Its seems impossible to center a fixed size image on a container

Question

Hi,
I have a fixed sized JPanel that I want to add to the center of a container. I would like my JPanel's center to be fixed to the center of the container.
I have tried to use GridBagLayout with a GridbagConstraint like follows:
GridBagLayout gridbag = new GridBagLayout();
getContentPane().setLayout(gridbag);
GridBagConstraints c = new GridBagConstraints();
c.fill = c.CENTER;
SubPanel subPanel = new SubPanel();
gridbag.setConstraints(subPanel, c);
getContentPane().add(subPanel);
setSize(400, 500);
setVisible(true);
But this results in the upper left corner of my JPanel being put in the center of the container. Does anyone know how to get the center of my JPanel being placed in the center?
The following two suggestions does not work:
1) This solution put my panel two the right of the center:
this.setLayout( new Flowlayout( Flowlayout.CENTER ) );
this.add( subPanel );
2) This does only work in the special case that the width of my panel is a third of the width of the container:
Use GridLayout and set the grids, when you add your panel it Should size the cells equal to your largest component which would be the JPanel, then just add blank JLabels to the other cells and add your subpanel to the center cell.

Answer 1

this is some sample code. It has not been compiled like this, but there should only be minor problems. if it does not work, just post a message...
ok, here we go:
import javax.swing.*;
import java.awt.*;
public class CenterLayout implements LayoutManager2 {
// make sure that only one child is added to the container
public void addLayoutComponent(Component comp, Object constraints) {
if( comp.getParent().getComponentCount() > 1 )throw new RuntimeException("only one child allowed for this layout");
// as big as you want...
public Dimension maximumLayoutSize(Container target) {
return new Dimension( Integer.MAX_VALUE, Integer.MAX_VALUE);
public float getLayoutAlignmentX(Container target) {
return .5f;
public float getLayoutAlignmentY(Container target) {
return .5f;
public void invalidateLayout(Container target) {
public void addLayoutComponent(String name, Component comp) {
addLayoutComponent(comp, name);
public void removeLayoutComponent(Component comp) {
// this assumes that the preferred size is the preferred size of the single child plus insets...
public Dimension preferredLayoutSize(Container parent) {
Insets insets = parent.getInsets();
int w = insets.left + insets.right;
int h = insets.top + insets.bottom;
if (parent.getComponentCount() > 0 ) {
Dimension childSize = parent.getComponent(0).getPreferredSize();
return new Dimension(childSize.width + w, childSize.height + h );
} else {
return new Dimension(w,h);
// this assumes that the minimum size is only the minimum size of the single child. if you want insets
// considered for minimum size, add them here
public Dimension minimumLayoutSize(Container parent) {
if (parent.getComponentCount() > 0 ) {
return parent.getComponent(0).getMinimumSize();
} else {
return new Dimension(0,0);
// this is the important stuff, calculating childs
// position. It always sets the child to its
// preferred size and centers it.
// there may be better strategies handling the case
// that the containers dimension are smaller than its
// childs dimensions. you may peek, if the child may be
// scaled to fit in considering the minimum size of the child.
// once started, layoutmanager can be improved with
// more sophisticated algorithms ever on, go and paly
// around with it :-)
public void layoutContainer(Container parent) {
if( parent.getComponentCount() > 0 ){
Component child = parent.getComponent(0);
Insets insets = parent.getInsets();
Dimension size = parent.getSize();
size.width = size.width - insets.left - insets.right;
size.height = size.height - insets.top - insets.bottom;
// put in child scaling here, if needed
child.setSize(child.getPreferredSize());
int x = insets.left + (int) ( (size.width - child.getWidth() )/2);
int y = insets.top + (int) ( (size.height - child.getHeight() )/2);
child.setLocation(x,y);
public static void main(String[] args ) {
JFrame f = new JFrame();
f.getContentPane().setLayout(new CenterLayout() );
f.getContentPane().add( new JButton("hello") );
f.setSize( 200, 200 );
f.setVisible(true);
}