel();
        String1 = "Beans";

        this.setSize(400,400);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("Hello there");

        Box theBox = Box.createHorizontalBox();

        Beans = new JButton("" + String1);
        Beans.addActionListener(this);

        theBox.add(Box.createHorizontalStrut(50));
        theBox.add(Beans);

        this.add(theBox);

        setVisible(true);
    }

    public void actionPerformed(ActionEvent e)
    {
        String1 = "Surprise!";
        things.revalidate();
        things.repaint();
    }
}

因此，为了澄清起见，我在JPanel内和JFrame内有一个JButton。该按钮在其内部显示一个字符串，该字符串最初为“ Beans”。当我按下按钮时，我希望字符串现在显示为“惊喜！”。

感谢您的宝贵时间。

Answer 1

您的问题是将对象与引用变量混淆的一种方式，认为更改String1的文本将神奇地导致JButton显示的文本发生更改，但这不是Java的OOP模型的工作原理。理解JButton正在显示一个String对象，该对象与String1最初引用的对象相同，但是当您更改String1引用的String时，这对原始String对象没有任何影响。为了更改显示的String，您必须通过调用JButton的setText(...)方法来更改JButton显示的String对象，并将新的String传递给它。这是唯一可行的方法。

public void actionPerformed(ActionEvent e) {
    Beans.setText("Surprise!");
}

查看评论：

// here are several reference variables
// all without assigned objects, and thus
// all holding "null" values:
JButton Beans;
String String1;
JPanel things;


public Actions()  {
    //..... 

    // here you assign the String object, "Beans" to the String1 variable
    String1 = "Beans";

    // .....

    // here you create a JButton and pass in String1's current object, "Beans"
    // into the constructor (note the "" + is NOT needed for Strings, only for numberrs)
    Beans = new JButton("" + String1);

    //.....
}

public void actionPerformed(ActionEvent e)  {
    // here you change the object that String1 refers to
    String1 = "Surprise!";

    // but this has no effect on the original String object, "Beans" displayed in the
    // JButton, but rather all it does is change the state of String1. 
    // To change the state of the JButton, you must explicitly do this 
    // by calling setText on it

    //....

顺便说一句，您将要学习和使用Java naming conventions。变量名都应以小写字母开头，而类名应以大写字母开头。学习并遵循此规则将使我们能够更好地理解您的代码，并使您能够更好地理解其他人的代码。

请注意第二点：如果您实际上绘制字符串，那么您的原始代码就可以使用。注意，在下面的代码中，我有一个String变量currentString，它最初引用String数组TEXTS中的第一项，即String "One"。在JButton的ActionListener中，我更新名为index的数组索引变量，并将currentString变量设置为数组中的下一个String项，然后调用repaint()。该代码起作用的原因是因为我正在JPanel的绘制方法currentString中绘制paintComponent(...)持有的文本：

import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

import javax.swing.*;

public class DrawStringPanel extends JPanel {
    private static final String[] TEXTS = {
            "One", "Two", "Three", "Four", "Five", 
            "Six", "Seven", "Eight", "Nine", "Ten"
            };
    private static final int PREF_W = 400;
    private static final int PREF_H = PREF_W;
    private static final Font TEXT_FONT = new Font(Font.SANS_SERIF, Font.BOLD, 40);
    private static final int TEXT_X = 150;
    private static final int TEXT_Y = 200;
    private int index = 0;

    // Note that this String variable holds the first item in the TEXTS array
    private String currentString = TEXTS[index];

    public DrawStringPanel() {
        setPreferredSize(new Dimension(PREF_W, PREF_H));
        JButton nextBtn = new JButton("Next");
        add(nextBtn);
        nextBtn.addActionListener(e -> {
            // update the array index
            index++;  // get next index
            index %= TEXTS.length;  // but don't let get bigger then array length

            // and in the ActionListener here I'm changing the variable and calling repaint
            // this works because this variable is actually painted within this JPanel's 
            // paintComponent method....
            currentString = TEXTS[index];
            repaint();
        });
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
        g2.setFont(TEXT_FONT);

        // ****** HERE ****** I draw the contents of the currentString variable
        g2.drawString(currentString, TEXT_X, TEXT_Y);
    }

    private static void createAndShowGui() {
        DrawStringPanel mainPanel = new DrawStringPanel();

        JFrame frame = new JFrame("DrawStringPanel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

通过EventListener调用repaint（）和/或revalidate（）-Swing

1 个答案: