java hashtable

7/30/2019 Java Hashtable

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javapapers.com/core-java/java-hashtable/

Java Hashtable

17/08/2011

Hashtable is an implementation of a key-value pair data structure in java. You can store and retrieve a value

using a key and it is an identifierof the value stored. It is obvious that the key should be unique.

java.util.Hashtable extends Dictionary and implements Map. Objects with non-null value can be used as a key or

value. Key of the Hashtable must implement hashcode() and equals() methods. By the end of this article you will

find out the reason behind this condition.

Generally a Hashtable in java is created using the empty constructorHashtable(). Which is a poor decision and

an often repeated mistake. Hashtable has two other constructors

and

. Initial capacity is number of buckets created at the time of Hashtable instantiation. Bucket is a logical space of

Hashtable(intinitialCapacity)

Hashtable(intinitialCapacity, floatloadFactor)
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javapapers.com/core-java/java-hashtable/ 2

storage for Hashtable.

Hashing and Hashtable

Before seeing javas Hashtable in detail you should understand hashing in general. Assume that, v is a value to be

stored and k is the key used for storage / retrieval, then h is a hash function where v is stored at h(k) of table. To

retrieve a value compute h(k) so that you can directly get the position of v. So in a key-value pair table, you need

not sequentially scan through the keys to identify a value.

h(k) is the hashing function and it is used to find the location to store the corresponding value v. h(k) cannot

compute to a indefinite space. Storage allocated for a Hashtable is limited within a program. So, the hasing

function h(k) should return a number within that allocated spectrum (logical address space).

Hashing in Java

Javas hashing uses uses hashCode() method from the key and value objects to compute. Following is the core

code from Hashtable where the hashCode h is computed. You can see that both keys and values hashCode(method is called.

It is better to have your hashCode() method in your custom objects. String has its own hashCode methode and i

computes the hashcode value as below:

If you dont have a hashCode() method, then it is derived from Object class. Following isjavadoc comment of

hashCode() method from Object class:

If you are going to write a custom hashCode(), then follow the following contract:

The following is to improve performance of the Hashtable.

h += e.key.hashCode() ^ e.value.hashCode();

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

* Returns a hash code value forthe object. This method is * supported forthe benefit of hashtables such as those provided by * java.util.Hashtable.

* The general contract of hashCode is: * * Whenever it is invoked on the same object more than once during

* an execution of a Java application, the hashCode method * must consistently returnthe same integer, provided no informatio * used in equals comparisons on the object is modified.

* If two objects are equal according to the equals(Object) * method, then calling the hashCode method on each of * the two objects must produce the same integer result.
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hashCode() guarantees distinct integers by using the internal address of the object.

Collision in Hashtable

When we try to restrict the hashing functions output within the allocated address spectrue limit, there is a

possibility of a collision. For two different keys k1 and k2, if we have h(k1) = h(k2), then this is called collision in

hashtable. What does this mean, our hashing function directs us store two different values (keys are also

different) in the same location.

When we have a collision, there are multiple methodologies available to resolve it. To name a few hashtable

collision resolution technique, separate chaining, open addressing, robin hood hashing, cuckoo hashing,

etc. Javas hashtable uses separate chaining for collision resolution in Hashtable.

Collision Resolution in javas Hashtable

Java uses separate chaining for collision resolution. Recall a point that Hashtable stores elements in buckets. In

separate chaining, every bucket will store a reference to a linked list. Now assume that you have stored anelement in bucket 1. That means, in bucket 1 you will have a reference to a linked list and in that linked list you

will have two cells. In those two cells you will have key and its corresponding value.

Why do you want to store the key? Because when there is a collision i.e., when two keys results in samehashcode and directs to the same bucket (assume bucket 1) you want to store the second element also in the

same bucket. You add this second element to the already created linked list as the adjacent element.

Now when you retrieve a value it will compute the hash code and direct you to a bucket which has two

elements. You scan those two elements alone sequentially and compare the keys using their equals() method.

