Classes and Objects in Java

Java's standard library is organized into packages, and the names of those packages often look longer than they first appear. The package that provides the Scanner class, for example, is not called util on its own. To read a package name correctly, you need to understand how one package can sit inside another. This article traces package names from the outside in, so that by the end you can read a name such as java.io and say exactly where it lives.

Before reading on, try this quick check. Suppose you see the name java.util and someone asks you which package is the outer one. Decide your answer before reading the next section, then compare it with the walkthrough that follows.

Read a full package name from left to right, and each dot marks one step further into the structure. Take java.util. The leftmost part, java, is the outer package. The part after the dot, util, is a package that sits inside it. Reading the name from the inside outward would lose the relationship, so keep the direction in mind: the name records the chain of containers from the outermost one down to the one you are looking at.

The diagram above shows the three visible parts of the name. The java segment is the container, the dot marks the step inward, and util is the thing held inside. Reading java.util therefore tells you that util is contained within java.

Now apply the same reading to java.io and java.net. Each has java on the left and a single additional segment on the right. In every case, the segment after the dot names a package that lives directly inside java. None of these three names needs more than one dot, because each sits exactly one level below java in the structure.

This is the central fact of this section. The contents of a package are not limited to classes. A single package may hold ordinary classes at its own level while also holding other packages inside it. The terminology stays simple: the container is the containing package, and the thing inside it is the subpackage.

The source gives two top-level packages as examples of this structure. The java package and the javax package both contain subpackages. The java package has many of them, and the source names java.util, java.io, and java.net as examples. It notes that java.util is a subpackage of java. Because the same containment rule applies at every level, a subpackage can itself contain other packages, although the source's examples stay at the level directly below java.

Read the diagram from top to bottom and it matches the naming rule from earlier. Each child box carries its parent's name followed by a dot. The java.util box holds utility classes, and the source connects it to the Scanner class discussed earlier in the textbook. The java.io box provides facilities for input and output, and the java.net box deals with network communication. The diagram shows only what the source describes. The source introduces its list of subpackages with the words such as, so java contains more subpackages than the four shown here.

Once you understand the containment chain, you can build a full package name from the position of a package in the hierarchy. Start at the outermost package and append each inner package, separated by dots, until you reach the one you want. This worked example extends the pattern of java.io and java.net one level deeper.

The source calls java.lang the most basic package. It contains fundamental classes, and the source lists four of them: String, Math, Integer, and Double. The source singles out this package because many other parts of Java programming rely on these kinds of classes.

The name java.lang still follows the containment pattern. It is written with java on the left, so it is one of the packages held inside java, alongside java.util, java.io, and java.net. Being the most basic package does not change how its name is built; it only describes what the package contains.

Several errors come up often when people first read package names. Each one usually comes from treating a package name as a flat label rather than as a path through containers.