required java.util.ArrayList<String>,found java.lang.Object : 我不明白这个错误的原因 [英] required java.util.ArrayList&lt;String&gt;,found java.lang.Object : I do not understand the reason for this error

问题描述

以下是jsp页面的一些片段:

Following are some snippets from a jsp page:

<%! ArrayList songList = new ArrayList<String>(); %>

    <%
        songList = StoreSongLink.linkList;
        // linkList is a static variable in the class StoreSongLink
        // There the linkList is defined as public static ArrayList<String> linkList = new ArrayList<String>();
    %>

<%}  else {
       for (ArrayList<String> list : songList){}

%>

else srciplet 中的代码产生错误 required java.util.ArrayList找到 java.lang.Object.为什么是这样 ?我不明白这是什么原因.

The code inside the else srciplet produces an error required java.util.ArrayList<String> found java.lang.Object. Why is this ? I do not understand the reason for this.

为什么编译器说 songList 是 Object 类型?

推荐答案

你应该在开始时明确声明:

You should declare it explicitly at the start:

ArrayList<String> songList = new ArrayList<String>();

如果你这样声明:

ArrayList songList = new ArrayList<String>();

那么你是说 songList 是 Object 的 ArrayList，不管 =.分配不会改变这一点，所以这个:

Then you are saying songList is an ArrayList of Objects, regardless of what is to the right of the =. Assignment doesn't change this, so this:

ArrayList songList = new ArrayList<String>();
songList = StoreSongLink.linkList;

不改变songList的类型，它仍然是有效的ArrayList