Lecture 04: Namespaces, Packages and the CLASSPATH

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Namespaces

Many programming languages, including Java incorporate the idea of a “namespace”. A namespace is a way to provide context to a particular name.

For a real-world analogy, you might think of a person’s name, say, “Zachary”. There might be more than one in this class, so we add some context (a surname, or a student ID, or an address, or all of the above) to disambiguate which we mean.

This is very similar to the idea of a variable’s scope in Java, but slightly different, as it’s how we precisely name and identify classes.

For example, we all write System.out.println() all the time, and we all know that System is probably a class, since it starts with a capital letter. Where does it come from, though?

Packages

It’s part of the java.lang package. Java organizes classes into packages; which are a hierarchical sequence of tokens (words), separated by dots. The built-in parts of the Java standard library all are part of the “java.” package, though it’s further subdivided.

For example, the aforementioned java.lang package defines the classes that are fundamental to the design of the language itself: things like System and String are defined here.

http://docs.oracle.com/javase/8/docs/api/java/lang/package-summary.html#package.description

By default, things in this namespace are automatically “imported” into the local namespace. That is, you don’t have to type java.lang.String to declare a String (though you can); String suffices.

Interestingly, it’s not against the rules to define your own System class. But it’s like if you have both an instance variable named x and a local variable named x. By default, Java assumes you want to “more local” one.

int x = 5;

void test() {
  x = 3;
  System.out.println(x);
  System.out.println(this.x);
}

To get the “outer” one, you need to prefix it with this., which tells Java you want the current instance variable with the same name.

Similarly, with a class, you need to fully specify the class if you want access to it. Inside your custom System class, if you want to access the “normal” System.out.println, you’ll need to refer to it by its full name, java.lang.System.out.println.

java and javax are reserved by the JVM for built-in and extensions classes, but much like anyone can register a domain name on the Internet, anyone can declare a package namespace. There’s a loose convention that you should use your reverse domain name as a prefix (for example, if our department released a package for autograding, we might put it in the edu.umass.cs.autograder package). But many modern java packages declare a top-level namespace – you’ll see this in Lab 4, where the Processing project just prefixes their namespace with “processing”. In practice, projects that do similar things don’t usually have the same name, and/or agree to avoid namespace collisions.

Importing packages

Sometimes you want to use something not in the current namespace. Then you need to “import” it.

For example, if I want to print a random number between one and six:

public class Die {
  public static void main(String[] args) {
    Random r = new Random();
    System.out.println(r.nextInt(6) + 1);
  }
}

because Random is not in the namespace. But I can use an import statement: import java.util.Random; to add it to the namespace.

Finding packages

Where does Java look for packages? By default the JVM has access to a set of “built-in” packages, that form the Java Platform API:

https://docs.oracle.com/javase/8/docs/api/index.html?overview-summary.html

Again, mostly in the java and javax namespace, but also some others.

But where do these classes, that is, the code, actually live? On my machine, a big chunk of them live in /Library/Java/JavaVirtualMachines/jdk1.8.0_102.jdk/Contents/Home/jre/lib in the JARs there.

JARs are essentially ZIPfiles of compiled Java classes with some extra stuff (a Java-specific manifest, describing their contents, that the JVM knows how to read). Let’s take a look in rt.jar. Hey look! Our friends System and String!

You may have noticed that the file, say String.class (which is the compiled representation of String.java) lives in a directory java/lang/. That looks a lot like java.lang., doesn’t it?

Not a coincidence! The JVM requires that packages map to (that is, directly correspond to) directories with the same name(s), and that classes map to .class files within those directories.

But there’s still a piece of the puzzle missing. How does the JVM know where to look for these directories? How did it know, for example, that /Library/Java/JavaVirtualMachines/jdk1.8.0_102.jdk/Contents/Home/jre/lib/rt.jar was a place to search?

CLASSPATH and friends

There are three mechanisms the JVM uses. One is under your control: the “CLASSPATH”. The other two you can’t (easily) change – there is a “bootstrap CLASSPATH” and an “extensions directory”.

The latter two are configured when your JVM is installed, and they contain classes that are part of the JRE, JDK platform, and vendor-distributed extensions.

But the CLASSPATH you do control. If you run from within Eclipse, you can add JARs (and directories) to the CLASSPATH by selecting the appropriate menu items, either “Build Path -> Add to Build Path” for JARs or “Build Path -> Use as Source Folder” for a directory. You can also manage the Build Path (“Configure Build Path”) and view what’s on it, which is how I found the rt.jar I showed you earlier.

Putting your own classes into packages

If you want to put a class into a package, like, for example, you want to move our Die class into the nerdy.gaming package, you need to explicitly declare the package at the top of the file, and move it into an appropriate directory, in order for it to compile and be recognized by the JVM. Eclipse will prompt you to do one if you do the other.

Class exercise

Suppose I had the following directory hierarchy:

src/marc/liberatore/Banana.java
support/marc/liberatore/Smoothie.java

and the following code in Smoothie.java:

class Smoothie {
  public static void main(String[] args) {
    System.out.println("Add a " + new Banana());
  }
}

What directories need to be on the classpath in order for this to compile?
What package should we declare at the top of this file?
Do we need to import anything? If so, what?