When the key mathches you get the respective value. Hope you have got the reason behind the condition that

your object must have hashCode() and equals() method.

Java has a private static class Entry inside Hashtable. It is an implementation of a list and you can see there, it


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stores both the key and value.

Hashtable performance

To get better performance from your java Hashtable, you need to

1) use the initialCapacity and loadFactor arguments

2) use them wisely

while instantiating a Hashtable.

initialCapacitiy is the number of buckets to be created at the time of Hashtable instantiation. The number of

buckets and probability of collision is inversly proportional. If you have more number of buckets than needed

then you have lesser possibility for a collision.

For example, if you are going to store 10 elements and if you are going to have initialCapacity as 100 then you

will have 100 buckets. You are going to calculate hashCoe() only 10 times with a spectrum of 100 buckets. The

possibility of a collision is very very less.

But if you are going to supply initialCapacity for the Hashtable as 10, then the possibility of collision is very largeloadFactor decides when to automatically increase the size of the Hashtable. The default size of initialCapacity is

11 and loadFactor is .75 That if the Hashtable is 3/4 th full then the size of the Hashtable is increased.

New capacity in java Hashtable is calculated as follows:

If you give a lesser capacity and loadfactor and often it does the rehash() which will cause you performance

issues. Therefore for efficient performance for Hashtable in java, give initialCapacity as 25% extra than you need

and loadFactor as 0.75 when you instantiate.

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101 comments on Java Hashtable

1. joshi said on 17/08/2011,

if youre really interested in performance you should stick to hashmap if multithreading is no issue. You

really should add a section about Hastable vs. Hashmap in this article

Reply

2. Joe said on 17/08/2011,

yes Joshi you are right. Between Hashtable and Hashmap, Hashmap is preferred in non-threaded

environment.

When you are forced to use Hashtable, consider these performance tips.

Sure I will come up with an article about Hashtable vs Hashmap.

Reply

3. Tito said on 18/08/2011,

good info. keep posting. can u write some info on the DAO patterns.

Reply

4. Jigar Joshi said on 18/08/2011,

Nicely explained..!

See About : Hash Table VS HashMap :http://stackoverflow.com/questions/40471/java-hashmap-vs-

hashtable

Reply

5. Jigar Joshi said on 18/08/2011,

and implementation of hashCode for String is

h = 31*h + val[off++];

in standard jdk 6 & 7
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Reply

6. Jacob said on 18/08/2011,

See this also ,,http://stackoverflow.com/questions/6493605/how-does-java-hashmap-work

Reply

7. Dev Ghotkule said on 18/08/2011,

Most use full information.

Reply

8. Praveen said on 18/08/2011,

Good article after a long time

Kindly start posting on JEE6 features.

Reply

9. Madhusudhan said on 18/08/2011,

Great. Clear & Simple. Thank you.

Reply

10. KiranJyothi said on 18/08/2011,

It is very nice and clear. Can you take some real time examples and explain (like yellow pages). That

would be great!!

Thank you for this post!!

Cheers,

Jyothi

Reply

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11. shashi said on 24/08/2011,

nice artical

Reply

12. shyam said on 24/08/2011,

this kind of explanation is clear

Reply

13. Born said on 25/08/2011,

Hashtable is preferred in non-threaded environment. you mixed it up? ;) hashtable is the

synchronized version, so use hashmap instead, if multi-threading is not needed. And besides one should

always consider the synchronizedXYZ Methods of the Collections class instead of using special

synchronized Collection types. So use: Collections.synchronizedMap if you want a synchronized version

of HashMap.

But nevertheless the theory the blog post is talking about is of course the same for the hashmap andconsidered as useful information ;)

Reply

14. Joe said on 25/08/2011,

Oops! Born, thats a typo (very costly one) :0 I have fixed it. Thanks for pointing out.

