Java: Packages - Importing

Multiple classes of larger programs are usually grouped together into a package. Packages correspond to directories in the file system, and may be nested just as directories are nested.

Using existing packages

Java libraries are organized in packages (directories). The most common way to get access to them is to use the import statement. For example,

import java.util.*;
. . .
ArrayList students;  // ArrayList is a class in java.util

gives your program access to the all (that's what the "*" means) classes in the package java.util. The source files for these classes are in a directory named util, which is in a directory named java.

Importing classes explictly

If you need only one class from a package, eg ArrayList, you can write

import java.util.ArrayList;
. . .
ArrayList students;  // same as above.

You might think that importing all classes from a package is inefficient, but there is no noticeable difference in my experience. Consequently most programs use the ".*" style of import.

Using package qualifiers instead of imports

The import statement is not required. Class references can be made but explicit qualification with the "." operator. For example,

java.util.ArrayList students;  // fully qualified.  No need for import.

The fully qualified style is used in some textbooks, but is generally not used when programming, where the import style is almost universal.

However, there is one situation where qualification is necessary - when two classes have the same name, but are in different packages. For example, there is both java.util.Timer and java.swing.Timer. Because it's common to import all classes in both java.util and java.swing, the name Timer is ambiguous and can't be used without qualification.

import java.util.*;
import java.swing.*;
. . .
Timer t;  //AMBIGUOUS - compilation error
java.util.Timer t;  // OK

Packages within packages require additional imports

The import statment gives access to classes in a package, but not to packages in that package. For example,

import java.util.*;

does not give access to the classes of the package java.util.regex. To access classes in java.util and java.util.regex, import both.

import java.util.*;
import java.util.regex.*;

Java's `import` is not the same as C++'s `#include`

C++'s #include is commonly used to for library headers, but the mechanism which is used is fundamentally different. #include inserts the entire source file that is referenced into your C++ program. In contrast, the Java import statement only looks up the the identifiers and their declarations from the compiled class file (not the source files).

Another difference is that Java imports are non-transitive. If class A imports packagex and packagex imports packagey, class A does NOT get access to packagey. In C++, the imports are transitive, which can lead to some unexpected effects.

A minor difference is that Java's import is a statement and requires a semicolon, unlike C++.