Question

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){}

%>

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.

Why does the compiler say songList to be of type Object ?

Answer 1

You should declare it explicitly at the start:

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

If you declare it like this:

ArrayList songList = new ArrayList<String>();

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;

does not change the type of songList, it’s still effectively ArrayList<Object>.

If you fix the declaration, then your for loop should look like:

for (String list : songList){}

Because songList is array list of Strings. As you have it, Java extracts each Object from songList and tries to assign it to what you have declared to the left of :, which you have told it is an ArrayList<String>. It cannot convert the Object to ArrayList<String> (especially since the Object is really just a String underneath), so it throws an error.