Table layout manager

TableLayout is a Java layout manager that tries to be both powerful and easy to use, unlike the standard layout managers supplied with the JDK AWT, which seem to be neither! It is supplied as a single pure Java source file, and has now been updated so no deprecated methods are used under JDK1.1.

The basic metaphor adopted is that of a table, laid out as rows and columns, similar to tables in HTML. Unlike the AWT GridLayout, each row and column is sized separately, and features such as row and column spanning are supported.

Basic use

The only mandatory parameter that needs to be supplied when creating a table layout is the number of columns. Adding successive components to a container will fill the first row from left to right until the last column is reached, then proceed with the next row down and so on.

For example, the following code will lay out a 2x2 table:

	setLayout(new TableLayout(2));
	add(new Label("Top left"));
	add(new Label("Top right"));
	add(new Label("Bottom left"));
	add(new Label("Bottom right"));

Cell alignment

Within each cell of the table, components can keep their preferred size and be aligned to any corner or side of their cell or be centred, or they can be made to expand and contract, filling the cell horiztontally or vertically (or both). A default alignment for the whole table can be specified in the TableLayout constructor, and individual components can be overridden when their added to the container. For example,

	setLayout(new TableLayout(3, "W")); // left aligned, by default
	add(new Label("Left"));
	add("FH", new Label("Filled horizontally"));
	add("SE", new Label("Bottom right"));

Spanning and positioning

You can control which cell a component goes in by skipping cells, forcing the component into a given column (which will skip cells or go to the next row, as required), by specifying SKIP=n or COL=n tags. You can also span more than one row or column with the RS=n and CS=n tags:

	setLayout(new TableLayout(2));
	add("CS=2", new Label("Long label across the top"));
	// bottom left is skipped
	add("SKIP=1", new Label("Bottom right"));

Cell sizing and weighting

TableLayout is well behaved with respect to a components minimum and preferred sizes. The preferred size of a table is big enough to accommodate all its contents at their own preferred size. If the table is resized then rows and columns expand to fill the table. Rows always keep their relative proportions, and columns do so too by default. However, it is also possible to specify a weighting factor for each column using the WT=n flag, in which case any extra space is distributed according to the column's weight. The weights of all the columns are added up and then divided by the total to give relative weights. By default, the weight of each column is its preferred size, so they expand proportionately. Setting the weight of a column to 0 means it doesn't expand and always keeps its preferred size. Note that it is possible to change the column weightings on different rows, in which case the columns may stop lining up.

TableLayout Reference

Constructors

TableLayout(int cols, String alignment, int hgap, int vgap);
TableLayout(int cols, String alignment);	// hgap=0, vgap=0
TableLayout(int cols);			// alignment="C"

cols	Number of columns, used when adding components to tell when to go to the next row
alignment	Default alignment for cells if not specified at the time of adding the component
hgap	Horizontal gap between cells and at edge (in pixels)
vgap	Vertical gap between cells and at edge (in pixels)

Alignment flags

N,NE,E,SE, S,SW,W,NW	align to top, top right, etc. of cell
FH	fill horizontally (may be combined with N, C, S or FV to specify vertical behaviour)
FV	fill vertically (may be combined with E, C, W or FH to specify horizontal behaviour)
F	fill entire cell
C	centre component within cell
RS=n	span a given number of rows
CS=n	span a given number of columns
COL=n	force to a given column (where 0 is the first column, ie. start a new row early)
SKIP=n	skip a given number of columns
WT=n	specify a horizontal weighting factor, used to decide how much to expand the width of each column when there is extra space. Applies to this and subsequent rows. WT=-1 means use the preferred size of each column as the weighting factor; if all columns have weight -1 each column grows proportionately (the default). A weight of 0 means don't expand the width above its preferred size.

Most flags can be combined together, eg. "RS=2,FH". If more than one alignment flag is specified they must be separated with commas. Case is significant, and spaces are not allowed.

Rolf Howarth
Last updated: 8 April 1998