Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win


map in C++ v/s HashMap in Java

Posted on 2011-03-01
Medium Priority
Last Modified: 2012-05-11
Could you tell me if the time complexity of operations with a map in C++ and HashMap in Java are the same?
IF not, please tell me the time complexity of operations such as Insert/Delete/Search in a map in C++.

Question by:dshrenik
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
LVL 86

Expert Comment

ID: 35006673
I would doubt they were any different. In theory both languages should use the best possible complexity
LVL 53

Accepted Solution

Infinity08 earned 1336 total points
ID: 35006744
std::map in C++ is NOT a hash table. Most implementations use a BST (binary search tree) to implement std::map, so its time complexity is quite different from that of a hash table.

For example : std::map has O(log n) search complexity for example, while a well-implemented hash table would have O(1) complexity for the same operation.

Author Comment

ID: 35006818
I'm sorry.

Can you tell if C++'s hash_map and Java's HashMap have the same time and space complexity?
Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

LVL 53

Expert Comment

ID: 35006854
>> Can you tell if C++'s hash_map and Java's HashMap have the same time and space complexity?

I'm not 100% sure about the Java HashMap implementation, but afaik there is a normal hash table behind it.
The std::hash_map in C++ is currently an extension, so the specific implementation depends on the implementer (but the general consensus seems to be a normal hash table).

So, yes : the complexities would be very similar, if not the same.

As always with hash tables though : a lot depends on your choice of hash function, and a proper dimensioning of the hash table (as well as collision strategy if appropriate).
LVL 35

Expert Comment

ID: 35008655
can you tell about your keys? and the number of entries now and in future? is your main concern a fast read and find of existing keys or a fast insert/update/delete ?

you may think of making test implementations for either map. if you can serve realistic test data you reliably will know what is the best container for your purpose.

LVL 28

Assisted Solution

dpearson earned 664 total points
ID: 35009443

Can you tell if C++'s hash_map and Java's HashMap have the same time and space complexity?

In terms of space, both will be O(n) - there is one entry in the map for each key and then a list of entries where the keys collide, but each (key,value) pair in the map will appear only once in the data structure so they're O(n) for space.

In terms of time complexity, inserts will be O(1) [locating the hashed entry is a constant time operation and adding to the end of the list of items for a key should also be a constant time operation].  Reads will be O(log n) because the process is locate the hashed entry (which is constant time) and then walk the list of items with the same hash value - which will tend to log n for a large n and a fixed map size.  (If you make your map as large as your key space then this will instead be O(1) - but that's not a typical usage pattern for maps).

C++ and Java will both have the same complexity.

LVL 53

Assisted Solution

Infinity08 earned 1336 total points
ID: 35009478
>> (If you make your map as large as your key space then this will instead be O(1) - but that's not a typical usage pattern for maps).

It is typical for many hash table needs to avoid collisions as much as possible though.

Featured Post

Enroll in October's Free Course of the Month

Do you work with and analyze data? Enroll in October's Course of the Month for 7+ hours of SQL training, allowing you to quickly and efficiently store or retrieve data. It's free for Premium Members, Team Accounts, and Qualified Experts!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

For beginner Java programmers or at least those new to the Eclipse IDE, the following tutorial will show some (four) ways in which you can import your Java projects to your Eclipse workbench. Introduction While learning Java can be done with…
This article shows you how to optimize memory allocations in C++ using placement new. Applicable especially to usecases dealing with creation of large number of objects. A brief on problem: Lets take example problem for simplicity: - I have a G…
This tutorial will introduce the viewer to VisualVM for the Java platform application. This video explains an example program and covers the Overview, Monitor, and Heap Dump tabs.
The goal of the video will be to teach the user the concept of local variables and scope. An example of a locally defined variable will be given as well as an explanation of what scope is in C++. The local variable and concept of scope will be relat…
Suggested Courses

618 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question