importstatement. For example,
import java.util.*; . . . ArrayList students; // ArrayList is a class in java.utilgives 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
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.
importstatement 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
Because it's common to import all classes in both
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
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.regex, import both.
import java.util.*; import java.util.regex.*;
importis not the same as C++'s
#includeis commonly used to for library headers, but the mechanism which is used is fundamentally different.
#includeinserts the entire source file that is referenced into your C++ program. In contrast, the Java
importstatement 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
packagey, class A does NOT get access to
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++.