[Q#2227845][A#2228469] How to find out the preferred size of a JPanel which is not displayed, according to its content?

https://stackoverflow.com/q/2227845

I am using a JPanel (with several labels inside) to add a dynamic information on a graph. This panel is dynamically created, it is not visible before I use it to draw. For this, I am using a BufferedImage, and I follow approximately the same steps as described on this other question. It works good, as long as I specify all sizes (the panel, and its components). Like asked as well in comments of the referred question, how can I determine the optimal size of this panel? The same operation would be done if this panel was displayed in a regular frame/layout setting. In my case, how can I "pack", in a way, this panel, so that its size, and size of its content are set to the optimal (determined by the size of labels, then)?

Answer

https://stackoverflow.com/a/2228469

Suraj and willcodejavaforfood put me on the good track. Checking what is actually done in a pack() method, I see that this is mostly setting the current size to the one returned by getPreferredSize(). From this, I managed to make such solution: This method works correctly, and adjusts the panel and its content to the correct size (and answers my question in a simplistic way: "how to find the preferred size? getPreferredSize()"). However, it requires to set the initial size to a large enough size, so that the content fits in, or they won't be put on the layout. This is a bit pity, and not really "clean", but I can't find a way to avoid that, for now. Edit: Actually, the default size was not necessary, because getPreferredSize() returns the correct value, even before calling doLayout(). As such, the panel can be set to its proper size before calling the layout method.

APIzation

// Creating the panel
JPanel lPanel = new JPanel();
//lPanel.setSize(1000, 1000); //default size, not needed anymore
lPanel.setLayout(new BoxLayout(lPanel, BoxLayout.PAGE_AXIS));

//Adding the content
lPanel.add(new JLabel("Blah"));
// etc...

//Adjust the panel to its preferred size
lPanel.setSize(lPanel.getPreferredSize());

//Call the layout method 
//(this will adjust the content components to their correct size and position)
lPanel.doLayout();

package com.stackoverflow.api;

import javax.swing.BoxLayout;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Human2228469 {

  public static void configureSizeForAPanel() {
    // Creating the panel
    JPanel lPanel = new JPanel();
    // lPanel.setSize(1000, 1000); //default size, not needed anymore
    lPanel.setLayout(new BoxLayout(lPanel, BoxLayout.PAGE_AXIS));

    // Adding the content
    lPanel.add(new JLabel("Blah"));
    // etc...

    // Adjust the panel to its preferred size
    lPanel.setSize(lPanel.getPreferredSize());

    // Call the layout method
    // (this will adjust the content components to their correct size and position)
    lPanel.doLayout();
  }
}

package com.stackoverflow.api;

import javax.swing.BoxLayout;
import javax.swing.JLabel;
import javax.swing.JPanel;

/**
 * How to find out the preferred size of a JPanel which is not displayed, according to its content?
 *
 * @author APIzator
 * @see <a href="https://stackoverflow.com/a/2228469">https://stackoverflow.com/a/2228469</a>
 */
public class APIzator2228469 {

  public static void find() throws Exception {
    // Creating the panel
    JPanel lPanel = new JPanel();
    // lPanel.setSize(1000, 1000); //default size, not needed anymore
    lPanel.setLayout(new BoxLayout(lPanel, BoxLayout.PAGE_AXIS));
    // Adding the content
    lPanel.add(new JLabel("Blah"));
    // etc...
    // Adjust the panel to its preferred size
    lPanel.setSize(lPanel.getPreferredSize());
    // Call the layout method
    // (this will adjust the content components to their correct size and position)
    lPanel.doLayout();
  }
}