Java HashMap: How to get a key and value by index?
Solution 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.
Solution 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);
Solution 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.
Solution 4
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();
}
Solution 5
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.
Derek
Updated on July 10, 2022Comments
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Derek almost 2 years
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?
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Derek over 13 yearsThanks to everyone who answered. Jjnguy, this is exactly what I wanted. You too, kkress.
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avalancha about 10 yearswell to be very strict... the OP asked for the value, not the key. So you might want to add
Object myValue = myMap.get(myKey);
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Matthieu about 9 yearsI'm not sure the (potential) overhead of
toArray()
beats theiterator().next()
...