In my case I have two children of a SashForm, but the question applies to all Composites and different layouts.

class MainWindow {
    Sashform sashform;
    Tree child1 = null;
    Table child2 = null;

    MainWindow(Shell shell) {
        sashform = new SashForm(shell, SWT.NONE);
    }

    // Not called from constructor because it needs data not available at that time
    void CreateFirstChild() {  
        ...
        Tree child1 = new Tree(sashform, SWT.NONE);
    }

    void CreateSecondChild() {
        ...
        Table child2 = new Table(sashform, SWT.NONE);
    }    
}

I don't know in advance in what order these methods will be called. How can I make sure that child1 is placed on the left, and child2 on the right? Alternately, is there a way to change their order as children of sashform after they are created?

Currently my best idea is to put in placeholders like this:

class MainWindow {
    Sashform sashform;
    private Composite placeholder1;
    private Composite placeholder2;
    Tree child1 = null;
    Table child2 = null;

    MainWindow(Shell shell) {
        sashform = new SashForm(shell, SWT.NONE);
        placeholder1 = new Composite(sashform, SWT.NONE);
        placeholder2 = new Composite(sashform, SWT.NONE);
    }

    void CreateFirstChild() {  
        ...
        Tree child1 = new Tree(placeholder1, SWT.NONE);
    }

    void CreateSecondChild() {
        ...
        Table child2 = new Table(placeholder2, SWT.NONE);
    }    
}