Fonts and the Font Class in Java

Drawing text in a Java window looks simple: you call drawString() with a string and a pair of coordinates. The difficulty is that a naive guess at those coordinates will almost always look wrong. A string that starts at x equals 100 occupies a different width on screen depending on the font and the characters in it, and the same is true vertically. If you want text centered in a window, or want two lines to sit at a predictable distance apart, you need to know how large the rendered text will be before you draw it.

Every font carries a family of vertical measurements, and they are all defined relative to one reference line. The baseline is the line that the letters stand on. The ascent is how far the tallest characters reach above that baseline. The descent is how far characters with tails, such as the letter y, reach below it. The suggested distance from one baseline to the next for single-spaced text is called the lineheight of the font. Between two adjacent lines there is also extra blank space called leading.

The four numbers do not simply add up to the lineheight. Ascent and descent together cover the characters on one line, but they leave out the gap that keeps the tops of one line's characters from colliding with the tails of the line above. That gap is the leading. So lineheight equals ascent plus descent plus leading, and the lineheight is not just the sum of the ascent and the descent.

Because the font's measurements are anchored to the baseline, drawString() treats its y-coordinate as the baseline. The letters are drawn so that they rest on that line. Their tallest parts extend upward from it by the ascent, and any tails extend downward from it by the descent. The top of the string therefore sits above the y-coordinate you passed in, not at it. This is why a value that looks like 'the top' in your head produces text that appears to float too low or too high.

A Font object describes the typeface and size you want. A FontMetrics object is always tied to one particular font and answers questions about text in that font only. To obtain one, ask the graphics context for the metrics of a font. If you already have a Graphics object named g and a Font object named font, the call is g.getFontMetrics(font). The result is commonly stored in a variable named fm, and that variable then provides the measuring methods.

Each method answers one specific question. getAscent(), getDescent(), getLeading(), and getHeight() describe vertical structure. charWidth(ch) gives the width of one character drawn in that font, and stringWidth(str) gives the width of a string drawn in that font. The width methods depend on the font, so the same string produces different widths when the font changes. That dependency is why measurements must be taken from the font you are actually going to draw with.

The main use of these measurements is centering. The procedure has four moves. First find the center of the component. Next find the size of the string. Then move back from the center by half the string's width to find the left edge, and move up from the center by half the string's height to find the top edge. The last move is the one that trips people up: drawString() needs the baseline, not the top, so you move down from the top by the ascent.

The height used for vertical centering depends on the characters in the string. The source example Hello World has no tails, so the ascent alone describes its visible height. If the string were Goodbye World, the letter y has a tail, so the vertical size must include the descent as well, and the height becomes the ascent plus the descent.