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So I need a 2-dimensional ConcurrentHashMap .
It has to be as blazing fast as possible, as I'm going to be adding to and updating its values extremely frequently. It's in a multithreaded application, hence the choice to use ConcurrentHashMap instead of...
Started by DanM on
, 5 posts
by 5 people.
Answer Snippets (Read the full thread at stackoverflow):
*/ void recordAccess(Int2DMap m) { } /** * This method is invoked whenever() - start) / 1 ); Map map2 = new HashMap....
However it will need that's already in the HashMap.
I find it is faster than using an IntPair as key .
Java HashMap.
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So I have 2 questions about HashMaps in Java:
(1) What is the correct way to initialize a HashMap? I think it might be best in my situation to use:
HashMap x = new HashMap();
But Eclipse keeps suggesting that I use:
HashMap<something, something>...
Started by hatorade on
, 8 posts
by 8 people.
Answer Snippets (Read the full thread at stackoverflow):
A HashMap can hold any object as a value, even if it is another ....
Eclipse is recommending that you declare the type of the HashMap because that enforces some type want to do the latter, try declaring map as HashMap<String,Object> .
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This is a Java related question. If I create a new HashMap and a new List, and then place the List inside the Hashmap with some arbitrary key and then later call List.clear() will it affect what I've placed inside the hashmap? The deeper question here...
Started by Diego on
, 6 posts
by 6 people.
Answer Snippets (Read the full thread at stackoverflow):
When you define
List<SomeType> list;
you're defining in Java either, though references... .
What's happening here is that you're placing a pointer to a list in the hashmap, not the list itself.
The list you're referencing in the hashmap.
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Ask your Facebook Friends
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I have two HashMaps: FOO & BAR.
HashMap FOO is a superset of HashMap BAR.
How do I find out what 'keys' are missing in HashMap BAR (i.e. exists in FOO but not BAR)?
Answer Snippets (Read the full thread at stackoverflow):
Set missing = new HashSet(foo.keySet()); missing.removeAll(bar.keySet());
If you're using google-collections (and realistically I think it should be on the classpath of more or less every non-trivial Java project) it's just:
Set<X> missing = Sets... .
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Hi,
does anyone know of a good library or method of converting the contents of a HashMap to XML and then parsing this to reconstruct the HashMap?
Started by Aly on
, 3 posts
by 3 people.
Answer Snippets (Read the full thread at stackoverflow):
Http://xstream.codehaus.org/
It has a HashMap converter.
The XStream library is what you want.
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What is the more efficient approach for using hashmaps?
A) Use multiple smaller hashmaps, or
B) store all objects in one giant hashmap?
(Assume that the hashing algorithm for the keys is fairly efficient, resulting in few collisions)
CLARIFICATION: Option...
Started by Jen on
, 4 posts
by 4 people.
Answer Snippets (Read the full thread at stackoverflow):
Keep one hashmap for each logical mapping from key to value.
Up a single map into multiple maps.
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Hi,
I have a pretty big (100'000s of entries) HashMap . Now, I need a HashSet containing all the keys from this HashMap . Unfortunately, HashMap only has a keySet() method which returns a Set but not a HashSet .
What would be an efficient way to generate...
Started by Haes on
, 6 posts
by 6 people.
Answer Snippets (Read the full thread at stackoverflow):
MapBackedHashSet extends HashSet { private HashMap theMap; public MapBackedHashSet(HashMap theMap.
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Hi all gurus!
I am facing a problem. I used the following hashmap to store some values
HashMap<String, Object> hm = new HashMap<String, Object>();
The object is another HashMap (yeah, you can say HashMap of HashMap). String is to store book...
Answer Snippets (Read the full thread at stackoverflow):
Maybe you already know them (if they are fixed), otherwise you have to loop over all of them... .
The first thing you need to do is to figure out the unique keys in your second-level map and assign them to columns .
Definitely no built-in function to do this.
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If I have the value "foo", and a hashmap ftw for which ftw.containsValue("foo") returns true , how can I get the corresponding key? Do I have to loop through the hashmap? What is the best way to do that?
Started by Rosarch on
, 8 posts
by 8 people.
Answer Snippets (Read the full thread at stackoverflow):
Yes, you have to loop through the hashmap, unless.
It sounds like the best way is for you));
Then when you have a value, you also have the key .
All the pairs in the hashmap, using map.entrySet().
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Could anyone please tell important use cases of Identity HashMap?
Started by Thomman on
, 6 posts
by 6 people.
Answer Snippets (Read the full thread at stackoverflow):
In a HashMap....
HashMap creates lots of objects.
This is about 33% faster than HashMap for gets! It probably uses less memory too.
Experience from me:
IdentityHashMap leaves a much smaller memory footprint compared to HashMap.
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