Reply

15. Anjali said on 07/09/2011,

Nice article Joe. Well explained. Thanks

Reply

16. amit said on 14/09/2011,

great post

Reply

17. Rajesh said on 28/09/2011,

Nice article

Reply

18. Elakkiya said on 29/09/2011,

Such a nice article i have ever seen..

Reply

19. Anant Choubey said on 08/10/2011,
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Worth Reading. Nice post. Looking forward for more :) keep posting

Reply

20. Partha said on 10/10/2011,

Instead of using HashTable, If we use HashMap and put those block in a synchronized block..what you

say on performance?

Reply21. Joe said on 10/10/2011,

@Partha, using HashMap enclosed in a synchronized block is costlier than using a Hashtable. Since, the

synchronization in Hashtable is fine-grained than this approach.

The best possible approach would be analyze the need for synchronization and avoid using Hashtable.

Reply

Ashok said on 17/02/2013,

Dear Joe,

Please explain how this hashCode() returns hash number and why attributes i and j are converted

into String

int i,j;

public int hashCode()

{

String s1 += Integer.toString(i);

String s2 += Integer.toString(j);

return hash;}

Reply

22. satesh said on 14/10/2011,

Very good explanation.Thanks alot :-)

Reply

23. ROCKS said on 15/10/2011,

Very Nice.. Keep it mate!!

Reply

24. Anonymous said on 24/10/2011,

awesome CSS !! i like it very much :D contents are lucid and to the point.. :)

Reply

25. Aman Prakash Mohla said on 29/10/2011,
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Really nice post, comprehensive and upto the mark. I have also covered some important aspects of the

Java Map interface on myblog . You would also find a detailed post on Map synchronization and

concurrent HashMaps.

Reply

26. prabha said on 04/11/2011,

It is very simple language that you used in this blog which makes me very impressive to read a lot of stuff

about java from this blog. I am fortunate to take look at this website. Hopefully you will be coming up with

better and essential information regarding Java.

Thank you so much

from

Prabha

Reply

neeraj said on 30/04/2013,

ok

Reply

27. Grasshopper Networksaid on 07/11/2011,

This is a lucid and nice Article. You can always keep adding the features and extra information. We are

also java developers and provide various projects for free. Would you give us an opportunity for a Guest

Posting to write about Image Processing in Java? We will wait for your positive response.

Reply

28. mangesh said on 15/11/2011,

Nice post.please try to elaborate it with some examples.

Reply

29. haripriya said on 16/11/2011,

very nice site and helpful for beginners :)

Reply

30. CAL said on 17/11/2011,

Hey joe the explanation is really excellent you can also post on struts, spring, hibernate.

Reply

31. muthu said on 22/11/2011,

Joe, Could you explain the Collision resolution for below scenario

Map map = new HashMap();

I- two keys are same .But it does not throw any exception. How it finds collision resolution?
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map.put(abc,12);

map.put(abc,13);

&

II- two keys are different .But hascode values are same for two keys. How it finds collision resolution?

map.put(abc,15);

map.put(cba,18);

Reply

32. JRZ said on 27/11/2011,

Your point to this line when discussing the use of value.hashCode():

h += e.key.hashCode() ^ e.value.hashCode();

A line much like this exists (Hashtable.java:948 in jdk 6u23). However, this line is NOT used in the

regular business of put() and get(). Rather, it is the hashCode Entry inner class, which must overridehashCode() because it overrides equals().

In normal put() and get() operations, the values hashCode is never used. (This must be the case, because

the value is not known in a get operation.) Look at the get() method on line 332 and note that it only calls

key.hashCode().

Thanks for posting this article! Since its getting plenty of readership, I wanted to make sure that people

dont get the wrong idea and think that the values hashCode plays a role in the put operation.

Reply33. Mariusz Lotko said on 28/11/2011,

To be honest, calculating hash using both key.hashCode() and value.hashCode() is done in

Map.Entry.hashCode().

