e1075. selecting from a set of child elements in a dom document using xpath.txt

这里面包含了一百多个JAVA源文件

XPath is an expression language for selecting nodes in an XML file. See e1074 Finding Elements by Absolute Location in a DOM Document Using XPath for common XPath expression for selecting elements. This example adds to those examples by demonstrating the ability to apply a selection filter to a set of child elements. For example, if an element A contained several B elements, you could include only the first B element or all but the first B element. 
Filtering in XPath is specified with a predicate after the element name. A predicate has the form [expression]. For example, 

    /book/chapter[2]/section[3]

selects the 3rd section element from the 2nd chapter element under the root element book. 
This example demonstrates some common filters; for more information on XPath, see the specification at http://www.w3c.org/TR/xpath. In the example, the result of an XPath expression is shown next to the expression; the numbers are ids of elements in the sample file shown at the end of the example. 

    // Get the first element under the root
    String xpath = "/*/*[1]";            // 2
    
    // Get the second elem1 element under the root
    xpath = "/root/elem1[2]";            // 8
    
    // Get all first-born e elements in the document; that is, for all
    // e elements with e element siblings, include only the first sibling
    xpath = "//e[1]";                    // 4 6 10 12

Note that //e[1] does not return the first e element in the document because the [1] predicate applies to e, which represents the set of e elements under one element and not to //e, which represents the set of e elements in the document. The following expression retrieves the first e element in the document: 
    // Get the first e element in the document
    xpath = "(//e)[1]";                  // 4
    
    // For all e elements with e element siblings, include only
    // the first 3 siblings
    xpath = "//e[position() <= 3]";      // 4 6 10 11 12
    
    // Get all last-born e elements in the document; that is, for all
    // e elements with e element siblings, include only the last sibling
    xpath = "//e[last()]";               // 4 6 11 12
    
    // Get the last e element in the document
    xpath = "(//e)[last()]";             // 12

To execute an XPath expression, see e1074 Finding Elements by Absolute Location in a DOM Document Using XPath. Here is the sample XML file used in the example: 
    <?xml version="1.0" encoding="UTF-8"?>
    <root id="1">
        <elem1 id="2">
            <elem2 id="3">
                <e id="4"/>
                <elem3 id="5">
                    <e id="6"/>
                </elem3>
                <elem3 id="7"/>
            </elem2>
        </elem1>
        <elem1 id="8">
            <elem2 id="9"/>
            <e id="10"/>
            <e id="11"/>
        </elem1>
        <e id="12"/>
    </root>