Question

考虑java中的以下代码片段： class：GraphPanel.java

package graph_draw;

import graph_draw.LocationPrinter;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.util.List;

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

@SuppressWarnings("serial")
class GraphPanel extends JPanel {

    private Color lineColor = new Color(44, 102, 230, 180);

    private List<Point> graphPoints = null;
    private static Graphics2D gra2;

    public GraphPanel(List<Point> gPoints) {
        graphPoints = gPoints;        
    }    

    @Override
    public void paintComponent(Graphics g) 
    {
        super.paintComponent(g);
        //gra2 = (Graphics2D)g;
          gra2 = (Graphics2D)g.create();

        gra2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        drawGraph(gra2, graphPoints);
    }

    public void drawGraph(Graphics2D g2, List<Point> graphPoints)
    {
        Graphics2D g = (Graphics2D)g2;

        g.setColor(lineColor);

        int x1 = graphPoints.get(0).x;
        int y1 = graphPoints.get(0).y;

        int x2 = graphPoints.get(1).x;
        int y2 = graphPoints.get(1).y;

        int x3 = graphPoints.get(2).x;
        int y3 = graphPoints.get(2).y;

        int x4 = graphPoints.get(3).x;
        int y4 = graphPoints.get(3).y;

        int x5 = graphPoints.get(4).x;
        int y5 = graphPoints.get(4).y;

        g.drawLine(x1, y1, x2, y2);
        g.drawLine(x2, y2, x3, y3);
        g.drawLine(x2, y2, x4, y4);
        g.drawLine(x3, y3, x4, y4);
        g.drawLine(x2, y2, x5, y5);
        g.drawLine(x3, y3, x5, y5);

    }

    public static void drawNewColoredLine(List<Point> lst)
    {
        Graphics2D g21 = (Graphics2D)gra2;  // gra2 is 'null' here. So g21 is null. Hence: NullPointerException.

        g21.setColor(Color.GREEN);
        int SIZE = lst.size();

        for(int i = 0; i < SIZE -1 ; i++)
        {
            int xa = lst.get(i).x;
            int ya = lst.get(i).y;

            int xb = lst.get(i+1).x;
            int yb = lst.get(i+1).y;

            g21.drawLine(xa, ya, xb, yb);
        }       
    }


    public static void createAndShowGui(List<Point> graphPoints) {        

        GraphPanel mainPanel = new GraphPanel(graphPoints);
        mainPanel.setPreferredSize(new Dimension(1366, 768));
        JFrame frame = new JFrame("DrawGraph");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);

    }
}

现在包含main的类 - class：DriverClass.java

package graph_draw;

import java.awt.Point;
import java.util.ArrayList;
import java.util.List;

public class DriverClass {

    public DriverClass() {
        // TODO Auto-generated constructor stub
    }

    public static void main(String[] args)
    {       
        List<Point> graphPoints = new ArrayList<>();

        // add points here.

        graphPoints.add(new Point(358, 237));
        graphPoints.add(new Point(366, 467));
        graphPoints.add(new Point(661, 468));
        graphPoints.add(new Point(636, 229));
        graphPoints.add(new Point(527, 648));

        GraphPanel.createAndShowGui(graphPoints);

        List<Point> ls = new ArrayList<Point>();

        ls.add(graphPoints.get(0));
        ls.add(graphPoints.get(1));
        ls.add(graphPoints.get(3));
        ls.add(graphPoints.get(2));
        ls.add(graphPoints.get(4)); 

        GraphPanel.drawNewColoredLine(ls); // not working.          
    }    
}

问题：在课程DriverClass.java的最后一行，静态方法＆＃39; drawNewColoredLine＆＃39;被调用包含列表作为参数。我想使用静态对象＆＃39; gra2＆＃39;作为＆＃39; drawNewColoredLine＆＃39;等所有方法的图形对象。

'gra2'初始化为方法'paintComponent'内的图形对象。 'gra2'适用于方法＆＃39; drawGraph'。但它并没有找到方法＆draw;抽奖彩色线＆＃39; drawNewColoredLine＆＃39; - 引发空指针异常。 [＆＃39; gra2＆＃39;在'drawNewColoredLine']中为空。那么如何让它在'drawNewColoredLine'内工作？

Answer 1

你有什么理由使用静态方法吗？你应该摆脱它。您创建了GraphPanel类的对象，但是您没有参考，所以在调用另一个静态方法之后，您没有像预期的那样初始化字段。尝试删除所有静态，删除createAndShowGui中创建新对象并调用以下内容：

....
GraphPanel graphPanel = new GraphPanel(graphPoints);
graphPanel.createAndShowGui(graphPoints); // actually you don't need to pass it again.
....
graphPanel.drawNewColoredLine(ls);
....

Answer 2

不应存储Graphics中收到的paintComponent个对象。相反，从paintComponent内调用所有绘画代码，并将Graphic2D对象传递给其他绘画函数，就像您对drawGraph所做的那样。

public void paintComponent(Graphics g) 
{
    super.paintComponent(g);
    //gra2 = (Graphics2D)g;
      gra2 = (Graphics2D)g.create();

    gra2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

    drawGraph(gra2, graphPoints);
}

public void drawGraph(Graphics2D g, List<Point> graphPoints)
{

    g.setColor(lineColor);
....
....

图形对象在方法内部不起作用

2 个答案: