Using the Scanner Class

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 library a shape that a programmer can navigate.

Keep one question in mind as you read this article: 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. You have probably already used a class from java.util without thinking about its full name, so that is a good place to start.

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 clearest way to see nesting is to follow it level by level. The java package is the top of one branch. It holds several subpackages of its own, and it also contains several non-GUI packages along with the original AWT graphical user interface classes. The package named javax is a separate top-level package. It holds the classes used by the Swing graphical user interface, along with many other classes. Both java and javax contain subpackages.

Read the tree downward to find what a package contains, and read it upward to find a subpackage's parent. The diagram shows java as the parent of lang, util, io, and net. The javax package sits at the same top level as java, and the source describes it as holding Swing classes and more without listing its subpackages in detail.

A subpackage is a package contained in another package, and its full name includes every parent package. The util subpackage sits inside java, so its full name is java.util, with the parent name written first and a dot separating each level. The dot marks the boundary between a container and something it contains.

The process reverses when you take a name apart. Reading from the outside in, you move through each dot: java contains util, and util contains Scanner. Each dot marks one boundary, so a name with more dots describes a deeper level of nesting.

The subpackages io and net also live inside java. They are siblings of util, not children of it. Sibling subpackages such as java.io and java.util are both contained in java. This is why their full names are java.io and java.net rather than java.util.io or anything similar. A subpackage's name always reflects its actual parent, not the sibling beside it.

The source describes util as holding utility classes, including Scanner, and describes network communication as one of the jobs a class can perform. The names io and net suggest input-output and networking work, which fits the examples the source gives of tasks classes handle. The source itself does not list every class inside io or net, so treat those descriptions as guides to the purpose the name signals.

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 that Java programs use most often, you rarely need an import statement for them. The javax package is different: it is the home of the Swing graphical user interface classes, and it is not a subpackage of java.

The source notes that javafx is not a standard part of Java. When you meet a name that starts with javafx, do not assume it belongs to the standard library just because it looks similar to java or javax.