Question

我试图在这个GUI中创建一个文本字段（下面的变量查询），如果它注意到按下某个键，GUI将绘制出键入文本字段的字符串。但是，如果我将我的字符串str初始化为“”它在GUI上保持为“”并且永远不会改变，但是当我在paintComponent方法中打印出字符串时，它具有正确的字符串（由于某种原因除了最后一个字符）。有小费吗？提前谢谢！

private String str;
private JTextField query;
public void keyPressed(KeyEvent e) {
        str = query.getText();
}
...
public void paintComponent(Graphics g){
        super.paintComponent(g);
        //System.out.println(str);
        g.drawString(str,20,30);

}

Answer 1

不要将KeyListener与任何JTextComponent一起使用，这不是监控这些组件更改的适当方法，而是使用DocumentListener，这将在字段修改时通知通过调用setText或者将文本粘贴到字段中以及用户键入内容时
确保在更新字段时重新绘制组件...

您还应该避免直接绘制到组件上的相同组件，因为布局管理器可以将内容放在自定义绘制的顶部

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;

public class Test1 {

    public static void main(String[] args) {
        new Test1();
    }

    public Test1() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private String paintText;

        public TestPane() {
            JTextField field = new JTextField(10);
            add(field);
            field.getDocument().addDocumentListener(new DocumentListener() {

                protected void update(DocumentEvent e) {
                    Document doc = e.getDocument();
                    try {
                        paintText = doc.getText(0, doc.getLength());
                        repaint();
                    } catch (BadLocationException ex) {
                        ex.printStackTrace();
                    }
                }

                @Override
                public void insertUpdate(DocumentEvent e) {
                    update(e);
                }

                @Override
                public void removeUpdate(DocumentEvent e) {
                    update(e);
                }

                @Override
                public void changedUpdate(DocumentEvent e) {
                    update(e);
                }
            });
            field.setText("This is some text");
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            FontMetrics fm = g2d.getFontMetrics();
            int x = (getWidth() - fm.stringWidth(paintText)) / 2;
            int y = ((getHeight() - fm.getHeight()) + fm.getAscent()) / 2;
            g2d.drawString(paintText, x, y);
            g2d.dispose();
        }

    }

}

drawString没有正确写入字符串

1 个答案: