调整JPanel大小后，组件消失
我试图创建JPanel与可拖动的十字架，鼠标点击后出现。 一切正常，但是当我调整JPanel的大小时，十字架就会消失。 我试图覆盖JPanel中的paintComponent方法，但是所有的十字都在坐标（0,0）处。 我该如何解决？
import java.awt.*; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import java.awt.event.MouseListener; import java.awt.event.MouseMotionAdapter; import java.util.ArrayList; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JPanel; public class CrossPanel extends JPanel implements MouseListener { private int orderOfCross = 0; private ArrayList crosses; private int defaultSizeOfCrosses = 10; CrossPanel() { setOpaque(false); addMouseListener(this); crosses = new ArrayList(); } @Override public void mouseClicked(MouseEvent e) { int x = e.getX(); int y = e.getY(); Cross cross = new Cross(orderOfCross++, defaultSizeOfCrosses); crosses.add(cross); cross.setLocation(x - defaultSizeOfCrosses, y - defaultSizeOfCrosses); add(cross); repaint(); } @Override public void paintComponent(Graphics g) { super.paintComponent(g); // for (int i = 0; i < crosses.size(); i++) { // crosses.get(i).paint(g); // } } @Override public void mousePressed(MouseEvent e) {} @Override public void mouseReleased(MouseEvent e) {} @Override public void mouseEntered(MouseEvent e) {} @Override public void mouseExited(MouseEvent e) {} public static void main(String[] args) { JFrame f = new JFrame(); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); CrossPanel crossPane = new CrossPanel(); f.getContentPane().add(crossPane); f.setSize(600, 500); f.setLocation(200, 200); f.setVisible(true); } } class Cross extends JComponent { private int order; protected Cursor draggingCursor = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR); private volatile int draggedAtX, draggedAtY; int size; public Cross(int order, int size) { this.order = order; this.size = size; this.setBounds(0, 0, 4 * size, 3 * size + 10); addDragListeners(); setCursor(draggingCursor); } @Override protected void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g; g2.setColor(Color.red); g2.setStroke(new BasicStroke(3)); g2.drawLine(0, size, size + size, size); g2.drawLine(size, 0, size, size + size); Font f = new Font("Monospaced", Font.BOLD, size + 10); g2.setFont(f); g2.drawString(String.valueOf(order), size - size / 2, 3 * size + 10); } private void addDragListeners() { addMouseListener(new MouseAdapter() { @Override public void mousePressed(MouseEvent e) { draggedAtX = e.getX(); draggedAtY = e.getY(); } }); addMouseMotionListener(new MouseMotionAdapter() { @Override public void mouseDragged(MouseEvent e) { Point newLocation = new Point(e.getX() - draggedAtX + getLocation().x, e.getY() - draggedAtY + getLocation().y); setLocation(newLocation); } }); } } 我很少看到使用null布局，因为所有感知的好处，只有许多缺点。
整个Swing API都是围绕布局管理器的使用而设计的，所以你会疯狂（恕我直言）简单地抛弃所有的工作。
如果您发现自己处于可用的布局管理器似乎没有按照您的意愿行事的位置，那么编写自己的布局管理器可能更有价值。
在这里，我提出了一个PropertionalLayoutManager其目的是提供布局function，将基于父组件的宽度/高度的百分比将组件放置在容器上。 这意味着，随着父组件的大小调整，子组件将以父级大小的百分比重新定位自己。
import java.awt.BasicStroke; import java.awt.Color; import java.awt.Component; import java.awt.Container; import java.awt.Cursor; import java.awt.Dimension; import java.awt.Font; import java.awt.FontMetrics; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.LayoutManager2; import java.awt.Point; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import java.awt.event.MouseListener; import java.awt.event.MouseMotionAdapter; import java.text.NumberFormat; import java.util.ArrayList; import java.util.HashMap; import java.util.Map; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JPanel; public class CrossPanel extends JPanel implements MouseListener { private int orderOfCross = 0; private ArrayList crosses; private int defaultSizeOfCrosses = 10; CrossPanel() { setOpaque(false); addMouseListener(this); crosses = new ArrayList (); setLayout(new PropertionalLayoutManager()); } @Override public void mouseClicked(MouseEvent e) { int x = e.getX(); int y = e.getY(); Cross cross = new Cross(orderOfCross++, defaultSizeOfCrosses); float xPos = (float)x / (float)getWidth(); float yPos = (float)y / (float)getHeight(); crosses.add(cross); add(cross, new PropertionalConstraints(xPos, yPos)); revalidate(); } public static String format(float value) { return NumberFormat.getNumberInstance().format(value); } @Override public void mousePressed(MouseEvent e) { } @Override public void mouseReleased(MouseEvent e) { } @Override public void mouseEntered(MouseEvent e) { } @Override