If you are using Eclipse on Ubuntu, the default color schema of Ubuntu uses white foreground on a black background for the tooltip colors. These colors are used by the Javadoc view and Declaration view and also for the tooltip hovers.
You can change these colors by editing editing the /usr/share/themes/Ambiance/gtk-2.0/gtkrc file. The relevant properties are tooltip_fg_color and tooltip_bg_color. Just search for these values, the position changes sometimes between releases, currently they are at the very top of the file.
The following setting uses more reasonable colors.