2.6. Further Important Notes

2.6.1. Setting the Class Path

Both javac and java allow you to specify the classpath using the -cp or -classpath command-line option. The usual syntax is as follows:

-cp some/path

If more than one default package is needed, then a colon : can be used to separate each path in a list of multiple paths:

-cp path1:path2

Each path can be a path to a directory or a .jar file (usually used for third-party libraries).

VERY IMPORTANT NOTE: The classpath should always point to a default package for compiled code. If you are compiling a .java file that depends on an already compiled class, then you will need to specify the classpath to the corresponding default package for that dependency when invoking javac.

2.6.2. Import Statements

In Java, you do not have to import classes that are in the same package. However, it’s interesting to note that import statements are never required in Java. We just use them for convenience. Assuming the corresponding default package for the class’s package is on the classpath when compiling and/or running, you can always refer to a class by its fully qualified name. Consider two uses of java.util.Random below:

// Assuming the class was imported
Random rng = new Random();

// Assuming the class was NOT imported
java.util.Random rng = new java.util.Random();

As you can imagine, the latter (without an import statement) might get annoying and repetitive. Therefore, we usually prefer to use an import statement for the convenience it provides. Why would anyone prefer to use the fully qualified name instead of the simple name for a class? It enables you to use two classes from different packages with the same simple name at the same time!

2.6.3. The java.lang Package

Java automatically performs a wildcard import of the java.lang package in every Java file, without requiring the programmer to explicitly write it. That is why you can use classes such as java.lang.String and java.lang.System by using their simple names without importing!