HashMap example in Java

By: Charles Printer Friendly Format    

The HashMap class uses a hash table to implement the Map interface. This allows the execution time of basic operations, such as get() and put(), to remain constant even for large sets.
The following constructors are defined: HashMap( )
HashMap(Map m)
HashMap(int capacity)
HashMap(int capacity, float fillRatio)

The first form constructs a default hash map. The second form initializes the hash map by using the elements of m. The third form initializes the capacity of the hash map to capacity. The fourth form initializes both the capacity and fill ratio of the hash map by using its arguments. The meaning of capacity and fill ratio is the same as for HashSet, described earlier.

HashMap implements Map and extends AbstractMap. It does not add any methods of its own. You should note that a hash map does not guarantee the order of its elements. Therefore, the order in which elements are added to a hash map is not necessarily the order in which they are read by an iterator.

The following program illustrates HashMap. It maps names to account balances. Notice how a set-view is obtained and used.

import java.util.*;
class HashMapDemo {
public static void main(String args[]) {
// Create a hash map
HashMap hm = new HashMap();
// Put elements to the map
hm.put("John Doe", new Double(3434.34));
hm.put("Tom Smith", new Double(123.22));
hm.put("Jane Baker", new Double(1378.00));
hm.put("Todd Hall", new Double(99.22));
hm.put("Ralph Smith", new Double(-19.08));
// Get a set of the entries
Set set = hm.entrySet();
// Get an iterator
Iterator i = set.iterator();
// Display elements
while(i.hasNext()) {
Map.Entry me = (Map.Entry)i.next();
System.out.print(me.getKey() + ": ");
// Deposit 1000 into John Doe's account
double balance = ((Double)hm.get("John Doe")).doubleValue();
hm.put("John Doe", new Double(balance + 1000));
System.out.println("John Doe's new balance: " +
hm.get("John Doe"));

Output from this program is shown here:

Ralph Smith: -19.08
Tom Smith: 123.22
John Doe: 3434.34
Todd Hall: 99.22
Jane Baker: 1378.0
John Doe's current balance: 4434.34

The program begins by creating a hash map and then adds the mapping of names to balances. Next, the contents of the map are displayed by using a set-view, obtained by calling entrySet(). The keys and values are displayed by calling the getKey() and getValue() methods that are defined by Map.Entry. Pay close attention to how the deposit is made into John Doe's account. The put() method automatically replaces any preexisting value that is associated with the specified key with the new value. Thus, after John Doe's account is updated, the hash map will still contain just one "John Doe" account.

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1. Great tutorial. THX.
View Tutorial          By: Velvetin at 2009-03-21 11:05:19

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7. Nice Example...!
View Tutorial          By: Kuldeep Singh at 2009-10-18 23:58:17

8. Hi there, what a nice example, very useful for me
View Tutorial          By: Yanny at 2009-12-10 12:34:26

9. nice article..
View Tutorial          By: yummy at 2010-01-05 21:18:35

10. It is very simple and understandable tutorial, the
View Tutorial          By: Syed Turab at 2010-03-09 07:27:15

11. gr8 tute dude.... ezy n nice..
View Tutorial          By: hErC!! at 2010-04-20 23:52:19

12. The last output is not :John Doe's current balance
View Tutorial          By: Santosh at 2010-06-01 22:54:21

13. This contents and example taken by java complete
View Tutorial          By: kumar at 2010-06-05 03:02:13

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View Tutorial          By: rida at 2010-07-03 22:49:04

15. Good Sample, thanks
View Tutorial          By: Tharani at 2010-08-26 23:10:59

16. Nice, Thanx a lot...
View Tutorial          By: Satish Puli at 2010-09-06 04:01:11

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View Tutorial          By: rakesh at 2010-09-18 01:49:25

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View Tutorial          By: Fernando at 2010-09-22 10:15:59

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View Tutorial          By: Sai at 2010-09-23 21:52:38

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View Tutorial          By: Sujoy Saha at 2010-10-01 03:27:57

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View Tutorial          By: Pravat at 2010-12-16 06:08:16

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View Tutorial          By: Rajesh B at 2011-02-01 21:59:56

26. Very Good Tutorial...
Very Good For Beginne

View Tutorial          By: Yatiraj Chakre at 2011-02-02 04:28:57

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View Tutorial          By: niyaz d at 2011-02-20 23:44:56

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View Tutorial          By: panda at 2011-07-14 09:35:36

34. easy.......
View Tutorial          By: Suman at 2011-07-20 07:04:28

35. Nice and easy to understand......Thanks for same
View Tutorial          By: vikas at 2011-07-21 07:44:30

36. thanks..very nice and easy to understand for new j
View Tutorial          By: Ranjan at 2011-07-22 08:34:50

37. Please help I want to map the data say username an
View Tutorial          By: akshat at 2011-07-25 12:49:06

38. Simple Example. Understood. Thanx.
In aksha

View Tutorial          By: Amila Nadanda at 2011-07-28 06:21:05

39. Thanks Its nice example for learn basic related to
View Tutorial          By: Milind Deshmukh at 2011-08-10 10:47:57

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View Tutorial          By: Milind Deshmukh at 2011-08-10 10:53:17

41. great.............
View Tutorial          By: chanti at 2011-08-21 07:09:28

42. thank you so much!
View Tutorial          By: anita at 2011-10-10 12:16:35

43. nice one....but i have a problem

View Tutorial          By: sami at 2011-10-23 15:32:44

44. I do not understand this line:
Map.Entry me

View Tutorial          By: hap Og at 2011-11-03 01:42:33

45. great example for beginners. Thank you
View Tutorial          By: kumar at 2011-11-24 08:15:14

46. this is simple example but need to more explanatio
View Tutorial          By: Ramalingam at 2012-01-05 06:25:12

47. How do you remove an element from the map?
View Tutorial          By: Shreejit at 2012-01-29 16:53:27

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View Tutorial          By: kathir at 2012-02-15 02:20:14

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View Tutorial          By: junnydc at 2012-02-16 04:04:55

50. Thx for a good one, Can we use anything else inste
View Tutorial          By: NagaSudheer at 2012-04-03 11:53:36

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View Tutorial          By: Sudhir singh at 2012-04-09 08:11:34

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View Tutorial          By: Rohit Bhojani at 2012-04-10 12:49:20

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View Tutorial          By: Swathi Kumar at 2012-04-15 10:44:14

54. Thanks.... Nice Example
View Tutorial          By: Nagarjuna at 2012-05-29 12:00:50

55. Do you have similar sample for nested HashMap?
View Tutorial          By: Vivian T. at 2012-06-14 17:47:20

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65. Why the output list in the wrong order?
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66. Plz catch the exception to make the program comple
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