我正在尝试使用数字字符串制作图表，但如果尝试重绘我的paintComponent（Graphics g）方法，将不会调用该方法

Question

I'm trying to make chart with a string of number but if I try to repaint my paintComponent(Graphics g) method won't be called. I debugged the file and i really don't understand why this happens. This class extends JPanel and the method: "verwerkData(String s)" is called when I press a button on my JFrame.

Try-catch is used because when I start the program paintComponent(Graphics g) is called and "punt" and "punti" aren't initialized yet that's why i throw a NullPointerException.

package grafiek;

import java.awt.Graphics;

public class Grafiek extends javax.swing.JPanel {

private String[] punt;
private int[] punti;
private int afstandX, afstandY, puntX1=0, puntY1=0, puntX2=0, puntY2=0;
private int max=0;

/**
 * Creates new form Grafiek
 */
public Grafiek() {
    initComponents();
}

@Override
public void paintComponent(Graphics g) {

    try{
        for(int i=0; i<punti.length; i++) {
            if(max >= punti[i]) {
                max = punti[i];
            }
        }

        afstandX = getWidth()/punt.length;
        afstandY = getHeight()/max;

        for(int i=0; i<punti.length; i++) {
            puntX1 = puntX2;
            puntY1 = puntY2;
            puntX2 += afstandX;
            puntY2 = punti[i]*afstandY;

            System.out.println(puntX1+" "+puntY1+" "+puntX2+" "+puntY2);

            g.drawLine(puntX1, puntY1, puntX2, puntY2);
        }
    }catch(java.lang.NullPointerException npe) {
        super.paintComponent(g);
    }
}

public void verwerkData(String s) {
    punt = s.split(" ");

    punti = new int[punt.length];

    for(int i=0; i<punt.length; i++) {
        punti[i] = Integer.parseInt(punt[i]);
    }

    repaint();
}

/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    setBackground(new java.awt.Color(204, 204, 204));

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
    this.setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 400, Short.MAX_VALUE)
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 300, Short.MAX_VALUE)
    );
}// </editor-fold>                        

}

Answer 1

EDIT : When you call repaint() , you only tell the system to call paintComponent() as soon as possible. But the actual call to paintComponent() is done by another thread (the Event dispatch thread). This is explained well in the Swing Custom Painting Tutorial

The computation of max was wrong: max remained zero, which caused an ArithmeticException due to division by zero. This may already have messed up some things. Fixing this and inserting a main method caused the program to run properly for me, and to display the data... somehow.

Apart from that: NEVER catch a NullPointerException . Instead, check whether the respective variables are null , and skip further computations if necessary.

The initComponents method is useless here (I don't really like these "Visual GUI Builders", but some may find them convenient).

Another very general hint: You should try to keep the scope of variables as small as possible !. Particularly, the variables afstandX .... puntY2 are only needed in paintComponent . So you should also declare them in paintComponent .

Your way of drawing the lines was not perfect. It worked partially, but some variables have not been reset to zero (which is related to the previous point that I mentioned), and the fact that all computations are made with int values may have undesirable effects.

A slightly cleaned up MCVE . One could go very far with further improvements, but then, this answer would become "A tutorial for writing a line-chart plotter", which is beyond the scope that such an answer should have. If you want to draw sophisticated line charts, you should consider using an appropriate library, as it was already suggested in the comments.

package stackoverflow;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class Grafiek extends javax.swing.JPanel {

    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run()
            {
                JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                Grafiek g = new Grafiek();
                g.verwerkData("50 100 60 90 70 80");
                f.getContentPane().add(g);
                f.setSize(300,300);
                f.setLocationRelativeTo(null);
                f.setVisible(true);
            }
        });
    }

    private String[] punt;
    private int[] punti;

    @Override
    protected void paintComponent(Graphics g) 
    {
        super.paintComponent(g);

        System.out.println("paintComponent is being called!");

        if (punt == null || punti == null)
        {
            return;
        }

        int max = Integer.MIN_VALUE;
        for(int i=0; i<punti.length; i++) {
            if(max < punti[i]) {
                max = punti[i];
            }
        }

        int afstandX = getWidth()/(punt.length-1);
        int afstandY = getHeight()/max;

        int puntX1 = 0; 
        int puntY1 = punti[0] * afstandY;

        for(int i=1; i<punti.length; i++) {
            int puntX2 = puntX1 + afstandX;
            int puntY2 = punti[i]*afstandY;

            System.out.println(puntX1+" "+puntY1+" "+puntX2+" "+puntY2);

            g.drawLine(puntX1, puntY1, puntX2, puntY2);

            puntX1 = puntX2;
            puntY1 = puntY2;
        }
    }

    public void verwerkData(String s) {
        punt = s.split(" ");

        punti = new int[punt.length];

        for(int i=0; i<punt.length; i++) {
            punti[i] = Integer.parseInt(punt[i]);
        }

        repaint();
    }
}