如何从HashMap列表中获取值? [英] How to Get Values from List of HashMap?

问题描述

我有一个 HashMap 的 List，它有 Integer 类型的键和 Long 类型的值.

I have a List of HashMap's which has key of type Integer and value of type Long.

List<HashMap<Integer, Long>> ListofHash = new ArrayList<HashMap<Integer, Long>>();
        for (int i = 0; i < 10; i++) {
            HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
            mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L+i));
            ListofHash.add(mMap);
        }

现在，我如何从 HashMap 的列表中检索键和值?

Now, how do I retrieve the key and value from the list of HashMap?

如果使用 Collection 类是解决方案，请赐教.

If using Collection class is the solution, please enlight me.

更新 1:

实际上我是从数据库中获取值并将其放入 HashMap

Actually i am getting the value from the database and putting that into a HashMap

public static List<Map<Integer, Long>> populateMyHashMapFromDB()
{

List<HashMap<Integer, Long>> ListofHash = new ArrayList<HashMap<Integer, Long>>();

for (int i = 0; i < 10; i++) {
                HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
                mMap.put(Integer.valueOf(getIntFromDB(i)), Long.valueOf(getLongFromDB(i)));
                ListofHash.add(mMap);
            }
return ListofHash;


}

其中 getIntFromDB 和 getLongFromDB 可以分别检索任何 int 和 long 值.

where getIntFromDB and getLongFromDB can retreive any int and long values respectively.

现在这是我想获取我从数据库中获得的值的地方

Now this is where i want to get my values that i got from DB both keys and values

public void getmyDataBaseValues()
{
List<HashMap<Integer,Long>> ListofHash=populateMyHashMapFromDB();

// This is where i got confused

}

ANSWER 这就是为什么 Peter Lawrey 建议

public class HashMaps {


    public static void main(String[] args) {
        // TODO Auto-generated method stub

        testPrintHashmapValues(putToHashMap());
        testPrintHashmapValuesWithList(putToHashMapWithList());
    }

    public static Map<Integer, Long> putToHashMap() {
        Map<Integer, Long> map = new LinkedHashMap<Integer, Long>();
        for (int i = 0; i < 10; i++) {
            map.put(Integer.valueOf(i), Long.valueOf(200000000000L + i));
        }
        return map;
    }

    public static void testPrintHashmapValues(Map<Integer, Long> map) {
        for (Map.Entry<Integer, Long> entry : map.entrySet()) {
            System.out.println("key: " + entry.getKey() + " value: "
                    + entry.getValue());
        }
    }

    public static List<Map<Integer, Long>> putToHashMapWithList() {
        List<Map<Integer, Long>> listOfHash = new ArrayList<Map<Integer, Long>>();
        for (int i = 0; i < 10; i++) {
            Map<Integer, Long> mMap = new HashMap<Integer, Long>();

            mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L + i));
            listOfHash.add(mMap);
        }
        return listOfHash;
    }

    public static void testPrintHashmapValuesWithList(
            List<Map<Integer, Long>> listOfHash) {

        for (int i = 0; i < 10; i++) {
            Map<Integer, Long> map = listOfHash.get(i);
            for (Map.Entry<Integer, Long> entry : map.entrySet()) {
                System.out.println(i + " hashMap");
                System.out.println("key: " + entry.getKey() + " value: "
                        + entry.getValue());
            }
        }
    }

}

即使没有创建列表，我的任务也完成了.

My Task is done even without creating a List.

推荐答案

我仍然不清楚你为什么想要一个列表.

Its still not clear to me why you want a list.

public static Map<Integer, Long> populateMyHashMapFromDB() {
    Map<Integer, Long> map = new LinkedHashMap<Integer, Long>();
    for (int i = 0; i < 10; i++) 
            map.put(getIntFromDB(i), getLongFromDB(i));
    return map;
}

Map<Integer, Long> map = populateMyHashMapFromDB();
Long value = map.get(key);

<小时>

此集合并非旨在轻松为您提供键/值对.如果您需要此功能，我建议您使用不同的结构.

---

This collection isn't designed to give you the key/values pairs easily. I fyou need to this functionality I would suggest using a different structure.

假设您有一些无法更改的错误代码，您可以这样做

Assuming you have a some bad code you cannot change, you can do

List<Map<Integer, Long>> maps = new ArrayList<Map<Integer, Long>>();

Map<Integer, Long> all = new HashMap<Integer, Long>();
for(Map<Integer, Long> map: maps)
    all.putAll(map);

for(Map.Entry<Integer, Long> entry: all.entrySet() {
    // do something which each key/value.
}

<小时>

在本例中，您不需要列表或地图.

---

In this example you don't need a List or a Map.

long[] longs = new long[10];
for (int i = 0; i < 10; i++) 
    longs[i] = i;

int key = 1;
int num = longs[key];

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