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javaswinglayoutjpanellayout-manager

java Swing: how to keep one component left aligned with another component centered with respect to parent

Say I have two components, A and B, in a JPanel. I want Component A to stay left aligned, while Component B does its best to stay in the middle of the panel. I mocked up the following demo (sorry for the quality, I made it in paint): Component movement demo

What I am doing now is using a GridBagLayout on the JPanel, and keeping A left aligned while keeping B centered, but B stays centered within the 2nd column, so it is centered in the space remaining after A is placed, instead of centered with respect to the panel as a whole.

I cannot use any 3rd party libraries for this. Is there a way to do this using pure Swing?

Edit:

Firefly's answer is correct (as marked) but I created an SSCCE showing my original problem (first row), Hovercraft Full Of Eels' attempted solution (second row), and the Firefly's correct solution (third row). I figured it can't hurt to post it:

package stackoverflow;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.LayoutManager2;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class StackOverflowTest extends JFrame
{

   public StackOverflowTest()
   {
      super("Stack Overflow Test");
      this.setDefaultCloseOperation(EXIT_ON_CLOSE);

      JPanel testPanel = new JPanel(new GridLayout(3, 1));

      // set up grid bag layout example
      JPanel gridBagPanel = new JPanel(new GridBagLayout());
      GridBagConstraints gridBagConstraints = new GridBagConstraints();
      gridBagConstraints.gridx = 0;
      gridBagConstraints.gridy = 0;
      gridBagConstraints.anchor = GridBagConstraints.LINE_START;
      gridBagPanel.add(getA(), gridBagConstraints);
      gridBagConstraints = new GridBagConstraints();
      gridBagConstraints.gridx = 1;
      gridBagConstraints.gridy = 0;
      gridBagConstraints.anchor = GridBagConstraints.CENTER;
      gridBagConstraints.weightx = 1.0;
      gridBagPanel.add(getB(), gridBagConstraints);
      testPanel.add(gridBagPanel);

      // set up border layout panel 
      JPanel borderPanel = new JPanel(new BorderLayout());
      borderPanel.add(getA(), BorderLayout.LINE_START);
      JPanel flowPanel = new JPanel(new FlowLayout(FlowLayout.CENTER, 0, 0));
      borderPanel.add(flowPanel, BorderLayout.CENTER);
      flowPanel.add(getB());
      testPanel.add(borderPanel);

      // set up sly493 layout panel
      JPanel sly493LayoutPanel = new JPanel(new Sly493LayoutManager());
      sly493LayoutPanel.add(getA(), Sly493LayoutManager.LEFT);
      sly493LayoutPanel.add(getB(), Sly493LayoutManager.CENTERED);
      testPanel.add(sly493LayoutPanel);

      // set up panel to act as the midpoint marker
      JPanel midpointMarkerPanel = new JPanel()
      {
         @Override
         public void paintComponent(Graphics g)
         {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;

            g2.setColor(Color.BLACK);

            int w = getWidth();
            int h = getHeight();

            int x = w / 2;
            g2.drawLine(x, 0, x, h);
            g2.drawLine(x, 0, x - (h / 5), (h / 5));
            g2.drawLine(x, 0, x + (h / 5), (h / 5));
         }
      };
      midpointMarkerPanel.setPreferredSize(new Dimension(1, 50));

      // setup up content pane
      JPanel contentPane = new JPanel(new BorderLayout());
      contentPane.add(testPanel, BorderLayout.NORTH);
      contentPane.add(midpointMarkerPanel, BorderLayout.CENTER);
      this.setContentPane(contentPane);

      pack();
   }

   private JPanel getA()
   {
      JPanel aPanel = new JPanel(new BorderLayout());
      JLabel aLabel = new JLabel("A", JLabel.CENTER);
      aLabel.setFont(aLabel.getFont().deriveFont(Font.BOLD, 36));
      aPanel.add(aLabel, BorderLayout.CENTER);
      aPanel.setBackground(Color.RED);
      aPanel.setPreferredSize(new Dimension(50, 50));
      return aPanel;
   }

   private JPanel getB()
   {
      JPanel bPanel = new JPanel();
      JLabel bLabel = new JLabel("B", JLabel.CENTER);
      bLabel.setForeground(Color.WHITE);
      bLabel.setFont(bLabel.getFont().deriveFont(Font.BOLD, 36));
      bPanel.add(bLabel, BorderLayout.CENTER);
      bPanel.setBackground(Color.BLUE);
      bPanel.setPreferredSize(new Dimension(50, 50));
      return bPanel;
   }

   public static void main(String[] args)
   {
      SwingUtilities.invokeLater(new Runnable()
      {

