GridBagLayout上下调整间隙

Question

I have following code:

public Frame() {
    super();
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();
    c.fill = GridBagConstraints.BOTH;
    c.gridx = 0;
    c.weightx = 1;
    c.weighty = 1;
    for (int i = 0; i < 10; i++) {
        c.gridy = i;
        panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        add(panel, c);
    }
}

Resulting in following frame:

Due to resizing (intended) you can see there is some space above and below the top and bottom element(marked by arrows).

Any idea how to get rid of both empty spaces or at the very least the top one?
Thanks

ps: i'm aware this question is similar to a previous question of mine however it is not the same!

Answer 1

The problem has to do with how GridBagLayout manages extra vertical space, through GridBagConstraints.weighty property (see API). I'll just quote this line:

If all the weights are zero, all the extra space appears between the grids of the cell and the top and bottom edges .

The same happens when all weights are 1 (your case). In fact this will happen unless the first or last row (or both of them) be explicitly set to fill the whole extra space because of the way on how the layout manager calculates the available space. Presumably there's always extra space unassigned due to rest of division between available space and rows number and this space is distributed at the top (and/or) bottom edges.

If you replace your loop by next lines, then the whole extra space will be distributed in top and bottom rows without any gap, (but middle rows won't take any extra space):

    //c.weighty = 1; comment this line

    for (int i = 0; i < 10; i++) {
        c.gridy = i;
        c.weighty = (i == 0 || i == 9) ? 1 : 0; // add this line here
        panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        frame.getContentPane().add(panel, c);
    }

You can play with this added line and you'll see what I mean.

Note : if you want a well distributed grid then GridLayout might be a better choice.

Off-topic

• When you add components to a container which has already been displayed you need to call revalidate() method.
• You should always call pack() method before make your top-level container visible.