Sunday, June 14, 2009

3 types of swing "classes" in containment hierarchy

Every (yes Every) swing application can be /usefully/ described as a hierarchy of containment, even if 2-level. I only encountered 3-levels or more. A hierarchy always has exactly one INSTANCE of a top-level
class.

Simplified classification -- each everyday swing class is exactly one of these 3 --
1) top-level container class, usually JFrame.java
2) branch container class, like JPanel.java, JScrollPane.java,JTextPane.java, JSplitPane.java
3) leaf class, like jtable

As you can see, swing provides exactly 2 types of container classes -- top-level vs branch. Both types are few in number and so widely used they are unavoidable. Better know then well.

Layout managers mostly target a branch container. I doubt they target top-levels since the top-level usually contains just a single content pane, so no layout required, presumably. I feel WPF is similar -- The WPF top-level window holds exactly one container, which is a layout-control-panel.

Q: can we make do without branch containers?
A: I don't think so, since a jframe has a content pane which is a jpanel.

A top-level swing class is always heavyweight; a branch or leaf class is lightweight. See P 362 [[java cookbook]]

Q: can a branch class be used as top-level or leaf?
A: I don't think so. A branch container class can't be top-level (not heavyweight) and should not be a leaf because it doesn't display useful content by itself.

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