checking if key exists in hashmap where the key is a nested hashmap

Question

I have the following HashMap:

private HashMap<HashMap<Integer,String>,ArrayList<String>> unique_schemas = new HashMap<HashMap<Integer,String>,ArrayList<String>>();

I am having no problems adding entries to it or printing out its contents but I'm not sure how to check for an existing entry.

I have tried:

//create temp HashMap to check against HashMap<Integer,String> mapkey = new HashMap<Integer,String>(); //populate it with the values to check for mapkey.put(parentId,text); if (unique_schemas.containsKey(mapkey.get(0))) { //do whatever when the entry exists }

This isn't working, I know it's wrong somewhere but I'm at a loss. Can anyone shed some light on my problem?

unique_schemas key is a HashMap<Integer,String>, and you are comparing it between mapkey.get(0) which is a String. — Héctor Valls
– Héctor Valls, Commented Aug 25, 2014 at 11:12
@bigdestroyer that makes sense, I'm not sure how the syntax of the comparison should be written. — C_B
– C_B, Commented Aug 25, 2014 at 11:13

Olimpiu POP · Accepted Answer · 2014-08-25 11:14:05Z

You should change the check to:

//create temp HashMap to check against HashMap<Integer,String> mapkey = new HashMap<Integer,String>(); //populate it with the values to check for mapkey.put(parentId,text); if (unique_schemas.containsKey(mapkey)) { //do whatever when the entry exists }

If you are sure that there is just one pair in the map you could also allocate just 1 element to the map:

private HashMap<HashMap<Integer,String>,ArrayList<String>> unique_schemas = new HashMap<HashMap<Integer,String>,ArrayList<String>>(1);

Thomas · Accepted Answer · 2014-08-25 11:14:03Z

If your key is of type HashMap<Integer, String> you can't query the map with a key of type String which unique_schemas.containsKey(mapkey.get(0)) would do, since mapKey.get(0) would return the string value for key 0, if it exists.

So try unique_schemas.containsKey(mapkey) instead, to use the entire map as a key, as you specified it.

Btw, I'd most probably refactor that whole construct, since using a map as a key seems quite odd to me. Can the key really contain multiple combinations of id and text? Would it be a problem to have a Map<KeyClass, List<String>> instead, where KeyClass would look like this:

class KeyClass { Integer id; String text; public int hashCode() { ... } public boolean equals(Object o) { ... } }

Note that this is just a stub and you'd need to expand that, especially the implementation of hashCode() and equals().

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