| JAVA | 绘制树，JPanel repaint（）不起作用

Question

I want to draw a tree but the repaint function does not call the paintComponent function. I save every node in an array.

My main class:

public class Main {

public static void main(String[] args) {
    GUI g = new GUI();
    g.setVisible(true);
}
}

My GUI class:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class GUI extends JFrame {

private JPanel cp;
private Node[] tree;
private displayTree dtree = new displayTree();

public GUI() {
    this.setTitle("Tree");
    this.setSize(900, 700);
    this.setLocation(300, 100);
    this.setResizable(false);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    cp = (JPanel) this.getContentPane();
    cp.setLayout(null);
    cp.setBackground(Color.WHITE);

    Node a = new Node("+");
    Node b = new Node("3");
    Node c = new Node("x");

    a.left = b;
    a.right = c;

    tree = new Node[3];
    tree[0] = a;
    tree[1] = b;
    tree[2] = c;

    System.out.print(tree[0].data);
    System.out.print(tree[0].left.data);
    System.out.print(tree[0].right.data);

    // Nothing like that works

    dtree.revalidate();
    dtree.repaint();
    this.repaint();
    System.out.print("a");
}

class displayTree extends JPanel {

    private int radius = 25;
    private int padding = 70;

    @Override
    public void paintComponent(Graphics g) {
        // Should be displayed but doesn't 
        System.out.print("It works");
        super.paintComponent(g);
        displayTree(g, tree[0], getWidth() / 2, 30, getWidth() / 4);
    }

    private void displayTree(Graphics g, Knoten k, int x, int y, int tempPadding) {
        g.drawOval(x - radius, y - radius, 2 * radius, 2 * radius);
        g.drawString(k.data, x - 6, y + 4);

        if (k.left != null) {
            displayNode(g, x - tempPadding, y + padding, x, y);
            displayTree(g, k.left, x - tempPadding, y + padding, tempPadding / 2);
        }

        if (k.right != null) {
            displayNode(g, x + tempPadding, y + padding, x, y);
            displayTree(g, k.right, x + tempPadding, y + padding, tempPadding / 2);
        }
    }

    private void displayNode(Graphics g, int x1, int y1, int x2, int y2) {
        double d = Math.sqrt(padding * padding + (x2 - x1) * (x2 - x1));
        int x11 = (int) (x1 - radius * (x1 - x2) / d);
        int y11 = (int) (y1 - radius * (y1 - y2) / d);
        int x21 = (int) (x2 + radius * (x1 - x2) / d);
        int y21 = (int) (y2 + radius * (y1 - y2) / d);
        g.drawLine(x11, y11, x21, y21);
    }
}

}

The Node class lokes like this:

import javax.swing.JPanel;

public class Node extends JPanel {

String data;
Knoten left;
Knoten right;

public Node(String data) {
    this.data = data;
}
}

Can you help me. How can i draw the Nodes ?

Answer 1

Two things:

You aren't adding dtree to the content pane, and you are setting the content pane's layout to null, which is useless in your case.

public GUI() {
    this.setTitle("Tree");
    this.setSize(900, 700);
    this.setLocation(300, 100);
    this.setResizable(false);
    setBackground(Color.red);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    cp = (JPanel) this.getContentPane();
    //REMOVE null layout.
    //cp.setLayout(null);
    cp.setBackground(Color.WHITE);

    Node a = new Node("+");
    Node b = new Node("3");
    Node c = new Node("x");

    a.left = b;
    a.right = c;

    tree = new Node[3];
    tree[0] = a;
    tree[1] = b;
    tree[2] = c;

    System.out.print(tree[0].data);
    System.out.print(tree[0].left.data);
    System.out.print(tree[0].right.data);

    // No useless repaints/revalidates
    //Add the tree to the content pane
    cp.add(dtree);


    System.out.print("a");
}

If you really need (no, you shouldn't) to have a null layout on the content pane you must size the tree panel:

[...]
// No useless repaints/revalidates

//Size the content pane, otherwise with null layout its size will be 0x0
dtree.setSize(900,700);
//Add the tree to the content pane
cp.add(dtree);