The Java Standard Library Overview

Java ships with a very large collection of ready-made classes. Each class offers tools for a specific job, such as reading user input, doing arithmetic, or communicating over a network. If every one of those classes sat in a single undivided list, finding the right one would be slow and confusing. Java solves this problem with packages, which give the standard library a shape that a programmer can navigate.

As you read this article, keep one question in mind: when you see a name such as java.util, what does each part of that name tell you about where a class lives? By the end you should be able to answer that question for any standard package you meet.

Think of a package as a labeled container that holds related material. Its job is to keep things that belong together in one place. Inside a package you will find classes, and a package may also hold other packages. That second possibility is what lets Java organize code at more than one level of detail.

The container analogy has a limit worth noticing. In Java, a package is a grouping of names and classes, and it does not perform any work by itself. Its value is organizational. It tells you which classes belong together and gives each class a complete name that is unambiguous, so two classes with the same short name can live in different packages without being confused.

The diagram shows the same nesting as a family tree. A package can hold classes at its own level and also hold subpackages, which are packages one level down. The java package holds several subpackages, and util is one of them. Scanner sits inside util, so it is contained at two levels: inside java.util, which is itself inside java.

A subpackage is a package contained in another package, and its full name includes every parent package. The java package holds a subpackage called util, which contains utility classes, including the Scanner class you have probably already used to read input. Because util sits inside java, its name is not just util. Its full name is java.util, with the parent name written first and a dot separating each level.

The entire standard Java API is spread across a few top-level packages, and two of them matter most at the start. The package named java is the largest of the group. It contains several non-GUI packages, along with the original AWT graphical user interface classes. The package named javax also appears in the standard library. It holds the classes used by the Swing graphical user interface, along with many other classes. Both java and javax contain subpackages.

Several of the most useful packages live inside java. Each one groups classes for a different kind of job. The sections below describe them in turn, and the comparison that follows puts them side by side.