Classes with the same functionality should be placed in one package with the package name, which justifies their functionality and uses.
Example:
your.company.app.service
Must contain all service interfaces
your.company.app.service.impl
Must contain all service implementation classes
your.company.app.utils
Must contain all utility classes.
From the standard API
java.lang — basic language functionality and fundamental types
java.util — collection data structure classes
java.io — file operations
java.math — multiprecision arithmetics
java.nio — the New I/O framework for Java
java.net — networking operations, sockets, DNS lookups, ...
java.security — key generation, encryption and decryption
java.sql — Java Database Connectivity (JDBC) to access databases
java.awt — basic hierarchy of packages for native GUI components
javax.swing — hierarchy of packages for platform-independent rich GUI components
java.applet — classes for creating an applet
Cm