public void mouseExited(MouseEvent e) { } public static void main(String[] args) { JFrame f = new JFrame(); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); CrossPanel crossPane = new CrossPanel(); f.getContentPane().add(crossPane); f.setSize(600, 500); f.setLocation(200, 200); f.setVisible(true); } public class Cross extends JComponent { private int order; protected Cursor draggingCursor = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR); private volatile int draggedAtX, draggedAtY; int size; public Cross(int order, int size) { this.order = order; this.size = size; // this.setBounds(0, 0, 4 * size, 3 * size + 10); addDragListeners(); setCursor(draggingCursor); Font f = new Font("Monospaced", Font.BOLD, size + 10); setFont(f); } @Override public Dimension getPreferredSize() { // This is dangrous, you are making assumptions about platforms // that you have no eviednce to support. FontMetrics fm = getFontMetrics(getFont()); return new Dimension(Math.max(fm.stringWidth(String.valueOf(order)), size), size + fm.getHeight()); } @Override protected void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g; g2.setColor(Color.red); g2.setStroke(new BasicStroke(3)); FontMetrics fm = g2.getFontMetrics(); int width = getWidth() - 1; int height = getHeight() - 1; int x = (width - fm.stringWidth(String.valueOf(order))) / 2; int y = fm.getAscent(); g2.drawString(String.valueOf(order), x, y); int crossSize = Math.min(width, height - fm.getHeight()); x = (width - crossSize) / 2; y = fm.getHeight(); g2.drawLine(x, y, x + crossSize, y + crossSize); g2.drawLine(x + crossSize, y, x, y + crossSize); } private void addDragListeners() { addMouseListener(new MouseAdapter() { @Override public void mousePressed(MouseEvent e) { draggedAtX = e.getX(); draggedAtY = e.getY(); } }); addMouseMotionListener(new MouseMotionAdapter() { @Override public void mouseDragged(MouseEvent e) { Point newLocation = new Point(e.getX() - draggedAtX + getLocation().x, e.getY() - draggedAtY + getLocation().y); setLocation(newLocation); } }); } } public class PropertionalConstraints { private float x; private float y; public PropertionalConstraints(float x, float y) { this.x = x; this.y = y; } public float getX() { return x; } public float getY() { return y; } public void setX(float x) { if (x > 1f) { x = 1f; } else if (x < -0f) { x = 0f; } this.x = x; } public void setY(float y) { if (y > 1f) { y = 1f; } else if (y < -0f) { y = 0f; } this.y = y; } } public class PropertionalLayoutManager implements LayoutManager2 { private Map mapConstraints; public PropertionalLayoutManager() { mapConstraints = new HashMap<>(25); } public PropertionalConstraints getConstraintsFor(Component comp) { return mapConstraints.get(comp); } public void setConstraintsFor(Component comp, PropertionalConstraints pc) { mapConstraints.put(comp, pc); } @Override public void addLayoutComponent(Component comp, Object constraints) { if (constraints instanceof PropertionalConstraints) { mapConstraints.put(comp, (PropertionalConstraints) constraints); } else { throw new IllegalArgumentException("Constraints must be PropertionalConstraints"); } } @Override public Dimension maximumLayoutSize(Container target) { return preferredLayoutSize(target); } @Override public float getLayoutAlignmentX(Container target) { return 0.5f; } @Override public float getLayoutAlignmentY(Container target) { return 0.5f; } @Override public void invalidateLayout(Container target) { } @Override public void addLayoutComponent(String name, Component comp) { } @Override public void removeLayoutComponent(Component comp) { mapConstraints.remove(comp); } @Override public Dimension preferredLayoutSize(Container parent) { return parent.getSize(); } @Override public Dimension minimumLayoutSize(Container parent) { return preferredLayoutSize(parent); } @Override public void layoutContainer(Container parent) { int width = parent.getWidth(); int height = parent.getHeight(); for (Component comp : parent.getComponents()) { PropertionalConstraints con = mapConstraints.get(comp); if (con != null) { int x = (int)(width * con.getX()); int y = (int)(height * con.getY()); comp.setSize(comp.getPreferredSize()); comp.setLocation(x, y); } else { comp.setBounds(0, 0, 0, 0); } } } } }
在某些方面，您使用“魔术”数字来确定某些元素的大小和渲染位置。 这是一个非常糟糕的主意。 特别是在绘画或印刷时，您应该将所有这些值基于经验值。
在这个例子中，我已经恢复使用FontMertrics提供所需的信息，以更准确地计算各种元素的大小和位置。 这将允许更好的跨平台支持，因为并非所有平台上的所有字体都呈现相同;）