cscope

Wikipedia defines cscope as "A console mode or text-based graphical interface that allows software developers to search source code".
http://en.wikipedia.org/wiki/Cscope

It is often used on very large projects to find source code, functions, declarations, definitions and regular expressions given a text string.

The official site for cscope is http://cscope.sourceforge.net/
cscope can be downloaded from http://cscope.sourceforge.net/#downloads

What it does:

• Allows searching code for:

all references to a symbol

global definitions

functions called by a function

functions calling a function

text string

regular expression pattern

a file

files including a file

• Curses based (text screen)
• An information database is generated for faster searches and later reference
• The fuzzy parser supports C, but is flexible enough to be useful for C++ and Java, and for use as a generalized 'grep database' (use it to browse large text documents!)
• Has a command line mode for inclusion in scripts or as a backend to a GUI/frontend
• Runs on all flavors of Unix, plus most monopoly-controlled operating systems.

GUI
kscope is a GUI for cscope on KDE Environment.

Tutorial
How to use cscope is a nice tutorial on using cscope. This explains how cscope can be used for java code.