         @Override
         public void run()
         {
            new StackOverflowTest().setVisible(true);
         }
      });
   }

   private static class Sly493LayoutManager implements LayoutManager2
   {

      public static final Integer LEFT = 0;

      public static final Integer CENTERED = 1;

      private Component leftComponent;

      private Component centeredComponent;

      @Override
      public void addLayoutComponent(String name, Component comp)
      {
      }

      @Override
      public void removeLayoutComponent(Component comp)
      {
         if (leftComponent == comp)
         {
            leftComponent = null;
         }
         else if (centeredComponent == comp)
         {
            centeredComponent = null;
         }
      }

      @Override
      public Dimension preferredLayoutSize(Container parent)
      {
         Dimension d = new Dimension();
         for (Component c : parent.getComponents())
         {
            //wide enough to stack the left and center components horizontally without overlap
            d.width += c.getPreferredSize().width;
            //tall enough to fit the tallest component
            d.height = Math.max(d.height, c.getPreferredSize().height);
         }
         return d;
      }

      @Override
      public Dimension minimumLayoutSize(Container parent)
      {
         return preferredLayoutSize(parent);
      }

      @Override
      public void layoutContainer(Container parent)
      {
         //in this method we will:
         //1) position the left component on the left edge of the parent and center it vertically
         //2) position the center component in the center of the parent (as long as it would not overlap
         //the left component) and center it vertically

         int leftComponentWidth = leftComponent.getPreferredSize().width;
         int leftComponentHeight = leftComponent.getPreferredSize().height;
         int centeredComponentWidth = centeredComponent.getPreferredSize().width;
         int centeredComponentHeight = centeredComponent.getPreferredSize().height;

         leftComponent.setBounds(0, (parent.getHeight() - leftComponentHeight) / 2, leftComponentWidth, leftComponentHeight);
         int leftComponentRightEdge = leftComponent.getX() + leftComponent.getWidth();
         int centerComponentLeftEdge = (parent.getWidth() - centeredComponentWidth) / 2;
         int centerComponentTopEdge = (parent.getHeight() - centeredComponentHeight) / 2;

         if (leftComponentRightEdge >= centerComponentLeftEdge)
         {
            //Center component will "do its best" to remain in the center
            //but it will not do so if it would cause it to overlap the left component
            centerComponentLeftEdge = leftComponentRightEdge;
         }

         centeredComponent.setBounds(centerComponentLeftEdge,
                                     centerComponentTopEdge,
                                     centeredComponentWidth,
                                     centeredComponentHeight);
      }

      @Override
      public void addLayoutComponent(Component comp, Object constraints)
      {
         if (LEFT.equals(constraints))
         {
            if (leftComponent != null)
            {
               throw new IllegalStateException("A left component has already been assigned to this layout.");
            }
            leftComponent = comp;
         }
         else if (CENTERED.equals(constraints))
         {
            if (centeredComponent != null)
            {
               throw new IllegalStateException("A centered component has already been assigned to this layout.");
            }
            centeredComponent = comp;
         }
         else
         {
            throw new IllegalStateException("Unexpected constraints '" + constraints + "'.");
         }
      }

      @Override
      public Dimension maximumLayoutSize(Container target)
      {
         return new Dimension(Integer.MAX_VALUE, Integer.MAX_VALUE);
      }

      @Override
      public float getLayoutAlignmentX(Container target)
      {
         return 0;
      }

      @Override
      public float getLayoutAlignmentY(Container target)
      {
         return 0;
      }

      @Override
      public void invalidateLayout(Container target)
      {

      }
   }
}
Solution

  • If I'm understanding your needs correctly, you want B centered relative to the parent as a whole, not centered in the space left over after A is positioned. That makes this problem interesting and after testing the other suggested answers, I don't believe they can meet that requirement.

    I'm having trouble thinking of a way to combine the built-in layout managers in a way that would achieve that. So, I've hacked up a custom implementation of LayoutManager2.

