java - ArrayList 中的元素消失

Question

我在使用这段代码时遇到了一些问题:

ArrayList<Shop> listShops = new ArrayList<Shop>();
Shop currShop = new Shop();

String query = "SELECT * FROM Shop WHERE... ";

try {
    PreparedStatement statement = connection.prepareStatement(query);
    ResultSet rs = statement.executeQuery();
    while(rs.next()) {
        currShop.setName(rs.getString(1));
        currShop.setDescription(rs.getString(2));
        System.out.println(listShops.add(currShop));
    }
} catch (SQLException e) {
    e.printStackTrace();
}

System.out.println("List size: "+listShop.getSize());
for(Shop s: listShop) {
    System.out.println(s.getName());
}

输出:

true
true
true 
[...]
List size: 78
[78 empty strings]

我不明白为什么它给我那些空行。我 100% 确定 currShop 工作正常，因为我打印了 currShop.getName() 和 currShop.getDescription() (在 rs.next() while 中)并且它们两者都有效。它还为我提供了“true” boolean 值，表明已成功插入 ArrayList，那么为什么它不打印任何内容？

Answer 1

您应该每次在 while 循环内创建一个新的 Shop 对象:

while(rs.next()) {
    Shop currShop = new Shop();
    currShop.setName(rs.getString(1));
    currShop.setDescription(rs.getString(2));
    System.out.println(listShops.add(currShop));
}

否则，您只需将单个 Shop 实例多次添加到列表中，并在每次迭代时覆盖名称和描述。