I can't get BoxLayout's setPreferredSize() method to work the way I want it to

I think I solved my problem, but I don't know why it works this way, so I'm hoping someone can explain it to me so I don't do the same mistake again in the future.

Here's a quick example that is compilable of what I'm trying to do:

public class BoxLayoutTest extends JFrame
{
  public BoxLayoutTest()
  {
    setSize(400,300);
    JPanel mainPanel = new JPanel(new FlowLayout());
    setContentPane(mainPanel);

    JPanel subPanel = new JPanel();
    subPanel.setLayout(new BoxLayout(subPanel, BoxLayout.PAGE_AXIS));
    subPanel.setBackground(Color.BLUE);

    JLabel labelTest = new JLabel("This is a test");
    subPanel.add(labelTest);

    labelTest.setPreferredSize(new Dimension(150, 20));
    mainPanel.add(subPanel);
    System.out.println(mainPanel.getSize());
  }

  public static void main( String[] args )
  {
    BoxLayoutTest testFrame = new BoxLayoutTest();
    testFrame.setVisible(true);
  }
}

At first, I had problems with the panel containing the JLabel not resizing like it should with the preferred size. I found out that it was because I was using some variation of mainPanel.getSize() as a preferred size for my subpanels. In this example, I'm using actual number values, which work.

The reason why it didn't work the old way (and that's actually the thing I'd like someone to explain), is why, as seen in the SOP line, mainPanel.getSize() returns a width and a height of 0 while it clearly takes the whole screen, which is 400x300.

Thanks @camickr for telling me I shouldn't set a preferredSize for 开发者_JAVA百科my Panels, this helped me figure out where the problem was coming from.

Why [does] mainPanel.getSize() returns a width and a height of 0?

Until pack() "causes this Window to be sized to fit the preferred size and layouts of its subcomponents," the dimensions will be zero.

System.out.println(mainPanel.getSize());
this.pack();
System.out.println(mainPanel.getSize());

Console:

java.awt.Dimension[width=0,height=0]
java.awt.Dimension[width=160,height=30]

Its hard to say why your code doesn't work, since you didn't post your code. A few random lines of code does not give us the context of how the code is used in your program.

When you post a question you need to post your SSCCE which demonstrates the problem.

The glue should not have fixed the problem. The panel should still display, its just that it may not display in the position you expect it to be. A BoxLayout will attempt to resize components added to it to fill up the entire space available to it.

You should not be using setPreferredSize() on a panel. It is the job of the layout manager to calculate the preferred size of the panel based on the preferred size of all the components added to it. So I would say your code is still wrong.

is there a difference between typing this and this.getContentPane()

Certain methods calls are automatically forwarded to the content pane of the frame, which is why the end result is the same. Read the JFrame API. This is addressed in the API description or for the method in question.