Hashtable uses _only_ the value of key.hashCode() to find value. It would be quite surprising to use

values hash for lookups.

I assume Map.Entry.hashCode() is implemented only for purpose of using Map.Entry as a key:

Map

Which maps par (String, Integer) -> Integer.

Reply

34. Anurag Bansal said on 03/12/2011,

Great post Buddy.

Keep posting :)
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Reply

35. Rajasekar said on 06/12/2011,

Very nice..

Reply

36. Ramesh said on 13/12/2011,

nice explanation! i was struggling to understand about hash table and hash map.Now i am clear.Thanks alot.looking for more stuff.

Reply

37. Thangavel said on 18/12/2011,

Hi Joe,

Its pretty good article for Hashtable. I have got more idea about on this. The Explanation is very nice..

Keep on posting more article, best of luck.

Thangavel L Nathan.

Reply

38. Maganti Suryanarayana Murthy said on 22/12/2011,

Nice Article joe !But if you go into still deeper with articles it will be really great !

Reply

39. Vishnu said on 29/12/2011,

Good work dude. very very easy to understand

Reply

40. Ghanshyam said on 29/12/2011,

Please give a same demo for HashMap also.

Reply

41. chandran said on 04/01/2012,

Really very happy to see this articleThanks a lot

Reply

42. pavan said on 08/01/2012,

Information is very clear and in simple terminology. Thanks a lot.

Reply

43. anonimous said on 10/01/2012,
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ok.thanks da keep posting

Reply

44. chandra said on 18/01/2012,

this blog is great but if u posting the examples with real time that would be better

Reply

45. shrinath said on 23/01/2012,

nice keep it up

Reply

46. Manjunath said on 08/02/2012,

why null is not allowed in Hastable..??

Reply

47. GeekDude said on 16/02/2012,

i think it should be edit.have a look on this code .,, java.util.Hashtable

Reply

48. Arvind Singh said on 21/02/2012,

Very nice discussionit has almost full coverage of the topic,which is most feared one in java..if you

please give some practical examples..then it will increase readers understanding

Reply

49. Raajesh said on 13/03/2012,

Hi Joe,

Worth going through this article well done

Regards

Raajesh

Reply

50. dimuthu said on 25/03/2012,

This is great!

Reply

51. ahmed farag said on 26/05/2012,

i want hashcode for long string without collision please.

Reply
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52. mr.hello said on 12/06/2012,

excellent and super

Reply

53. BLOB said on 23/06/2012,

You are awesome Last thing I am gonna say (Actually the first thing too)..

Reply

54. sweta said on 20/07/2012,

Thanks Joe for making Hashtable simple for me.All my queries are resolved after reading this short and

simple article.You rock :)

Reply


your explanation was good..also pl try to put simple example to construct a hashtable program in java..

by

Reader

Reply


Nice explanation, Ive one question. Plz give two different Keys which produces a same hashcode?

Reply

57. Bhushan said on 03/08/2012,

Please Post article on HashMap vs Hashtable

Reply


Sir,

Please write an article on Thread and Comparable/comparator. I like the the way you are explaining everytopic. This blog became as one of good references.

Reply

59. Muthuraj said on 27/08/2012,

Interesting.please write Hashtable vs Hashmap also. it will really help us

Reply

60. convex.naresh said on 30/08/2012,
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14/20



Thank you for this post. Very useful article for me.

Reply

61. Swati said on 05/09/2012,

Thanks Joe,

can you please provide a full article on java collection framework with example.

Reply62. Anonymous said on 13/09/2012,

http://javapapers.com/wp-content/themes/papers/images/pencil.png

Reply


awesum

thanks for the details.

Reply

64. Bhanu Tekula said on 17/09/2012,

nice article..

Reply

65. Pankaj said on 23/09/2012,

Very good article Joe!

