Java - Swing Layout + AWT - 如何在使用布局定位的 JPanel 中绘制一条线？

Question

For the purpose of another program I need to draw a few lines on the screen which has a image built with incremental painting and layouts. Since the lines were very straight forward and just a few, I thought using the JPanel's painting space to do this. Since this was not working, I made an testing program to try to make it work, but I still can't seem to figure out why this isn't working.

Here is my testing program:

import javax.swing.JPanel;
import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JFrame;

import java.awt.GridLayout;
import java.awt.Toolkit;
import java.awt.Dimension;


class TestMain{
public static void main(String[]args){
    TestingComponent tester = new TestingComponent();
}
}
class Block extends JPanel{
Block(){
        setOpaque(true);
}
public void paintComponent(Graphics gr){
    super.paintComponent(gr);
    if(getBackground() == Color.white){
        System.out.println("Reached");
        gr.drawLine(getX(), getY(), getX() + getWidth(), getY() + getHeight());
    }
}
}
class TestingComponent{
TestingComponent(){
    JFrame frmMain = new JFrame("testing");
    frmMain.setSize(Toolkit.getDefaultToolkit().getScreenSize());
    JPanel pnlMain = new JPanel();
    Block block = new Block();
    Dimension dmsDim = new Dimension(16, 16);
    pnlMain.setPreferredSize(dmsDim);
    frmMain.add(pnlMain);
    pnlMain.setLayout(new GridLayout(2, 2));
    for(int index = 0; index < 4; index++){
        block = new Block();
        switch(index){
            case 0:
                block.setBackground(Color.black);
                break;
            case 1:
                block.setBackground(Color.blue);
                break;
            case 2:
                block.setBackground(Color.green);
                break;
            case 3:
                block.setBackground(Color.white);
                break;
        }
        pnlMain.add(block);
    }
    frmMain.setVisible(true);
}
}

Versions/Programs I'm using (in the testing program) -

Java 8

Notepad

Command Prompt

Seeing how simple the drawLine method is, I'm almost certain it is due to the layouts, but other than that one point, I haven't a clue why the line isn't drawing. The System.out.println("Reached"); is being out-putted, so the program definitely reachs the gr.drawLine().

To clearly state the question... Why isn't the line being drawn? How do I fix this?

Answer 1

you are using the wrong points to draw your line

getX() and getY() return the location of your Component ( Block ) on the parent component.

and because you divide your drawings into two pieces, getX() returns the very right coordinate of your panel, getY() returns the very upper coordinate.

use gr.drawLine(0, 0, getWidth(), getHeight()); to draw the lines