import static 语法

今天读Spring源码时，在类DomContentHandlerTest发现这么一段：

import static org.custommonkey.xmlunit.XMLAssert.assertXMLEqual;

import是天天见，但后面跟个 static 倒是头一遭。翻查资料后，了解到原来是为了方便调用一些静态方法。如果多次用到同一个类的同一个静态方法，每次都要引用类名就显得麻烦了。比如DomContentHandlerTest里就多次用到assertXMLEqual这个方法。

看看这个英文例子吧。

This tutorial introduces a new convenience feature of Java 5.0—the static import declaration—which enables programmers to refer to the imported static members of a class as if they were declared in the class that uses them. Thus, it is not necessary to qualify the imported static members with the name of the class in which they were declared. This tutorial is intended for students and professionals who are familiar withstatic class members in Java.

Download the code for this tutorial here.

[Note: This tutorial is an excerpt (Section 8.12) of Chapter 8, Class and Objects: A Deeper Look, from our textbook Java How to Program, 6/e. This tutorial may refer to other chapters or sections of the book that are not included here. Permission Information: Deitel, Harvey M. and Paul J., JAVA HOW TO PROGRAM, ?2005, pp.380-382. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.]

8.12 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 them—the class name and a dot (.) are not required to use an imported static member.

    A static import declaration has two forms—one that imports a particular static member (which is known as singlestatic 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) andstaticMemberName 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 9–12 access the Math class’s static field E (line 11) and the staticmethods 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.

Fig. 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\n", sqrt( 900.0 ) );
10 System.out.printf( "ceil( -9.8 ) = %.1f\n", ceil( -9.8 ) );
11 System.out.printf( "log( E ) = %.1f\n", log( E ) );
12 System.out.printf( "cos( 0.0 ) = %.1f\n", 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

原文地址：http://www.deitel.com/articles/java_tutorials/20060211/index.html。