Swing : BoxLayout fill the entire JPanel

Question

I am adding list of JTree items inside JPanel. I want the parent JPanel to have BoxLayout so that the tree can be added vertically one after another.

The parent JPanel is initialized using :

holder.setLayout(new BoxLayout(holder, BoxLayout.Y_AXIS));
        holder.setMaximumSize(new java.awt.Dimension(32767, 24000));
        holder.setMinimumSize(new java.awt.Dimension(600, 100));
        holder.setPreferredSize(new java.awt.Dimension(600, 100));

The multiple JTree components are added inside :

holder.add(tree);

So i expect the JTree nodes to be occupying the entire width of my parent JPanel but somehow it is coming like this

So as you can see it coming in some portion of the parent JPanel. I want it to fill the entire parent Panel(width wise) and be aligned towards left.

EDIT

After trying the top-aligned approach mentioned by VGR I got this :

So the tree are not occupying the entire space still. And when i expand any tree then everything disappears.

I should have also mentioned this earlier that when the initilization of the parent panel(holder) is done in some other code like this

holder.setMaximumSize(new java.awt.Dimension(32767, 24000));
holder.setMinimumSize(new java.awt.Dimension(600, 100));
holder.setPreferredSize(new java.awt.Dimension(600, 100));
holder.setLayout(new java.awt.GridLayout(1, 0));
add(holder); // add to the top parent

This part is not reachable :( for me. I can only re-change the parent holder as per my requirement.

SSCCE after suggested changes from VGR. This is not compilable but i hope SSC.

public BasePanel() extends JPanel{

private javax.swing.JPanel holder;
private GridBagConstraints gbc;
private JPanel treesPanel;

BasePanel(){
  init();
}

public init(){ can't access this method
        holder.setMaximumSize(new java.awt.Dimension(32767, 25000));
        holder.setMinimumSize(new java.awt.Dimension(600, 0));
        holder.setPreferredSize(new java.awt.Dimension(600, 0));
        holder.setLayout(new java.awt.GridLayout(1, 0));
        add(holder);
}

public initTreeComponents(){ // i need to call this for each tree
        this.holder.setLayout(new BorderLayout());
        treesPanel = new JPanel(new GridBagLayout());
        this.holder.add(treesPanel, BorderLayout.PAGE_START);
        gbc = new GridBagConstraints();
        gbc.gridwidth = GridBagConstraints.VERTICAL;
        gbc.anchor = GridBagConstraints.FIRST_LINE_START;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.weightx = 1;
}

public addTree(JTree tree){// to be called for each tree
  treesPanel.add(tree,gbc);
}

Answer 1

I don't think BoxLayout makes child components fill the container. From the documentation:

… for a vertical layout, BoxLayout attempts to make all components in the column as wide as the widest component.

So your JTrees will all be the same width, but that doesn't guarantee they'll be as wide as the container.

Instead, I would use a GridBagLayout:

holder.setLayout(new GridBagLayout());

// etc.

GridBagConstraints gbc = new GridBagConstraints();
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbc.anchor = GridBagConstraints.FIRST_LINE_START;
gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.weightx = 1;
holder.add(tree, gbc);

That will result in the JTrees being vertically centered. If you want them top-aligned, you should put a GridBagLayout panel inside another panel:

holder.setLayout(new BorderLayout());

JPanel treesPanel = new JPanel(new GridBagLayout());
holder.add(treesPanel, BorderLayout.PAGE_START);

// etc.

GridBagConstraints gbc = new GridBagConstraints();
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbc.anchor = GridBagConstraints.FIRST_LINE_START;
gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.weightx = 1;
treesPanel.add(tree, gbc);

Answer 2

Try adding JTrees inside a JScrollPane, ie

holder.add(new JScrollPane(tree));