JTable为Graphics返回null？

Question

我正在尝试使用我制作的Painter对象在我的JTable上绘制线条,但由于某种原因table.getGraphics()返回null.

画家类:

import java.awt.BasicStroke;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JTable;

public class Painter extends JTable {

    public Painter(){

    }

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

    public void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setStroke(new BasicStroke(3));
        g2.drawLine(0, 300, 400, 250);
    }
}

我用以下方法调用方法:

private Painter paint = new Painter();
paint.paintSudokuLines(table.getGraphics());

我不知道为什么会这样,所以我需要一些解释.

Answer 1

永远不要使用getGraphics,它可以返回null(如你所见),而不是如何定制绘画.你永远不应该打电话给paintComponent自己.如果在打印时,你不会直接调用它,最好是打破画颜链并且不会引入额外的问题和工件.

为了更好地理解绘画作品,请参阅AWT和Swing中的绘画.

现在,让我快速演示一下为什么你不应该尝试画一个JTable像你想要做的那样......

现在,不要误解我的意思,我确信通过一些非常聪明的演绎工作,你可以计算行/列位置,以便能够正确地绘制指南,但这是一项额外的工作.

更好的解决方案可能是使用自定义TableCellRenderer...

import java.awt.Color;
import java.awt.Component;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.LineBorder;
import javax.swing.border.MatteBorder;
import javax.swing.table.DefaultTableCellRenderer;
import javax.swing.table.DefaultTableModel;

public class SudokuTest {

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

    public SudokuTest() {
        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 JScrollPane(new SudokuTable()));
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class SudokuTable extends JTable {

        public SudokuTable() {
            super(new DefaultTableModel(9, 9));
            setDefaultRenderer(Object.class, new SudokuCellRender());
        }

    }

    public class SudokuCellRender extends DefaultTableCellRenderer {

        @Override
        public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected, boolean hasFocus, int row, int column) {

            super.getTableCellRendererComponent(table, value, isSelected, hasFocus, row, column);

            int left = 0;
            int top = 0;
            int right = 0;
            int bottom = 0;

            if ((column % 3) == 0) {
                left = 2;
            } else if ((column % 3) == 2) {
                right = 2;
            }

            if ((row % 3) == 0) {
                top = 2;
            }
            if ((row % 3) == 2) {
                bottom = 2;
            }

            setBorder(new MatteBorder(top, left, bottom, right, Color.RED));

            return this;
        }

    }

}

现在,这只是一个概念证明.您可能需要使用复合边框来偏移单元格属性(因此所有单元格周围至少有一个2像素边框和/或减小边框宽度;))

另外,请查看如何使用表格以获取更多详细信息