随机访问 HashMap 个密钥

Question

I need to randomly access keys in a HashMap. Right now, I am using Set 's toArray() method on the Set that HashMap 's keySet() returns, and casting it as a String[] (my keys are Strings). Then I use Random to pick a random element of the String array.

public String randomKey() {
    String[] keys = (String[]) myHashMap.keySet().toArray();
    Random rand = new Random();
    return keyring[rand.nextInt(keyring.length)];
}

It seems like there ought to be a more elegant way of doing this, I've read the following post. but it seems even more convoluted than the way I'm doing it, If the following solution is the better? why is that so? Selecting random key and value sets from a Map in Java

Answer 1

There is no facility in a HashMap to return an entry without knowing the key so, if you want to use only that class, what you have is probably as good a solution as any.

Keep in mind however that you're not actually restricted to using a HashMap .

If you're going to be reading this collection far more often than writing it, you can create your own class which contains both a HashMap of the mappings and a different collection of the keys that allows random access (like a Vector ).

That way, you won't incur the cost of converting the map to a set then an array every time you read, it will only happen when necessary (adding or deleting items from your collection).

Unfortunately, a Vector allows multiple keys of the same value so you would have to defend against that when inserting (to ensure fairness when selecting a random key). That will increase the cost of insertion.

Deletion would also be increased cost since you would have to search for the item to remove from the vector.

I'm not sure there's an easy single collection for this purpose. If you wanted to go the whole hog, you could have your current HashMap , a Vector of the keys, and yet another HashMap mapping the keys to the vector indexes.

That way, all operations (insert, delete, change, get-random) would be O(1) in time, very efficient in terms of time, perhaps less so in terms of space :-)

Or there's a halfway solution that still uses a wrapper but creates a long-lived array of strings whenever you insert, change or delete a key. That way, you only create the array when needed and you still amortise the costs. Your class then uses the hashmap for efficient access with a key, and the array for random selection.

And the change there is minimal. You already have the code for creating the array, you just have to create your wrapper class which provides whatever you need from a HashMap (and simply passes most calls through to the HashMap ) plus one extra function to get a random key (using the array).

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Now, I'd only consider using those methods if performance is actually a problem though. You can spend untold hours making your code faster in ways that don't matter :-)

If what you have is fast enough, it's fine.

Answer 2

Why not use the Collections.shuffle method, saved to a variable and simply pop one off the top as required.

http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#shuffle(java.util.List)

Answer 3

You could avoid copying the whole keyset into a temporary data structure, by first getting the size, choosing the random index and then iterating over the keyset the appropriate number of times.

This code would need to be synchronized to avoid concurrent modifications.

Answer 4

If you really just want any element of a set, this will work fine.

String s = set.iterator().next();

If you are unsure whether there is an element in the set, use:

String s;
Iterator<String> it = set.iterator();
if (it.hasNext()) {
    s = it.next();
}
else {
    // set was empty
}