Java HashMap: How to get a key and value by index?

问题:

I am trying to use a HashMap to map a unique string to a string ArrayList like this:

HashMap<String, ArrayList<String>>

Basically, I want to be able to access the keys by number, not by using the key's name. And I want to be able to access said key's value, to iterate over it. I'm imagining something like this:

for(all keys in my hashmap) {
    for(int i=0; i < myhashmap.currentKey.getValue.size(); i++) {
        // do things with the hashmaps elements
    }
}

Is there an easy way to do this?

回答1:

You can iterate over keys by calling map.keySet(), or iterate over the entries by calling map.entrySet(). Iterating over entries will probably be faster.

for (Map.Entry<String, List<String>> entry : map.entrySet()) {
    List<String> list = entry.getValue();
    // Do things with the list
}

If you want to ensure that you iterate over the keys in the same order you inserted them then use a LinkedHashMap.

By the way, I'd recommend changing the declared type of the map to <String, List<String>>. Always best to declare types in terms of the interface rather than the implementation.

回答2:

Here is the general solution if you really only want the first key's value

Object firstKey = myHashMap.keySet().toArray()[0];
Object valueForFirstKey = myHashMap.get(firstKey);

回答3:

HashMaps are not ordered, unless you use a LinkedHashMap or SortedMap. In this case, you may want a LinkedHashMap. This will iterate in order of insertion (or in order of last access if you prefer). In this case, it would be

int index = 0;
for ( Map.Entry<String,ArrayList<String>> e : myHashMap.iterator().entrySet() ) {
    String key = e.getKey();
    ArrayList<String> val = e.getValue();
    index++;
}

There is no direct get(index) in a map because it is an unordered list of key/value pairs. LinkedHashMap is a special case that keeps the order.

回答4:

for (Object key : data.keySet()) {
    String lKey = (String) key;
    List<String> list = data.get(key);
}

回答5:

A solution is already selected. However, I post this solution for those who want to use an alternative approach:

// use LinkedHashMap if you want to read values from the hashmap in the same order as you put them into it
private ArrayList<String> getMapValueAt(LinkedHashMap<String, ArrayList<String>> hashMap, int index)
{
    Map.Entry<String, ArrayList<String>> entry = (Map.Entry<String, ArrayList<String>>) hashMap.entrySet().toArray()[index];
    return entry.getValue();
}

回答6:

You can do:

for(String key: hashMap.keySet()){
    for(String value: hashMap.get(key)) {
        // use the value here
    }
}

This will iterate over every key, and then every value of the list associated with each key.

回答7:

HashMaps don't keep your key/value pairs in a specific order. They are ordered based on the hash that each key's returns from its Object.hashCode() method. You can however iterate over the set of key/value pairs using an iterator with:

for (String key : hashmap.keySet()) 
{
    for (list : hashmap.get(key))
    {
        //list.toString()
    }
}

回答8:

If you don't care about the actual key, a concise way to iterate over all the Map's values would be to use its values() method

Map<String, List<String>> myMap;

for ( List<String> stringList : myMap.values() ) {
    for ( String myString : stringList ) {
        // process the string here
    }
}

The values() method is part of the Map interface and returns a Collection view of the values in the map.