Inserting enum values into HashMap

Question

I am doing a program in which i need to insert enum values into a HashMap. Can we really do that? I tried out it in many ways, but failed.

Can anyone please help me? Through the program I need to implement a HashMap containing 4 threadpools (whose names act as key) corresponding to which i have a ThreapoolExcecutor object.

Below given is my code :

public class MyThreadpoolExcecutorPgm {
    enum ThreadpoolName
    {
        DR,
        PQ,
        EVENT,
        MISCELLENEOUS;
    }
    private static String threadName;
    private static HashMap<String, ThreadPoolExecutor> threadpoolExecutorHash;

    public MyThreadpoolExcecutorPgm(String p_threadName) {
        threadName = p_threadName;

    }

    public static void fillthreadpoolExecutorHash() {
        int poolsize = 3;
        int maxpoolsize = 3;
        long keepAliveTime = 10;
        ThreadPoolExecutor tp = null;
        threadpoolExecutorHash = new HashMap<String, ThreadPoolExecutor>();

        ThreadpoolName poolName ;
        tp = new ThreadPoolExecutor(poolsize, maxpoolsize, keepAliveTime,
                TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(5));

        threadpoolExecutorHash.put(poolName,tp);    //Here i am failing to implement currect put()


    }

Answer 1

You may want to consider using an EnumMap instead of a HashMap here. EnumMap is much faster and more space-efficient than a HashMap when using enumerated values, which seems to be precisely what you're doing here.

Answer 2

Sure, it's possible to have enums as keys in a Map.

You get an error because the threadpoolExecutorHash maps from String s to ThreadPoolExecutor s, and it fails because you're trying to insert a String ( poolName ) as key.

Just change from

threadpoolExecutorHash = new HashMap<String, ThreadPoolExecutor>();

to

threadpoolExecutorHash = new HashMap<ThreadpoolName, ThreadPoolExecutor>();

---

As mentioned by @templatetypedef, there is even a special Map implementation, EnumMap tailored for using enums as keys.

Answer 3

You're using a String as the key of your HashMap, you should be using the Enum class instead. Your code should look like this :

public class MyThreadpoolExcecutorPgm {
enum ThreadpoolName
{
    DR,
    PQ,
    EVENT,
    MISCELLENEOUS;
}
private static String threadName;
private static HashMap<ThreadpoolName, ThreadPoolExecutor> threadpoolExecutorHash;

public MyThreadpoolExcecutorPgm(String p_threadName) {
    threadName = p_threadName;

}

public static void fillthreadpoolExecutorHash() {
    int poolsize = 3;
    int maxpoolsize = 3;
    long keepAliveTime = 10;
    ThreadPoolExecutor tp = null;
    threadpoolExecutorHash = new HashMap<ThreadpoolName, ThreadPoolExecutor>();

    ThreadpoolName poolName ;
    tp = new ThreadPoolExecutor(poolsize, maxpoolsize, keepAliveTime,
            TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(5));

    threadpoolExecutorHash.put(poolName,tp);    //Here i am failing to implement currect put()
}