j2se jxta 强制类型转换

PeerID st = (PeerID)it.next();//报错：
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to net.jxta.peer.PeerIDSystem.out.println(it.next());
输出是字符串：urn:jxta:uuid-59616261646162614E504720503250330438C2E9E82C4E60A444A202FC14C41103莫非是这个强制类型转换不行？谁知道JXTA PeerID 类型变量如何赋值？