集合Map删除问题

上代码
public static String BulidXml(Map TotalMap,Map map){
String XmlStr="";
for(Iterator it = TotalMap.entrySet().iterator();it.hasNext();){
 Map.Entry mapentry = (Map.Entry) it.next(); 
    Map maptemp=(Map)mapentry.getValue();
if((String.valueOf(maptemp.get("parent_id"))).equals(String.valueOf(map.get("id")))){
XmlStr+=getYesSonEle(map);
XmlStr+=BulidXml(TotalMap,maptemp);
XmlStr+="</pc>";
it.remove();
}else if(!(String.valueOf(maptemp.get("parent_id"))).equals(String.valueOf(map.get("id")))){
XmlStr+=getNoSonEle(map);
it.remove();
}
}
return XmlStr;
}这个一个迭代组装xml字符串的方法
可是每当到最后一个元素的时候
it.remove();就提示下面错误，
可是我打印it有值啊 郁闷啊
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.remove(Unknown Source)
at org.apache.jsp.TreeVml_jsp.BulidXml(TreeVml_jsp.java:30)
at org.apache.jsp.TreeVml_jsp.BulidXml(TreeVml_jsp.java:28)
at org.apache.jsp.TreeVml_jsp._jspService(TreeVml_jsp.java:145)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:331)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:874)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Unknown Source)