Import Statements in Java

Java ships with a very large collection of ready-made classes. Each one 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 library a shape that a programmer can navigate.

Keep one question in mind as you read: 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 of this article 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. The container's 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 analogy has a limit worth noticing. In Java, a package is a grouping of names and classes, and it does not itself perform any work. Its value is organizational: it tells you which classes belong together and gives each class a complete name that is unambiguous.

The java package is the clearest case of nesting. It holds several subpackages of its own. One of them is util, which contains a variety of utility classes, including the Scanner class that 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 diagram shows the nesting as a family tree. The subpackages io and net also live inside java. They are siblings of util, not children of it. That is why their full names are java.io and java.net rather than java.util.io or anything similar. The diagram shows only the parts the source describes; other classes and subpackages exist, but they are outside this material.

The procedure for names runs in two directions. When you build a name, you start from the outermost container and add each inner level in order. When you take a name apart, you read from the outside in. Each dot in a full name marks a boundary between a container and something it contains. The last segment is the class, and everything before it is the chain of packages that holds it.

The entire standard Java API is spread across a few top-level packages. 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.

Within java, the most basic package is java.lang. It contains fundamental classes such as String, Math, Integer, and Double. Because these are the classes Java programs use most often, you rarely need an import statement for them.

An import statement tells the compiler which package a class lives in, so the class can be used by its short name. The import line uses the full name you have been learning to build. The example below uses java.util.Scanner, a class from the java.util subpackage described in the source.