static Import

In Section 6.3, you learned about the static fields and methods of class Math. We invoked class Math's static fields and methods by preceding each with the class name Math and a dot (.). A static import declaration (a new feature of J2SE 5.0) enables programmers to refer to imported static members as if they were declared in the class that uses themthe class name and a dot (.) are not required to use an imported static member.

A static import declaration has two formsone that imports a particular static member (which is known as single static import) and one that imports all static members of a class (which is known as static import on demand). The following syntax imports a particular static member:


       import static packageName.ClassName.staticMemberName;

where packageName is the package of the class (e.g., java.lang), ClassName is the name of the class (e.g., Math) and staticMemberName is the name of the static field or method (e.g., PI or abs). The following syntax imports all static members of a class:


       import static packageName.ClassName.*;

where packageName is the package of the class (e.g., java.lang) and ClassName is the name of the class (e.g., Math). The asterisk (*) indicates that all static members of the specified class should be available for use in the class(es) declared in the file. Note that static import declarations import only static class members. Regular import statements should be used to specify the classes used in a program.

Figure 8.14 demonstrates a static import. Line 3 is a static import declaration, that imports all static fields and methods of class Math from package java.lang. Lines 912 access the Math class's static field E (line 11) and the static methods sqrt (line 9), ceil (line 10), log (line 11) and cos (line 12) without preceding the field name or method names with class name Math and a dot.

Figure 8.14. Static import Math methods.

 1 // Fig. 8.14: StaticImportTest.java
 2 // Using static import to import static methods of class Math.
 3 import static java.lang.Math.*;
 4
 5 public class StaticImportTest
 6 {
 7 public static void main( String args[] )
 8 {
 9 System.out.printf( "sqrt( 900.0 ) = %.1f
", sqrt( 900.0 ) );
10 System.out.printf( "ceil( -9.8 ) = %.1f
", ceil( -9.8 ) );
11 System.out.printf( "log( E ) = %.1f
", log( E ) );
12 System.out.printf( "cos( 0.0 ) = %.1f
", cos( 0.0 ) );
13 } // end main
14 } // end class StaticImportTest
 
sqrt( 900.0 ) = 30.0
ceil( -9.8 ) = -9.0
log( E ) = 1.0
cos( 0.0 ) = 1.0
 

Common Programming Error 8.9

A compilation error occurs if a program attempts to import static methods that have the same signature or static fields that have the same name from two or more classes.


Introduction to Computers, the Internet and the World Wide Web

Introduction to Java Applications

Introduction to Classes and Objects

Control Statements: Part I

Control Statements: Part 2

Methods: A Deeper Look

Arrays

Classes and Objects: A Deeper Look

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

GUI Components: Part 1

Graphics and Java 2D™

Exception Handling

Files and Streams

Recursion

Searching and Sorting

Data Structures

Generics

Collections

Introduction to Java Applets

Multimedia: Applets and Applications

GUI Components: Part 2

Multithreading

Networking

Accessing Databases with JDBC

Servlets

JavaServer Pages (JSP)

Formatted Output

Strings, Characters and Regular Expressions

Appendix A. Operator Precedence Chart

Appendix B. ASCII Character Set

Appendix C. Keywords and Reserved Words

Appendix D. Primitive Types

Appendix E. (On CD) Number Systems

Appendix F. (On CD) Unicode®

Appendix G. Using the Java API Documentation

Appendix H. (On CD) Creating Documentation with javadoc

Appendix I. (On CD) Bit Manipulation

Appendix J. (On CD) ATM Case Study Code

Appendix K. (On CD) Labeled break and continue Statements

Appendix L. (On CD) UML 2: Additional Diagram Types

Appendix M. (On CD) Design Patterns

Appendix N. Using the Debugger

Inside Back Cover

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Java(c) How to Program
Java How to Program (6th Edition) (How to Program (Deitel))
ISBN: 0131483986
EAN: 2147483647
Year: 2003
Pages: 615
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