Java 如何从HashMap列表中获取值？
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How to Get Values from List of HashMap?
提问by Ads
I have a Listof HashMap's which has key of type Integerand value of type Long.
我有一个Listof HashMap，它具有 type 的键和 type 的Integer值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);
}
Now, how do I retrieve the key and value from the list of HashMap?
现在，如何从 的列表中检索键和值HashMap？
If using Collectionclass is the solution, please enlight me.
如果使用Collection类是解决方案，请启发我。
Update 1:
更新 1：
Actually i am getting the value from the database and putting that into a HashMap
实际上我是从数据库中获取值并将其放入 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;
}
where getIntFromDB and getLongFromDB can retreive any int and long values respectively.
其中 getIntFromDB 和 getLongFromDB 可以分别检索任何 int 和 long 值。
Now this is where i want to get my values that i got from DB both keys and values
现在这是我想获取我从数据库获得的值的地方，包括键和值
public void getmyDataBaseValues()
{
List<HashMap<Integer,Long>> ListofHash=populateMyHashMapFromDB();
// This is where i got confused
}
ANSWERThis is why Peter Lawreysuggested
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());
}
}
}
}
My Task is done even without creating a List.
即使没有创建列表，我的任务也已完成。
采纳答案by Peter Lawrey
Its still not clear to me why you want a list.
我仍然不清楚你为什么想要一个列表。
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);
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.
此集合并非旨在轻松为您提供键/值对。如果你需要这个功能，我建议使用不同的结构。
Assuming you have a some bad code you cannot change, you can do
假设您有一些无法更改的错误代码，您可以这样做
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.
}
In this example you don't need a List or a Map.
在此示例中，您不需要 List 或 Map。
long[] longs = new long[10];
for (int i = 0; i < 10; i++)
longs[i] = i;
int key = 1;
int num = longs[key];
回答by MarcoS
You iterate over the list to get the maps, and then iterate over their key set:
您遍历列表以获取映射，然后遍历它们的键集：
public static void main(String[] args) throws NoSuchAlgorithmException {
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);
}
for (Map<Integer, Long> map : ListofHash) {
for (Integer key : map.keySet()) {
System.out.println(map.get(key));
}
}
}
Note: I've also changed a little your code, using Mapinstead of HashMapwhen possible
注意：我还对您的代码进行了一些更改，尽可能使用Map代替HashMap
回答by TeaCupApp
I am assuming you are asking how to get it from ArrayList,
我假设你在问如何从 ArrayList 中获取它，
ListofHash.get(0).get(Object key);
Object key means whatever Integer key you want
对象键意味着您想要的任何整数键
Good Luck!
祝你好运！
回答by Buhake Sindi
Something of this effect:
这种效果的一些东西：
public static Long getValue(Integer key)) {
for (HashMap<Integer, Long> entry : ListofHash) {
if (entry.containsKey(key)) {
return entry.get(key);
}
}
return null;
}
//Retrieve all key/value
for (HashMap<Integer, Long> entry : ListofHash) {
for (Integer key : entry.keySet()) {
System.out.println("Key : " + key + ", value: " + entry.get(key));
}
}
PSUntested.
PS未经测试。
回答by Manuel Selva
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);
}
回答by Alfredo Osorio
You have a total of 10 Maps with only one element stored. And those Maps are stored in a List. You can retrieve those Maps and its elements doing this:
您总共有 10 张地图，其中只存储了一个元素。这些地图存储在一个列表中。您可以通过以下方式检索这些 Maps 及其元素：
public void testPrintHashmapValues() {
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);
}
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());
}
}
}
Also the variables should not start with uppercase (ListOfHash). Even though java won't complain it is a bad practice and difficult to read code like that because one might think it's a class name instead of a variable.
此外，变量不应以大写字母 ( ListOfHash)开头。尽管 java 不会抱怨它是一种不好的做法并且难以阅读这样的代码，因为人们可能认为它是一个类名而不是一个变量。
