JPanel的位置和大小变化很奇怪

Question

I'm about to program a little version of "Pong". But I've already got a problem with my JPanels . The size of the Shields is changing pretty weird to 10 from 100. And there is no method that changes the size of it! Also the Shields jump to the upper middle of the JPanel . They aren't expect to do that. Seen on the two screenshots at the ent of the post. (Puu.sh)

Sorry if I'm just missing something obvious, but as I said, I'm training.

Main with psvm()

public class Main {

public static void main(String[] args) {
    new Field();
}

}

Field (Frame with the Game in it)

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Toolkit;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JFrame;

public class Field extends JFrame {

private Room room = new Room();

public Field() {

    this.setSize((int) (Toolkit.getDefaultToolkit().getScreenSize().getWidth() / 1.5), (int) (Toolkit.getDefaultToolkit().getScreenSize().getHeight() / 1.5));
    this.setLayout(null);
    this.setLocation((int) ((Toolkit.getDefaultToolkit().getScreenSize().getWidth() - (Toolkit.getDefaultToolkit().getScreenSize().getWidth() / 1.5)) / 2), (int) ((Toolkit.getDefaultToolkit().getScreenSize().getHeight() - (Toolkit.getDefaultToolkit().getScreenSize().getHeight() / 1.5)) / 2));
    this.getContentPane().setBackground(Color.GRAY);
    this.setResizable(true);
    this.setEnabled(true);
    this.setMinimumSize(new Dimension(1000, 455));
    if (getHeight() < 455) {
        this.setSize(getWidth(), 455);
    } else if (getWidth() < 1000) {
        this.setSize(1000, getHeight());
    }

    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


    this.add(room);

    this.setVisible(true);
    addMouseListener(this);

    this.addComponentListener(new ComponentAdapter() {
        public void componentResized(ComponentEvent evt) {
            room.updateLocation(getSize());
        }
    });


}

}

Room

import javax.swing.*;
import java.awt.*;
public class Room extends JPanel {


private Shield shieldL = new Shield(0); // LEFT side
private Shield shieldR = new Shield(1); // RIGHT side

public Room() {

    this.setBackground(new Color(122, 197, 205));

    this.add(shieldL);
    this.add(shieldR);
}

public void updateLocation(Dimension d) {
    this.setSize((int) d.getWidth(), (int) (d.getHeight() - (d.getHeight() / 5)));
    this.setLocation(0, (int) (d.getHeight() / 10));
    shieldL.updateLocation(getSize());
    shieldR.updateLocation(getSize());
    this.setVisible(true);

}
}

Shield

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

public class Shield extends JPanel {

private int side = 0;

public Shield(int s) { //0 = Left  |  1 = Right

    side = s;
    this.setSize(10, 100);
    this.setBackground(Color.white);

}

public void updateLocation(Dimension d) {   //Bug found here
    System.out.println(getSize());          //Bug seems not to be created here?
    int xPos;
    if (side == 0) {
        xPos = (int) (d.getWidth() / 8);
    } else {
        xPos = (int) (d.getWidth() / 8 * 7 - this.getWidth() / 2);
    }
    this.setLocation(xPos, (int) (d.getHeight() / 2 - (getHeight() / 2)));
    this.setVisible(true);
}

}

They end up like this:, switching the location at randome framesize changings.

http://puu.sh/ceyE0/ef15ee1748.png

http://puu.sh/ceyFN/122174db48.png

Thanks! Julian :)

Answer 1

In Shield , change the line

setSize(10, 100);

to

setPreferredSize(new Dimension(10, 100));

Do not use setSize methods, they are called internally.

Edit:

Try only these 2 classes:

public class Field extends JFrame {

    private Room room = new Room();

    public Field() {

        //Mock panel
        JPanel filler = new JPanel();
        filler.setOpaque(false);

        getContentPane().setBackground(Color.GRAY);
        getContentPane().add(filler, BorderLayout.PAGE_START);
        getContentPane().add(room);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setMinimumSize(new Dimension(1000, 455));
        setLocationRelativeTo(null);
        pack();
        room.init();
        setVisible(true);
    }
}

---

public class Room extends JPanel {

    private static int leftWidth = 100;
    private static int leftHeight = 100;
    private static int leftX;
    private static int leftY;

    private static int rightWidth = 100;
    private static int rightHeight = 100;
    private static int rightX;
    private static int rightY;

    public Room() {

        setBackground(new Color(122, 197, 205));
    }

    void init() {

        leftX = getWidth() / 8;
        leftY = getHeight() / 2 - leftHeight / 2;
        rightX = getWidth() * 7 / 8;
        rightY = getHeight() / 2 - rightHeight / 2;
    }

    @Override
    protected void paintComponent(Graphics g) {

        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setColor(Color.WHITE);
        g2d.fillRect(leftX, leftY, leftWidth, leftHeight);
        g2d.fillRect(rightX, rightY, rightWidth, rightHeight);
    }
}

Answer 2

Your problem is that you do not understand Layout Managers and how they treat or use setSize, setPreferredSize, setMinimumSize, and setMaximumSize methods.

For your panel class, setSize does not work because when you create a JPanel, it uses FlowLayout as the default layout manager and layout managers ignore size specified by the setSize method. You must use the others in some combination (at least one). The only way you can use setSize is for Components that do not assign a layout manager by default OR, by invoking setLayout(null). This is called Absolute Positioning. This is where you MUST specify the location AND size of each component.

Search the Java Swing tutorial and search for "How to use Layout Managers." If you are using Swing, it is IMPERATIVE that you understand this concept because different Layout Managers obey different size and positioning rules. For example, Flow Layout and Grid Layout behave very differently.

Of course, Absolute Positioning is a pain, but it is the method that offers you absolute control of size and positioning of Swing components. But understanding Layout Managers and using them properly can save you a lot of time creating your GUI's.

***** UPDATED INFO ***** The problem with resizing the frame is due to the fact your panel is using FlowLayout. This layout manager allows the rearrangement of components in the panel. There are several ways to fix this problem. One is to use a layout manager that preserves the aspect ratio of the component when it is resized. For example, GridLayout "breaks down" a component into grids. If you think of a checker board, it is an 8x8 grid. If you make the panel a perfect square, then all grids will be squares of equal size. When you resize a panel, the aspect ratio of the panel (not necessarily the inner components), is kept. Another way, and maybe your best alternative while you learn how to use layout managers properly, is to prevent resizing of the frame.