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<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Projecting Across Collections</title><link href="../docbook.css" rel="stylesheet" type="text/css"><meta content="DocBook XSL Stylesheets V1.65.0" name="generator"><link rel="home" href="index.html" title="OGNL Language Guide"><link rel="up" href="basicExpressions.html" title="Chapter 4. Expressions"><link rel="previous" href="collectionConstruction.html" title="Collection Construction"><link rel="next" href="selection.html" title="Selecting From Collections"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">Projecting Across Collections</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="collectionConstruction.html"><img src="../images/navigation/prev.gif" alt="Prev"></a> </td><th align="center" width="60%">Chapter 4. Expressions</th><td align="right" width="20%"> <a accesskey="n" href="selection.html"><img src="../images/navigation/next.gif" alt="Next"></a></td></tr></table><hr></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="projection"></a>Projecting Across Collections</h2></div></div><div></div></div><p><span class="acronym">OGNL</span> provides a simple way to call the same method or extract the same property from each element in a collection and store the results in a new collection. We call this "projection," from the database
term for choosing a subset of columns from a table. For example, this expression:</p><pre class="programlisting">listeners.{delegate}</pre><p>returns a list of all the listeners' delegates. See the coercion section for how <span class="acronym">OGNL</span> treats various kinds of objects as collections.</p><p>During a projection the <tt class="varname">#this</tt> variable refers to the current element of the iteration.</p><pre class="programlisting">objects.{ #this instanceof String ? #this : #this.toString()}</pre><p>The above would produce a new list of elements from the objects list as string values.</p></div><div class="navfooter"><hr><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="collectionConstruction.html"><img src="../images/navigation/prev.gif" alt="Prev"></a> </td><td align="center" width="20%"><a accesskey="u" href="basicExpressions.html"><img src="../images/navigation/up.gif" alt="Up"></a></td><td align="right" width="40%"> <a accesskey="n" href="selection.html"><img src="../images/navigation/next.gif" alt="Next"></a></td></tr><tr><td valign="top" align="left" width="40%">Collection Construction </td><td align="center" width="20%"><a accesskey="h" href="index.html"><img src="../images/navigation/home.gif" alt="Home"></a></td><td valign="top" align="right" width="40%"> Selecting From Collections</td></tr></table></div></body></html>⌨️ 快捷键说明
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