Key value
----- ------
car toyota
car bmw
car honda
fruit apple
fruit banana
computer acer
computer asus
computer ibm
...
(以上数据的每一行都是一个带有字段“键”和“值”的对象,全部在一个列表
我想将数据构造为List<DataObject>
中]Map<String, List<String>>
,如下所示:
"car" : ["toyota", "bmw", "honda"] "fruit" : ["apple","banana"] "computer" : ["acer","asus","ibm"]
如何从数据对象实现以上Map
结构?[ ** * *** 其他 * ** * **
我对使用纯JDK提供的类或接口来实现结果而不是使用外部库更感兴趣。有帮助吗?
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());
}
Multimap
实现。但是使用标准的JDK也是容易做到的。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(两个实现):
{car=[toyota, bmw, honda], fruit=[apple, banana], computer=[acer, asus, ibm]}
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 );
或使用Guava's ListMultimap,它将为您完成此操作。
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"));
Map<String, List<String>> dataMap = new HashMap<String, List<String>>();
伪代码:
for (String key : keys) { if (!dataMap.containsKey(key)) { dataMap.put(key, new ArrayList<String>()); } dataMap.get(key).add(getValue(key)); }
或者,使用番石榴ListMultiMap
。