Is there a way to get the value of a HashMap randomly in Java?

Is there a way to get the value of a HashMap randomly in Java?

Solution 1:

This works:

Random generator = new Random();
Object[] values = myHashMap.values().toArray();
Object randomValue = values[generator.nextInt(values.length)];

If you want the random value to be a type other than an Object simply add a cast to the last line. So if myHashMap was declared as:

Map<Integer,String> myHashMap = new HashMap<Integer,String>();

The last line can be:

String randomValue = (String) values[generator.nextInt(value.length)];

The below doesn't work, Set.toArray() always returns an array of Objects, which can't be coerced into an array of Map.Entry.

Random generator = new Random();
Map.Entry[] entries = myHashMap.entrySet().toArray();
randomValue = entries[generator.nextInt(entries.length)].getValue();

Solution 2:

Since the requirements only asks for a random value from the HashMap, here's the approach:

1. The HashMap has a values method which returns a Collection of the values in the map.
2. The Collection is used to create a List.
3. The size method is used to find the size of the List, which is used by the Random.nextInt method to get a random index of the List.
4. Finally, the value is retrieved from the List get method with the random index.

Implementation:

HashMap<String, Integer> map = new HashMap<String, Integer>();
map.put("Hello", 10);
map.put("Answer", 42);

List<Integer> valuesList = new ArrayList<Integer>(map.values());
int randomIndex = new Random().nextInt(valuesList.size());
Integer randomValue = valuesList.get(randomIndex);

The nice part about this approach is that all the methods are generic -- there is no need for typecasting.