如何实现这个Map <String,List <>>结构

Question

我有如下数据:

 Key       value
-----      ------
car         toyota
car         bmw
car         honda

fruit       apple
fruit       banana

computer    acer
computer    asus
computer    ibm 
...

(上述数据中的每一行是一个对象与字段"键"和"值",都在一个列表List<DataObject>)

我想将数据构建成Map<String, List<String>>如下所示:

 "car"      : ["toyota", "bmw", "honda"]
 "fruit"    : ["apple","banana"]
 "computer" : ["acer","asus","ibm"]

如何Map从数据对象中实现上述结构？

******除了******

我更感兴趣的是使用纯JDK提供的类或接口来实现结果,而不是使用外部库.有帮助吗？

Answer 1

    Map<String, List<String>> myMaps = new HashMap<String, List<String>>();
    for (DataObject item : myList) {
        if (!myMaps.containsKey(item.getKey())) {
            myMaps.put(item.getKey(), new ArrayList<String>());
        }
        myMaps.get(item.getKey()).add(item.getValue());
    }

Answer 2

我会使用guavas Multimap实现.但是使用标准JDK也很容易.

示例标准JDK:

public static void main(String[] args) {
    Scanner s = new Scanner(
            "car         toyota\n" +
            "car         bmw\n" +
            "car         honda\n" +
            "fruit       apple\n" +
            "fruit       banana\n" +
            "computer    acer\n" +
            "computer    asus\n" +
            "computer    ibm");

    Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();

    while (s.hasNext()) {

        String key = s.next();
        if (!map.containsKey(key))
            map.put(key, new LinkedList<String>());

        map.get(key).add(s.next());
    }

    System.out.println(map);
}

番石榴示例:

public static void main(String[] args) {
    Scanner s = new Scanner(
            "car         toyota\n" +
            "car         bmw\n" +
            "car         honda\n" +
            "fruit       apple\n" +
            "fruit       banana\n" +
            "computer    acer\n" +
            "computer    asus\n" +
            "computer    ibm");

    Multimap<String, String> map = LinkedListMultimap.create();

    while (s.hasNext()) 
        map.put(s.next(), s.next());

    System.out.println(map);
}

输出(两种实现):

{car=[toyota, bmw, honda], fruit=[apple, banana], computer=[acer, asus, ibm]}

Answer 3

Map<String, List<String>> data = new HashMap<String, List<String>>();
data.put("car", Arrays.asList("toyota", "bmw", "honda"));
data.put("fruit", Arrays.asList("apple","banana"));
data.put("computer", Arrays.asList("acer","asus","ibm"));

Answer 4

以下代码段将为您提供帮助.

        HashMap<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();

        ArrayList<String> carList = new ArrayList<String>();
        carList.add("toyota");
        carList.add("bmw");
        carList.add("honda");

        map.put("car", carList);

        ArrayList<String> fruitList = new ArrayList<String>();
        fruitList .add("apple");
        fruitList .add("banana");

        map.put("fruit", fruitList );

Answer 5

迭代对象。对于每个对象，从地图中获取其对应的列表。如果为空，则创建一个新列表并将其放入地图中。然后将该值添加到列表中。

或者使用Guava 的 ListMultimap，它会为您完成此操作。