Language Fundamentals - Import Declarations

Syntax

    import packageName.*;               // type-import-on-demand
    import packageName.ClassName;       // single-type-import
    import packageName.InterfaceName;   // single-type-import

• the import statement is used to reference classes and interfaces declared in other packages
• the type-import-on-demand import statement will cause the package to be searched when a type is declared for a class which has not been declared within the source file
• duplicate type-import-on-demand statements are ignored (JLS §7.5.2)
• the java.lang package is automatically imported in every compilation unit, it does not have to be specifically imported
• you can access classes and interfaces from other packages without first importing them but you must use their fully qualified names For example:

    If you import the java.awt.Button class by using:
        import java.awt.*;      ,or,
        import java.awt.Button;
    
    You can create a Button by coding:
        
        Button myButton = new Button();

    Without the package import you'd need to code:

        java.awt.Button myButton = new  java.awt.Button();       

• imported types are available to all classes and interfaces within the same compilation unit (JLS §7.5)
• it is legal to import a single-type and a package having the same names (JLS §7.5.4)

  i.e.
      import java.util.Vector;
      import Vector.pest;         // no compile error
  

Also see

• Sun Tech Tip: Using Import Declarations

Tips

• a single-type import will take precedence over an import-on-demand
• import-on-demand types do not increase the size of the compiled code ie only the types actually used are added to the code
• I've read that while import-on-demand adds no overhead to the compiled code, they can slow down the speed of the compile; however, Peter van der Linden, in Just Java 2, 4th Edition says it ain't so and my guess is he knows ... he's a kernel programmer for Sun

Traps

• single-type imports for two classes in different packages but with the same simple name
• single-type import with the same simple name as a class defined in the source file
• attempting to import a package vs a type ie import java.util vs import java.util.*