Panel.setPreferredSize() doesn't work

Question

I have a internal frame. In this internal frame, I need to dynamic generate panels and put labels data in each panel.
The panels and labels are generate correctly, but panel.setPreferredSize() doesn't work.
I hope each panel can have the same width. can someone take a look for me? Thanks a lot~~
Here is my code. When the internal frame is initialized, it will call addPanel() function.
public void addPanel(int[][] cluster, String[] labels, String[][] searchResult){
         JPanel mainPanel = new JPanel();
         mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.PAGE_AXIS));
        mainPanel.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
         //panel for clusters
         JPanel[] clusterPanel = new JPanel[cluster.length];
         docsLabel = new JLabel[searchResult[3].length];
         int docIndex, docsLabelIndex=0;
         for(int i=0;i<cluster.length;i++){
              //set up border for cluster panel
              clusterPanel=new JPanel();
               clusterPanel[i].setBorder(BorderFactory.createCompoundBorder(
BorderFactory.createTitledBorder(labels[i]),
BorderFactory.createEmptyBorder(3,3,3,3)));
               clusterPanel[i].setLayout(new BoxLayout(clusterPanel[i],BoxLayout.Y_AXIS));
          for(int j=0;j<cluster[i].length;j++){
               docIndex=cluster[i][j];
               labelsIndex[docIndex]=docsLabelIndex;
               docsLabel[docsLabelIndex]= new JLabel(url[docIndex]);
               docsLabel[docsLabelIndex].setToolTipText(searchResult[3][docIndex]);
               docsLabel[docsLabelIndex].setHorizontalTextPosition(JLabel.LEFT);
               docsLabel[docsLabelIndex].setOpaque(true);
               docsLabel[docsLabelIndex].addMouseListener(this);
               clusterPanel[i].add(docsLabel[docsLabelIndex]);
               clusterPanel[i].addMouseListener(this);
               clusterPanel[i].setName(Integer.toString(i));
               docsLabelIndex++;
               clusterPanel[i].setPreferredSize(new Dimension(200, 50)); ----------------->this doesn't work
          clusterPanel[i].setOpaque(true);
               mainPanel.setOpaque(true);
               mainPanel.add(clusterPanel[i]);
     ScrollPane sp = new ScrollPane(ScrollPane.SCROLLBARS_AS_NEEDED);
          sp.setBounds(2,2,2,2);
          sp.setVisible(true);
          sp.add(mainPanel);
     this.add(sp);

Answer 1

are you sure that the panels are the ones that are the wrong size? Maybe the contents of the panels sizes only make it look that way. Try setting different types of components to have different background colors to see what's what.
Keep in mind that preferred size only specifies the preferred size. It doesn't mean that a layout manager has to respect that. Although, I would expect BoxLayout to do so.