java - 对象到 ArrayList未经检查的演员表

Question

用 -Xlint 编译我的代码我发现了这个：

warning: [unchecked] unchecked cast
found   : java.lang.Object
required: java.util.ArrayList<java.lang.String>
                    ArrayList<String> msgData = (ArrayList<String>)element;

这是代码：

ArrayList<ArrayList<String>> msg = new ArrayList<ArrayList<String>>();
//add some data to msg
Iterator i = msg.iterator();
while(i.hasNext()) {

    Object element = i.next();
    ArrayList<String> msgData = (ArrayList<String>)element;     
}

演员应该怎么做？

Answer 1

您的迭代器也需要一个类型，如下所示：

ArrayList<ArrayList<String>> msg = new ArrayList<ArrayList<String>>();
//add some data to msg
Iterator<ArrayList<String>> i = msg.iterator();
while(i.hasNext()) {
    ArrayList<String> element = i.next();
}

顺便说一句，最好对接口进行编程，而不是对实现进行编程。因此，你会写：

List<List<String>> msg = new ArrayList<List<String>>();
//add some data to msg
Iterator<List<String>> i = msg.iterator();
while(i.hasNext()) {
    List<String> element = i.next();
}

Answer 2

您的迭代器应该类似于

Iterator<ArrayList<String>> i = msg.iterator();

顺便说一句，您可以使用 for-each 循环：

for (ArrayList<String> sublist : msg) {
    ....            
}

Answer 3

ArrayList<ArrayList<String>> msg = new ArrayList<ArrayList<String>>();
//add some data to msg
for (ArrayList<String> msgData : msg) {
   //do something
}