    The following executable example may meet your needs. The implementation is quick and dirty and is in no way an example of a good generalized layout manager, but it appears to meet your requirements and behaves like your drawings made me think it should. I interpreted your requirement that "B does its best to stay in the middle of the panel" to mean that B should try to remain centered relative to the panel as a whole, but not at the expense of overlapping A.

    package com.example;

import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.LayoutManager2;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Example  {

    public Example() {

        JPanel a = new JPanel();
        a.setBackground(Color.RED);
        a.setPreferredSize(new Dimension(128, 128));

        JPanel b = new JPanel();
        b.setBackground(Color.BLUE);
        b.setPreferredSize(new Dimension(128, 128));

        JPanel panel = new JPanel(new Sly493LayoutManager());
        panel.add(a, Sly493LayoutManager.LEFT);
        panel.add(b, Sly493LayoutManager.CENTERED);

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        new Example();
    }

    private static class Sly493LayoutManager implements LayoutManager2 {

        public static final Integer LEFT = 0;

        public static final Integer CENTERED = 1;

        private Component leftComponent;

        private Component centeredComponent;

        @Override
        public void addLayoutComponent(String name, Component comp) { }

        @Override
        public void removeLayoutComponent(Component comp) { 
            if (leftComponent == comp) {
                leftComponent = null;
            } else if (centeredComponent == comp) {
                centeredComponent = null;
            }           
        }

        @Override
        public Dimension preferredLayoutSize(Container parent) {
            Dimension d = new Dimension();
            for (Component c : parent.getComponents()) {
                //wide enough to stack the left and center components horizontally without overlap
                d.width += c.getPreferredSize().width;
                //tall enough to fit the tallest component
                d.height = Math.max(d.height, c.getPreferredSize().height);
            }
            return d;
        }

        @Override
        public Dimension minimumLayoutSize(Container parent) {
            return preferredLayoutSize(parent);
        }

        @Override
        public void layoutContainer(Container parent) {     
            //in this method we will:
            //1) position the left component on the left edge of the parent and center it vertically
            //2) position the center component in the center of the parent (as long as it would not overlap
            //the left component) and center it vertically

            int leftComponentWidth = leftComponent.getPreferredSize().width;
            int leftComponentHeight = leftComponent.getPreferredSize().height;
            int centeredComponentWidth = centeredComponent.getPreferredSize().width;
            int centeredComponentHeight = centeredComponent.getPreferredSize().height;

            leftComponent.setBounds(0, (parent.getHeight() - leftComponentHeight) / 2, leftComponentWidth, leftComponentHeight);
            int leftComponentRightEdge = leftComponent.getX() + leftComponent.getWidth();
            int centerComponentLeftEdge = (parent.getWidth() - centeredComponentWidth) / 2;
            int centerComponentTopEdge = (parent.getHeight() - centeredComponentHeight) / 2;        

            if (leftComponentRightEdge >= centerComponentLeftEdge) {
                //Center component will "do its best" to remain in the center
                //but it will not do so if it would cause it to overlap the left component
                centerComponentLeftEdge = leftComponentRightEdge;
            }

            centeredComponent.setBounds(centerComponentLeftEdge, 
                    centerComponentTopEdge, 
                    centeredComponentWidth, 
                    centeredComponentHeight);
        }

        @Override
        public void addLayoutComponent(Component comp, Object constraints) {
            if (LEFT.equals(constraints)) {
                if (leftComponent != null) {
                    throw new IllegalStateException("A left component has already been assigned to this layout.");
                }
                leftComponent = comp;
            } else if (CENTERED.equals(constraints)) {
                if (centeredComponent != null) {
                    throw new IllegalStateException("A centered component has already been assigned to this layout.");
                }
                centeredComponent = comp;
            } else {
                throw new IllegalStateException("Unexpected constraints '" + constraints + "'.");
            }
        }

        @Override
        public Dimension maximumLayoutSize(Container target) {
            return new Dimension(Integer.MAX_VALUE, Integer.MAX_VALUE);
        }

        @Override
        public float getLayoutAlignmentX(Container target) {
            return 0;
        }

        @Override
        public float getLayoutAlignmentY(Container target) {
            return 0;
        }

        @Override
        public void invalidateLayout(Container target) {

        }       
    }
}