SpringLayout not working on Java 5 (mac) VERTICAL_CENTER

Question

The documentation of SpringLayout says that it´s available since java 1.4, I have like 7 old macs that can´t be upgraded to any newer version of mac os 10.5 which leads me to have only java 1.5.0_30 at most. The problem is that I have an app that works with spring layout and I'm always getting this stack back trace:

Exception in thread "main" java.lang.NullPointerException
at javax.swing.SpringLayout$SpringProxy.getValue(SpringLayout.java:674)
at javax.swing.Spring$CompoundSpring.getValue(Spring.java:420)
at javax.swing.Spring$CompoundSpring.getValue(Spring.java:420)
at javax.swing.SpringLayout.layoutContainer(SpringLayout.java:1004)
at java.awt.Container.layout(Container.java:1401)
at java.awt.Container.doLayout(Container.java:1390)
at java.awt.Container.validateTree(Container.java:1473)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validate(Container.java:1448)
at java.awt.Window.show(Window.java:516)
at java.awt.Component.show(Component.java:1317)
at java.awt.Component.setVisible(Component.java:1270)

Is there any solution, any jar or something similar?

Please avoid answers like "Upgrade you computers"

UPDATE: I dicovered that there is SpringLayout on Java, but you cant use VERTICAL_LAYOUT with SOUTH. I don´t know why.

UPDATE 2: Following the instruction of Ian I did the following code and it is still not working on Java 5

springLayout.putConstraint(SpringLayout.NORTH, myImage, 
            Spring.sum(springLayout.getConstraint(SpringLayout.SOUTH, myPanel) ,
                    Spring.sum(Spring.scale(Spring.minus(springLayout.getConstraint(SpringLayout.HEIGHT, myImage)),0.5f),

                            Spring.scale(Spring.minus(springLayout.getConstraint(SpringLayout.HEIGHT, myPanel)),0.5f)
                    )
             ), SpringLayout.NORTH, myPanel);

UPDATE 3: Replacing springLayout.getConstraint(SpringLayout.HEIGHT, myImage) with springLayout.getConstraints(myImage).getHeight() it worked perfectly fine. :)

Answer 1

The VERTICAL_CENTER constraint type was introduced by Java 6 which is why it doesn't work for you in Java 5. But the invariants specify that

VERTICAL_CENTER = NORTH + HEIGHT/2
SOUTH = NORTH + HEIGHT

Substituting and re-arranging these equations we get

                     NORTH = SOUTH - HEIGHT

           VERTICAL_CENTER = SOUTH - HEIGHT/2
VERTICAL_CENTER + HEIGHT/2 = SOUTH
                  HEIGHT/2 = SOUTH - VERTICAL_CENTER
                    HEIGHT = 2*(SOUTH - VERTICAL_CENTER)

So if you are currently setting SOUTH and VERTICAL_CENTER then you can achieve the same effect by setting the height to a spring of size 2*(SOUTH - VERTICAL_CENTER). So replace

constr.putConstraint(SpringLayout.SOUTH, southSpring);
constr.putConstraint(SpringLayout.VERTICAL_CENTER, vcSpring);

with

constr.putConstraint(SpringLayout.SOUTH, southSpring);
constr.setHeight(
  Spring.scale(Spring.sum(southSpring, Spring.minus(vcSpring)),
               2.0f));