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   7 Constraint (Google Collections Library - snapshot-20080820 (ALPHA))
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  87 com.google.common.collect</FONT>
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  89 Interface Constraint&lt;E&gt;</H2>
  90 <HR>
  91 <DL>
  92 <DT><PRE>public interface <B>Constraint&lt;E&gt;</B></DL>
  93 </PRE>
  94
  95 <P>
  96 A constraint that an element must satisfy in order to be added to a
  97  collection. For example, <A HREF="../../../../com/google/common/collect/Constraints.html#notNull()"><CODE>Constraints.notNull()</CODE></A>, which prevents a
  98  collection from including any null elements, could be implemented like this:
  99  <pre>   <code>public Object checkElement(Object element) {
 100      if (element == null) {
 101        throw new NullPointerException();
 102      }
 103      return element;
 104    }</code></pre>
 105
 106  In order to be effective, constraints should be deterministic; that is,
 107  they should not depend on state that can change (such as external state,
 108  random variables, and time) and should only depend on the value of the
 109  passed-in element. A non-deterministic constraint cannot reliably enforce
 110  that all the collection's elements meet the constraint, since the constraint
 111  is only enforced when elements are added.
 112 <P>
 113
 114 <P>
 115 <DL>
 116 <DT><B>Author:</B></DT>
 117   <DD>Mike Bostock</DD>
 118 <DT><B>See Also:</B><DD><A HREF="../../../../com/google/common/collect/Constraints.html" title="class in com.google.common.collect"><CODE>Constraints</CODE></A>,
 119 <A HREF="../../../../com/google/common/collect/MapConstraint.html" title="interface in com.google.common.collect"><CODE>MapConstraint</CODE></A></DL>
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 130 <B>Method Summary</B></FONT></TH>
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 133 <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
 134 <CODE>&nbsp;<A HREF="../../../../com/google/common/collect/Constraint.html" title="type parameter in Constraint">E</A></CODE></FONT></TD>
 135 <TD><CODE><B><A HREF="../../../../com/google/common/collect/Constraint.html#checkElement(E)">checkElement</A></B>(<A HREF="../../../../com/google/common/collect/Constraint.html" title="type parameter in Constraint">E</A>&nbsp;element)</CODE>
 136
 137 <BR>
 138 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Throws a suitable <code>RuntimeException</code> if the specified element is
 139  illegal.</TD>
 140 </TR>
 141 <TR BGCOLOR="white" CLASS="TableRowColor">
 142 <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
 143 <CODE>&nbsp;<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/String.html" title="class or interface in java.lang">String</A></CODE></FONT></TD>
 144 <TD><CODE><B><A HREF="../../../../com/google/common/collect/Constraint.html#toString()">toString</A></B>()</CODE>
 145
 146 <BR>
 147 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Returns a brief human readable description of this constraint, such as
 148  "Not null" or "Positive number".</TD>
 149 </TR>
 150 </TABLE>
 151 &nbsp;
 152 <P>
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 163
 164 <A NAME="checkElement(java.lang.Object)"><!-- --></A><A NAME="checkElement(E)"><!-- --></A><H3>
 165 checkElement</H3>
 166 <PRE>
 167 <A HREF="../../../../com/google/common/collect/Constraint.html" title="type parameter in Constraint">E</A> <B>checkElement</B>(<A HREF="../../../../com/google/common/collect/Constraint.html" title="type parameter in Constraint">E</A>&nbsp;element)</PRE>
 168 <DL>
 169 <DD>Throws a suitable <code>RuntimeException</code> if the specified element is
 170  illegal. Typically this is either a <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/NullPointerException.html" title="class or interface in java.lang"><CODE>NullPointerException</CODE></A>, an
 171  <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/IllegalArgumentException.html" title="class or interface in java.lang"><CODE>IllegalArgumentException</CODE></A>, or a <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/ClassCastException.html" title="class or interface in java.lang"><CODE>ClassCastException</CODE></A>, though
 172  an application-specific exception class may be used if appropriate.
 173 <P>
 174 <DD><DL>
 175 <DT><B>Parameters:</B><DD><CODE>element</CODE> - the element to check
 176 <DT><B>Returns:</B><DD>the provided element</DL>
 177 </DD>
 178 </DL>
 179 <HR>
 180
 181 <A NAME="toString()"><!-- --></A><H3>
 182 toString</H3>
 183 <PRE>
 184 <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/String.html" title="class or interface in java.lang">String</A> <B>toString</B>()</PRE>
 185 <DL>
 186 <DD>Returns a brief human readable description of this constraint, such as
 187  "Not null" or "Positive number".
 188 <P>
 189 <DD><DL>
 190 <DT><B>Overrides:</B><DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html#toString()" title="class or interface in java.lang">toString</A></CODE> in class <CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html" title="class or interface in java.lang">Object</A></CODE></DL>
 191 </DD>
 192 <DD><DL>
 193
 194 <DT><B>Returns:</B><DD>a string representation of the object.</DL>
 195 </DD>
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