Question

I have some map with values:

private static Map<String, Long> hodnotyUdaju = new HashMap<String, Long>();

static {
    hodnotyUdaju.put("a", 1L);
    hodnotyUdaju.put("b", 2L);
    hodnotyUdaju.put("b", 4L);
    hodnotyUdaju.put("d", 8L);
    hodnotyUdaju.put("e", 16L);
    hodnotyUdaju.put("f", 32L);
    hodnotyUdaju.put("g", 64L);
    hodnotyUdaju.put("h", 128L);
    hodnotyUdaju.put("i", 256L);
    hodnotyUdaju.put("j", 512L);
    .
    .
    .
}

and method which convert set of value to long

public static long bitovaHodnotaSeznamuUdaju(Set<String> udaje) {
    long bitovaHodnota = 0;

    for (String udaj : udaje) {
        Long tmp = hodnotyUdaju.get(udaj);
        if (tmp != null)
            bitovaHodnota += tmp;
    }

    return bitovaHodnota;
}

how I can implement this method vice versa ? I need from long to create set set of String

UPDATE

methos should looks like:

public static Set<String> bin2Udaje(long binUdaje) {
    Set<String> udaje = new HashSet<String>();
    for (Map.Entry<String, Long> entry : hodnotyUdaju.entrySet()) {
        if (udaje some operation entry.getValue() ) {
            udaje.add(entry.getKey());
        }
        return udaje;

    }
}

Answer 1

It sounds like you should really be using an Enum for this instead of a map. For example, while you’re guaranteed to use values 1, 2, 4, 8… you’ll end up with an unambiguous answer – but doing that on a general map won’t work.

I would suggest you use an enum which has an extra value for the string if you need it, and you can use 1L << value.ordinal() to get a long value if you really want. To represent a general set of these values, however, you can simply use EnumSet.

If this isn’t appropriate, please give us more information.

EDIT: If you really want to stick to your existing structure, you could use:

public static Set<String> valueToKeySet(long value) {
    Set<String> keys = new HashSet<String>();
    for (Map.Entry<String, Long> entry : hodnotyUdaju.entrySet()) {
        if ((value & entry.getValue()) != 0) {
            keys.add(entry.getKey());
        }
    }
    return keys;
}

This will give unexpected results if your map doesn’t only contain single-bit-set values.