Enumeration<> 枚举差异
[英]Enumeration<> Enumeration difference
问题描述
I'm a biginner in JSP and I'm confused about the difference between Enumeration and Enumeration<type> I'm learning with this Korean book and the example source in it says Enumeration with the eclipse neon version it doesn't work.
我是 JSP 的大人物，我对Enumeration和Enumeration<type>之间的区别感到困惑我正在学习这本韩文书籍，其中的示例源说Enumeration与 eclipse neon 版本不起作用。 It works only when it write Enumeration<String> .它仅在写入Enumeration<String> 。 Can someone tell me the difference?有人能告诉我有什么区别吗？
<%@page import="java.util.Enumeration"%>
<%@ page language="java" contentType="text/html;
charset=UTF-8"pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>헤더 목록 출력</title>
</head>
<body>
<%
Enumeration<String> headerEnum = request.getHeaderNames();
while(headerEnum.hasMoreElements()){
String headerName = (String)headerEnum.nextElement();
String headerValue = request.getHeader(headerName);
%>
<%=headerName %> = <%=headerValue %> <br>
<%
}
%>
</body>
</html>
1 个解决方案
解决方案1
1 2017-01-12 09:38:59
Just give it a look at the Enumeration documentation .只需查看枚举文档即可。 Also review the generic types documentation .另请查看泛型类型文档。
By using Enumeration you are using Enumeration<Object> as it is the default.通过使用Enumeration您将使用Enumeration<Object>因为它是默认设置。 What this <Object> does is just indicate the Enumeration class that in that particular instance, the type that it calls E (in the Enumeration Documentation) will be resolved to Object.这个<Object>所做的只是指示枚举类，在该特定实例中，它调用 E（在枚举文档中）的类型将被解析为 Object。 By using <String> happens the same: the type called E will be resolved to String.通过使用<String>会发生相同的情况：称为 E 的类型将被解析为 String。
If you check the nextElement() signature it returns E. So, by using Enumeration or Enumeration<Object> that method will return Object and you will need the cast you did:如果您检查nextElement() 签名，它将返回 E。因此，通过使用Enumeration或Enumeration<Object>该方法将返回 Object 并且您将需要您所做的强制转换：
String headerName = (String)headerEnum.nextElement();
By using Enumeration<String> the method will return an String, so you can directly do this:通过使用Enumeration<String>该方法将返回一个字符串，因此您可以直接执行以下操作：
String headerName = headerEnum.nextElement();
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