JavaJScrollPane不会在JPanel上滚动

Question

我有一个JFrame，在其中我在一个大网格中绘制了很多图像。我的窗口不够大，无法容纳整个网格，因此我想使用一个JScrollPane，它允许我在窗口中显示网格的某些部分

我的问题是我不能滚动。当我这样做时，滚动条总是出现，它们似乎已经覆盖了整个区域

如您所见，滚动条给人的印象是整个区域都显示在窗口中。然而，如果我调整它的大小，你会注意到根本不是这样的

滚动条继续显示它们覆盖了整个区域

（请注意，网格中的数字实际上是16×16像素的图像，但为了制作一个最小、完整且可验证的示例，我将图像替换为坐标字符串）

以下是我的代码，用于最小、完整和可验证的示例：

import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;


public class Main {
    public static int[][] tiles;
    public static void main(String[] args) {
        Foo foo = new Foo();
        foo.run();
    }
}

class Foo {
    public void run(){
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        Panel panel = new Panel();

        panel.setPreferredSize(new Dimension(512,448));
        JScrollPane scrollPane = new JScrollPane(panel);

        scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

        frame.add(scrollPane);
        frame.setVisible(true);
        frame.pack();

        Main.tiles = new int[32][32];

        while (true) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            panel.repaint();    
        }
    }

}

class Panel extends JPanel {

    public void paintComponent(Graphics g){ 
        paintScreen(g);
    }

    private void paintScreen(Graphics g){
        int x =0, y =0;
        for (int i = 0; i < Main.tiles.length; i++) {
            for (int j = 0; j < Main.tiles[i].length; j++){
                g.drawString(i + " " + j, x*32, y*32);
                y++;
            }
            y=0;
            x++;
        }
    }
}

所以我的问题是：

Q:为什么JScrollPane不允许我在JPanel上滚动

Answer 1

你的问题似乎就在这里：

panel.setPreferredSize(new Dimension(512,448));

将JScrollPane持有的JPanel约束为特定大小，不允许其扩展。永远不要这样做。而是约束JScrollPane或其视图端口

例如

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

public class Main {
   public static int[][] tiles;

   public static void main(String[] args) {
      Foo foo = new Foo();
      foo.run();
   }
}

class Foo {
   public void run() {
      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);

      Panel panel = new Panel();

      // !! panel.setPreferredSize(new Dimension(512,448));
      JScrollPane scrollPane = new JScrollPane(panel);
      scrollPane.getViewport().setPreferredSize(new Dimension(512, 448));

      scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
      scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

      frame.add(scrollPane);
      frame.setVisible(true);
      frame.pack();

      Main.tiles = new int[32][32];

      while (true) {
         try {
            Thread.sleep(500);
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
         panel.repaint();
      }
   }

}

class Panel extends JPanel {

   private static final int PREF_W = 1200;
   private static final int PREF_H = 1200;

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

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   private void paintScreen(Graphics g) {
      int x = 0, y = 0;
      for (int i = 0; i < Main.tiles.length; i++) {
         for (int j = 0; j < Main.tiles[i].length; j++) {
            g.drawString(i + " " + j, x * 32, y * 32);
            y++;
         }
         y = 0;
         x++;
      }
   }
}

---

编辑
尝试根据程序员的建议改进版本：

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Rectangle;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.Scrollable;

public class Main {
   public static int[][] tiles;

   public static void main(String[] args) {
      Foo foo = new Foo();
      foo.run();
   }
}

class Foo {
   public void run() {
      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);

      NumberPanel panel = new NumberPanel();

      // !! panel.setPreferredSize(new Dimension(512,448));
      JScrollPane scrollPane = new JScrollPane(panel);
      // !! scrollPane.getViewport().setPreferredSize(new Dimension(512, 448));

      scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
      scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

      frame.add(scrollPane);
      frame.setVisible(true);
      frame.pack();

      Main.tiles = new int[32][32];

      while (true) {
         try {
            Thread.sleep(500);
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
         panel.repaint();
      }
   }

}

class NumberPanel extends JPanel implements Scrollable {

   private static final int PREF_W = 1200;
   private static final int PREF_H = 1200;
   private static final int VP_WIDTH = 512;
   private static final int VP_HEIGHT = 448;

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

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   private void paintScreen(Graphics g) {
      int x = 0, y = 0;
      for (int i = 0; i < Main.tiles.length; i++) {
         for (int j = 0; j < Main.tiles[i].length; j++) {
            g.drawString(i + " " + j, x * 32, y * 32);
            y++;
         }
         y = 0;
         x++;
      }
   }

   @Override
   public Dimension getPreferredScrollableViewportSize() {
      return new Dimension(VP_WIDTH, VP_HEIGHT);
   }

   @Override
   public int getScrollableBlockIncrement(Rectangle arg0, int arg1, int arg2) {
      // TODO Consider improving
      return 0;
   }

   @Override
   public boolean getScrollableTracksViewportHeight() {
      return false;
   }

   @Override
   public boolean getScrollableTracksViewportWidth() {
      return false;
   }

   @Override
   public int getScrollableUnitIncrement(Rectangle arg0, int arg1, int arg2) {
      // TODO Consider improving
      return 0;
   }
}