map with 1 million string in it

Question

I'd like to know the total size of a map like this

Map<String, String> map = new HashMap<>();
map.put(string1, string1);
map.put(string2, string2);
//many more lines following until 1 million entries populated to map

as you can see, the key and value always points to the same string and each string is guaranteed to be 24 char long containing lowercase letters and numeric (0-9) only. is it feasible to store this map (in heap I assume) for long term lookup table? how big is the size of this map?

What do you want to do with the map after you fill it with million values? — Gavriel, Jan 21 '16 at 15:11
@Manu, since it is tagged with memory and heap-memory, I assume he is after the memory size — Viktor Mellgren, Jan 21 '16 at 15:15
If you don't give a correct beginning size to your hashMap, adding the million value will be very slow. — JFPicard, Jan 21 '16 at 15:15
http://stackoverflow.com/questions/52353/in-java-what-is-the-best-way-to-determine-the-size-of-an-object — Nathaniel Johnson, Jan 21 '16 at 15:16
@JFPicard, yes and if you give a large from the beginning it will store more efficiently than if it performs automatic rehashing itself — Viktor Mellgren, Jan 21 '16 at 15:25
By "same string" can you guarantee `key == value` or do u mean just `key.equals(value)`? — SGM1, Jan 21 '16 at 15:29
Also you may want to consider using `HashSet` instead of reinventing the wheel. — SGM1, Jan 21 '16 at 15:58

score 0 · Answer 1 · answered Jan 21 '16 at 15:12

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Assuming your computer has enough free memory, yes. A hashmap can store over 1 billion entries (2³⁰ == 1073741824).

answered Jan 21 '16 at 15:12

Will

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score 0 · Answer 2 · edited May 23 '17 at 12:07

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If you want to check out the approx size of an object in Java, you can check out these questions:

Calculate size of Object in Java

In Java, what is the best way to determine the size of an object?

You could also use a profiling tool to look at the size in runtime. VisualVM, Java Flight Recorder or Java Mission Control

edited May 23 '17 at 12:07

Community

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answered Jan 21 '16 at 15:14

Viktor Mellgren

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score 0 · Answer 3 · answered Jan 21 '16 at 15:15

Hello if you are curious how much memory you java app is consuming, try java visual vm. If you have JDK its located in it's bin folder.

As for you question, this is very bad idea (if its not for testing purposes), it will store (size of the map)*(24 characters in each string) * (2 size of char), because you key and value are always the same and String is immutable (all equal string pointers point to the same object in memory).

map with 1 million string in it

3 Answers3