GroupLayout alignment

Question

Why does ...

Group horizontalGroup = groupLayout.createSequentialGroup()
    .addGroup(groupLayout.createParallelGroup()
            .addComponent(aTextArea)
            .addComponent(aButton,GroupLayout.Alignment.CENTER));

Group verticalGroup = groupLayout.createSequentialGroup()
    .addComponent(aTextArea)
    .addComponent(aButton);

... give this (as expected)

+--------------------------------------------------------------+
|+-------------------------------------------------------+     |
||                       aTextArea                       |     |
||                                                       |     |
|+-------------------------------------------------------+     |
|                        [aButton]                             |
|                                                              |
+--------------------------------------------------------------+

but ...

Group horizontalGroup = groupLayout.createSequentialGroup()
    .addGroup(groupLayout.createParallelGroup()
            .addComponent(aTextArea)
            .addGroup(groupLayout.createParallelGroup
                                         (GroupLayout.Alignment.CENTER)
                    .addComponent(aButton)));

Group verticalGroup = groupLayout.createSequentialGroup()
    .addComponent(aTextArea)
    .addComponent(aButton);

gives this?

+--------------------------------------------------------------+
|+-------------------------------------------------------+     |
||                       aTextArea                       |     |
||                                                       |     |
|+-------------------------------------------------------+     |
|[aButton]                                                     |
|                                                              |
+--------------------------------------------------------------+

Why does wrapping the aButton in a Parallel Group cause it to ignore the alignment? Adding the alignment also to the aButton itself has no effect.

I want to do something like this in order to have a mixture of groups of leading, trailing and central alignments beneath the aTextArea. It seems to me GroupLayout is rather limited IF it cannot cope with this.

Answer 1

This ...

.addGroup(groupLayout.createParallelGroup(GroupLayout.Alignment.CENTER)
    ...

specifies that components WITHIN THE GROUP are aligned centrally WITH EACH OTHER.

To align the Group (all elements within the Group) against the parent Group specify the alignment as a parameter of the addGroup() method not of createParallelGroup():

.addGroup(GroupLayout.Alignment.CENTER, groupLayout.createParallelGroup()
    ...

I've not found this documented but it seems the addGroup(GroupLayout.Group group) form (no alignment specification) applies a LEADING alignment.

In this way SequentialGroups can also be aligned against a parent group.