Java equivalent of Perl's hash

Question

I've been using a lot Perl hashes due to super flexibility and convenient. for instance, in Perl I can do the following:

$hash{AREA_CODE}->{PHONE}->{STREET_ADDR}

I wondering how can I accomplish the same thing with Java, I guess it has something to do with HashMap?

Thanks,

Answer 1

I've been using a lot Perl hashes due to super flexibility and convenient. for instance, in Perl I can do the following: $hash{AREA_CODE}->{PHONE}->{STREET_ADDR} I wondering how can I accomplish the same thing with Java, I guess it has something to do with HashMap?

The Java code which approximates the following Perl code:

my %hash;
$hash{AREA_CODE}{PHONE}{STREET_ADDR} = "221B Baker Street";
printf "Street address is %s\n", $hash{AREA_CODE}{PHONE}{STREET_ADDR};

is

HashMap<String, HashMap<String, HashMap<String, String>>> hash =
    new HashMap<String, HashMap<String, HashMap<String, String>>>();

hash.put("AREA_CODE", new HashMap<String, HashMap<String, String>>());
hash.get("AREA_CODE").put("PHONE", new HashMap<String, String>());
hash.get("AREA_CODE").get("PHONE").put("STREET_ADDR", "221B Baker Street");

System.out.printf("Street address is %s\n",
    hash.get("AREA_CODE").get("PHONE").get("STREET_ADDR"));

Isn’t that special? :)

I say ‘approximates’ for many reasons. One of these is that in Java you’ll be frustrated to the point of extreme apoplexy merely for wanting to then do on the next line of Java the equivalent of this perfectly straightforward Perl code:

$hash{AREA_CODE}{PREFIX} = 800;

If you want Perl’s flexibility and convenience in things like this, Java simply isn’t going to give it to you. Even worse, its partisans will often berate you for even expressing such a desire.

Answer 2

First of all, your specific example ($hash{AREA_CODE}->{PHONE}->{STREET_ADDR}), with hard-coded strings as hash keys, is not really a useful data structure in Java as Michael Carman pointed out - it should be stored as a class with attributes (and to be honest it's a bad data structure in concept - data like this is more likely to be an array of phones, not hash of phones).

Second, assuming you actually meant $hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR}, it looks like everyone's Java code so far was NOT implementing a generic equivalent code - the code all assumed that the Java hash is newly initialized for storing example OR fully populated for retrieval example (in other words, as leonbloy's answer noted, is missing autovivification feature).

The correct code mimiquing autovivification is:

// This method will ensure that hash-of-hash-of-hashes structure exists of a given set of 3 keys.
public HashMap<String, HashMap<String, HashMap<String, Object>>>
 autovivification_3rd_level (
           HashMap<String, HashMap<String, HashMap<String, Object>>> hash
         , String AREA_CODE, String PHONE, String STREET_ADDR) {
    if (hash == null) {
        hash = new HashMap<String, HashMap<String, HashMap<String, Object>>>();
    }
    if (!hash.contains(AREA_CODE) || hash.get(AREA_CODE) == null) {
        hash.put(new HashMap<String, HashMap<String, Object>>());
    }
    HashMap<String, HashMap<String, Object>> AREA_CODE_hash
         = (HashMap<String, HashMap<String, Object>>) hash.get(AREA_CODE);
    if (!AREA_CODE_hash.contains(PHONE) || AREA_CODE_hash.get(PHONE) == null) {
        AREA_CODE_hash.put(new HashMap<String, Object>());
    }
    return hash;
}

////////////////////////////////////////////////////////////////////////////////////    

// Equivalent to Perl's "$hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR} = value;"
public Object put_3d_level_hash(
          HashMap<String, HashMap<String, HashMap<String, Object>>> hash
        , String AREA_CODE, String PHONE, String STREET_ADDR,
        , Object value) {
    hash = autovivification_3rd_level(hash, AREA_CODE, PHONE, STREET_ADDR);
    return hash.get(AREA_CODE).get(PHONE).put(STREET_ADDR, value);
}
put_3d_level_hash(hash, AREA_CODE, PHONE, STREET_ADDR, obj);

////////////////////////////////////////////////////////////////////////////////////    

// Equivalent to Perl's "$var = $hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR}"
public Object get_3d_level_hash(HashMap<String, HashMap<String, HashMap<String, Object>>> hash
                       , String AREA_CODE, String PHONE, String STREET_ADDR) {
    hash = autovivification_3rd_level(hash, AREA_CODE, PHONE, STREET_ADDR);
    return hash.get(AREA_CODE).get(PHONE).get(STREET_ADDR);
}
Object obj = get_3d_level_hash(hash, AREA_CODE, PHONE, STREET_ADDR);