Java JPanel 不会滚动

Question

我正在尝试使用 java 中的 jPanel 和 jFrame 使图像可滚动。我在drawQuestion类中绘制图像，然后将drawQuestion添加到jScrollPanel并且它不滚动。请告诉我我的错误在哪里，我已经尝试了好几天了，但仍然找不到它。 抱歉我的英语不好。

main.java:

import java.awt.Dimension;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

public class main{

private static int width = 800;
private static int height = 450;

public static void main(String[] args){
    JFrame window = new JFrame("DPA Physics 2013 9 class");
    drawQuestion question = new drawQuestion();
    JScrollPane scroll = new JScrollPane(question);
    scroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setSize(width,height);
    window.setLocationRelativeTo(null);

    window.add(scroll);

    window.setVisible(true);
}
}

drawQuestion.java:

import java.awt.Graphics;
import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class drawQuestion extends JPanel{
private static final long serialVersionUID = 1L;

public void paintComponent(Graphics g){ 
    super.paintComponent(g);

    Image image1 = new ImageIcon(this.getClass().getResource("Page1.png")).getImage();

    g.drawImage(image1,0,0,this);
}
}

Answer 1

您尚未为 drawQuestion 面板指定(首选)尺寸，这意味着所有布局现在都认为它的尺寸为 0x0...

您永远不应该在任何 paint 方法中加载或执行任何可能需要少量时间才能完成的任务...

您需要重写getPreferredSize方法以返回所需的面板大小。这将允许滚动 Pane 确定面板是否需要滚动...

public class DrawQuestion extends JPanel{
    private static final long serialVersionUID = 1L;

    private Image image1;
    public DrawQuestion() {
        image1 = new ImageIcon(this.getClass().getResource("Page1.png")).getImage();
    }

    public Dimension getPreferredSize() {
        return image1 == null ? super.getPreferredSize() : new Dimension(image1.getWidth(this), image1.getHeight(this));
    }

    public void paintComponent(Graphics g){ 
        super.paintComponent(g);

        if (image1 != null) {
            g.drawImage(image1,0,0,this);
        }
    }
}

当我在墙上时...我还推荐使用 ImageIO 而不是 ImageIcon，因为如果图像 ImageIO 会抛出异常无法加载...