Graphics - Use a parameter (getSize()) to draw a line throughout the whole window-CodePudding

I need help here. I want to give a parameter to the drawLine() method which I get from getSize(). I want to draw a line throughout the whole window by using the getSize() method.

package PROG2;

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

class MyComponent extends JComponent {

    @Override
    public void paintComponent(Graphics g) {
        g.drawLine(100, 100, 200, 200);
    }
}
    
public class Übung1 extends JFrame{

    public static void berechnen() {
        int height = frame.getHeight(); //Here it says it doesn't know "frame" variable but I don't know how to declare it here.
        int width = frame.getWidth();
    }

    public static void main(String[] args){
            JFrame frame = new JFrame("First window");
            berechnen();
            frame.add(new MyComponent());
            frame.setSize(400, 400);
            frame.setVisible(true);
            frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            Graphics g = frame.getGraphics();

            // int width = frame.getWidth();
            // int height = frame.getHeight();
            System.out.println("Größe:"   frame.getSize());
            //System.out.println(width);
    }
}

CodePudding user response：

As Andrew already stated,

you don't want to get the dimensions or size of the JFrame but rather the component that is being displayed within the JFrame's contentPane, here your MyComponent instance.
The best place to get that information is inside of the paintComponent method itself, just prior to drawing the line.
And always call the super's painting method first

I also recommend:

Draw within a JPanel's paintComponent method, not a JComponent, if you want an opaque image
Avoid static methods and fields unless absolutely needed

Note that in the code below, the red line draws through the JPanel's diagonal, and continues to draw the diagonal, even when the JFrame is manually resized:

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.Stroke;

import javax.swing.*;

public class DrawLine extends JPanel {
    private static final Stroke LINE_STROKE = new BasicStroke(15f);
    private static final Dimension PREF_SIZE = new Dimension(800, 650);

    public DrawLine() {
        setPreferredSize(PREF_SIZE);
    }
    
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        
        // code to make the line smooth (antialias the line to remove jaggies)
        // and to make the line thick, using a wider "stroke"
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setStroke(LINE_STROKE);

        // if we want a red line
        g2.setColor(Color.RED);

        // this is the key code here:
        int width = getWidth();
        int height = getHeight();
        
        // draw along the main diagonal:
        g2.drawLine(0, 0, width, height);
    }
    
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            DrawLine mainPanel = new DrawLine();

            JFrame frame = new JFrame("GUI");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(mainPanel);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }

}