Question

我做错了什么？有人可以帮忙吗？在“if（result.get（0）...）”中，它表示“结果无法解决”。我坚持这个。

<%
    if(session != null){
        ArrayList<Prize> result = new ArrayList<Prize>();
        result = (ArrayList<Prize>) session.getAttribute("result");  
    }
%>

<%
    if(result.get(0).getIdPrize() != null){
        prize = result.get(0);
        out.println(prize.getLottery(prize.getIdLottery()) + 
        " - " + prize.getHour(prize.getIdHour()) +
        " - " + prize.getDate(prize.getDatePrize()));
    }
%>

Answer 1

变量结果在第一个if块中声明，在

之后不再可见

尝试一下

    <%
    ArrayList<Prize> result = new ArrayList<Prize>();
    if(session != null){
        result = (ArrayList<Prize>) session.getAttribute("result");  
    }
    %>

<%
    if(result.size() > 0 && result.get(0).getIdPrize() != null){
        prize = result.get(0);
        out.println(prize.getLottery(prize.getIdLottery()) + 
        " - " + prize.getHour(prize.getIdHour()) +
        " - " + prize.getDate(prize.getDatePrize()));
    }
%>

Answer 2

正如@Vyncent所说，将结果声明移到if结构之外。

Answer 3

常见的newbiew错误是不知道变量的范围。括号中的任何内容都是变量生命的cicle

    {
     int a = 3; // You can use this variable inside this brackets
      System.out.println(a);
    }
    System.out.println(a); => Java Compiler will throw an error.

这适用于所有if / while / for语句。因此，对于解决方案，只需知道如何使用变量的范围

   ArrayList<Prize> result = new ArrayList<Prize>(); 
        if(session != null){
            result = (ArrayList<Prize>) session.getAttribute("result"); 
        }

    if(result.get(0).getIdPrize() != null){
        prize = result.get(0);
        out.println(prize.getLottery(prize.getIdLottery()) + 
        " - " + prize.getHour(prize.getIdHour()) +
        " - " + prize.getDate(prize.getDatePrize()));
    }

无法解析变量ArrayList

3 个答案: