When is preferredLayoutSize called?

Question

preferredLayoutSize(Container parent) is required by all java layout manager, but when is this method called?

The following code is found in GridLayout.java:

public Dimension preferredLayoutSize(Container parent) {
  synchronized (parent.getTreeLock()) {

      System.out.println(parent.getWidth() + " " + parent.getHeight());

    Insets insets = parent.getInsets();
    int ncomponents = parent.getComponentCount();
    int nrows = rows;
    int ncols = cols;

    if (nrows > 0) {
        ncols = (ncomponents + nrows - 1) / nrows;
    } else {
        nrows = (ncomponents + ncols - 1) / ncols;
    }
    int w = 0;
    int h = 0;
    for (int i = 0 ; i < ncomponents ; i++) {
        Component comp = parent.getComponent(i);
        Dimension d = comp.getPreferredSize();
        if (w < d.width) {
            w = d.width;
        }
        if (h < d.height) {
            h = d.height;
        }
    }
    return new Dimension(insets.left + insets.right + ncols*w + (ncols-1)*hgap,
                         insets.top + insets.bottom + nrows*h + (nrows-1)*vgap);
  }
}

But what will be different if I change all the code above into the following?

public Dimension preferredLayoutSize(Container parent) {
  return new Dimension(parent.getWidth(), parent.getHeight());
}

Thank you.

Answer 1

Just press Method usage hotkey in your IDE...

com.sun.java.swing.plaf.motif - rt.jar
   -MotifFileChooserUI
      --getPreferredSize(JComponent)
   -MotifPopupMenuUI
      --getPreferredSize(JComponent)
com.sun.java.swing.plaf.windows - rt.jar
   -WindowsFileChooserUI
      --getPreferredSize(JComponent)
java.awt - rt.jar
   -Container
      --getPreferredSize() (2 matches)
      --preferredSize()
javax.swing.plaf.basic - rt.jar
   -BasicDesktopIconUI
      --getPreferredSize(JComponent)
   -BasicInternalFrameUI
      --getPreferredSize(JComponent)
   -BasicOptionPaneUI
      --getPreferredSize(JComponent)
javax.swing.plaf.metal - rt.jar
   -MetalFileChooserUI
      --getPreferredSize(JComponent)

Answer 2

preferredLayoutSize() is the default preferred size of Containers. That is, Container.getPreferredSize() will return that value if the container has a layout manager, unless it's overridden to return something else, or has been specified another value with setPreferredSize().

Changing it to

public Dimension preferredLayoutSize(Container parent) {
    return new Dimension(parent.getWidth(), parent.getHeight());
}

would mean that the container would make no attemp at taking in account the contents, but would always return its current size.