Confusion regarding the working of flowlayout

Question

I have developed a small swing application in which there is a square rotating in the upper half and there is a button in the lower half which can stop/run the square to rotate. I have used the GridLayout to place the rotating square and the button. (Another alternative is to use 2 JPanels ,one is with rotating square and second contains the button.Using this button appears of proper size.)

Here is the code :-

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;


public class Rotation {
JButton jbtn=new JButton("Stop");
component jpn2=new component();    //created a JPanel named jpn2 and got a reference to its timer object.
Timer timer=jpn2.timer;
Rotation()
{


    JFrame jfrm=new JFrame("Rotating a square about a center");
    jfrm.setSize(400,400);
    jfrm.setLayout(new GridLayout(2,1));
    jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //JPanel jpnl=new JPanel();

    //jpnl.add(jbtn);

    jbtn.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent e){
        if(e.getActionCommand().equals("Stop"))
        {
            timer.stop();
            jbtn.setText("Spin");
        }
        if(e.getActionCommand().equals("Spin"))
        {
            timer.start();
            jbtn.setText("Stop");
        }

    }});

    jfrm.add(jpn2);
    jfrm.add(jbtn);

    //jfrm.add(new JButton("Click"));
    jfrm.setVisible(true);
    //jfrm.setOpacity(0.8f);
}
public static void main(String args[]) throws ClassNotFoundException, InstantiationException, IllegalAccessException, UnsupportedLookAndFeelException
{
    //JFrame.setDefaultLookAndFeelDecorated(true);
    UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
    SwingUtilities.invokeLater(new Runnable(){public void run(){new Rotation();}});
}

}

class component extends JPanel implements ActionListener
{
Timer timer;
int theta=0;
component()
{
    timer=new Timer(10,this);
    timer.start();
}
public void paintComponent(Graphics g)
{
    super.paintComponent(g);
    Graphics2D g2=(Graphics2D)g;
    g2.rotate(theta,100,100);
    g2.fillRect(50, 50, 100,100);
}
public void actionPerformed(ActionEvent e)
{
    //Changing a global variable and then drawing the rectangle again and hence indirectly the square rotates.
    theta=theta+10;
    if(theta==360)
        theta=0;
    repaint();
}
}

Here is the output:-

But my confusion is when i decided to use FlowLayout instead of GridLayout i'm getting only the button and no rotating square. By,as far as i have read,FlowLayout places components in a row and if space is less than it uses multiple rows. Can anyone resolve this small stupid problem of mine which currently i am not able to resolve.

Answer 1

As others have said (+1 to mKorbel and HFOE)

• The problem is you use setSize(..) rather call pack() before setting JFrame to visible.

• You will also have to override getPrefferedSize(..) in JPanel class which will return the size of your square multiplied by 2 (or else when it rotates it wont fit).

• On a side note dont throw any excpetion in main(..) never a good thing.

see below for the code (uses FlowLayout but also works with GridLayout):

Using new FlowLayout():

Using new GridLayout(2,1):

import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Rotation {

    JButton jbtn = new JButton("Stop");
    component jpn2 = new component();    //created a JPanel named jpn2 and got a reference to its timer object.
    Timer timer = jpn2.timer;

    Rotation() {


        JFrame jfrm = new JFrame("Rotating a square about a center");

        // jfrm.setLayout(new FlowLayout());
        jfrm.setLayout(new GridLayout(2, 1));

        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        jbtn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (e.getActionCommand().equals("Stop")) {
                    timer.stop();
                    jbtn.setText("Spin");
                }
                if (e.getActionCommand().equals("Spin")) {
                    timer.start();
                    jbtn.setText("Stop");
                }

            }
        });

        jfrm.add(jpn2);
        jfrm.add(jbtn);

        jfrm.pack();
        jfrm.setVisible(true);
    }

    public static void main(String args[]) {
        try {
            UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {

            ex.printStackTrace();
        }

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Rotation();
            }
        });
    }
}

class component extends JPanel implements ActionListener {

    Timer timer;
    int theta = 0;
    int width = 100, height = 100;

    component() {
        timer = new Timer(10, this);
        timer.start();
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.rotate(theta, 100, 100);
        g2.fillRect(50, 50, width, height);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(width * 2, height * 2);//multiply by 2 to fit while rotating
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        //Changing a global variable and then drawing the rectangle again and hence indirectly the square rotates.
        theta = theta + 10;
        if (theta == 360) {
            theta = 0;
        }
        repaint();
    }
}