Reply

66. azer said on 26/09/2012,

very awesum. article joe

Reply


wow nice jun jun

Reply68. victor said on 26/09/2012,

wow nice jun jun

Reply

69. Ekanth said on 05/10/2012,

well expland dude.
http://javapapers.com/core-java/java-hashtable/#comment-16915http://javapapers.com/core-java/java-hashtable/?replytocom=16487#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16487http://javapapers.com/core-java/java-hashtable/?replytocom=16486#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16486http://javapapers.com/core-java/java-hashtable/?replytocom=16485#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16485http://javapapers.com/core-java/java-hashtable/?replytocom=16320#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16320http://javapapers.com/core-java/java-hashtable/?replytocom=16046#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16046http://javapapers.com/core-java/java-hashtable/?replytocom=16031#respondhttp://javapapers.com/core-java/java-hashtable/#comment-16031http://javapapers.com/core-java/java-hashtable/?replytocom=15840#respondhttp://javapapers.com/wp-content/themes/papers/images/pencil.pnghttp://javapapers.com/core-java/java-hashtable/#comment-15840http://javapapers.com/core-java/java-hashtable/?replytocom=15501#respondhttp://javapapers.com/core-java/java-hashtable/#comment-15501http://javapapers.com/core-java/java-hashtable/?replytocom=15260#respond


15/20



Reply


first time i understood the hashcode and equalsmethod in java good work it helps lot of people

Reply

71. Ramesh said on 09/10/2012,

Great post Joe :) Keep posting more..

Reply

72. gopal said on 11/10/2012,

gr8

Reply

73. ankit said on 15/10/2012,

its really very gud articlebut it cover only 1 topic..should post more article.

Reply

74. Pankaj said on 29/10/2012,

yeah site is very useful for beginner as well as for those who want to know about any specific topic.

great! Thank you very much

Reply

75. Neeraj said on 31/10/2012,

Also put all Hashing Techniques like Bucket all that.

Reply

76. Manish Kumar said on 06/11/2012,

very very clear explanation on hash table..

Thanks a lot

Reply

77. cc said on 07/11/2012,

wonderful

i love you!

Reply

78. Tamawy said on 15/11/2012,

Quick and briefly useful. Thank you for that. We need easy examples to start with and increase our skills

in hashing.
http://javapapers.com/core-java/java-hashtable/#comment-18934http://www.etaworx.com/http://javapapers.com/core-java/java-hashtable/?replytocom=18661#respondhttp://javapapers.com/core-java/java-hashtable/#comment-18661http://javapapers.com/core-java/java-hashtable/?replytocom=18611#respondhttp://javapapers.com/core-java/java-hashtable/#comment-18611http://javapapers.com/core-java/java-hashtable/?replytocom=18332#respondhttp://javapapers.com/core-java/java-hashtable/#comment-18332http://javapapers.com/core-java/java-hashtable/?replytocom=18164#respondhttp://javapapers.com/core-java/java-hashtable/#comment-18164http://javapapers.com/core-java/java-hashtable/?replytocom=17373#respondhttp://javapapers.com/core-java/java-hashtable/#comment-17373http://javapapers.com/core-java/java-hashtable/?replytocom=17202#respondhttp://javapapers.com/core-java/java-hashtable/#comment-17202http://javapapers.com/core-java/java-hashtable/?replytocom=17059#respondhttp://javapapers.com/core-java/java-hashtable/#comment-17059http://javapapers.com/core-java/java-hashtable/?replytocom=17056#respondhttp://javapapers.com/core-java/java-hashtable/#comment-17056http://javapapers.com/core-java/java-hashtable/?replytocom=16915#respond


16/20



Thank you

Reply

79. kamal sharma said on 30/11/2012,

Thank you very much.

very well explained !!!

Reply80. Anonymous said on 15/12/2012,

Very wonder full article.

I like very much

Reply

81. Srinu D said on 15/12/2012,

Very good article..

I like that.Thank you very much

Reply

82. neha said on 04/01/2013,

thank you.

Reply

83. venkat said on 07/01/2013,

Hi Joe,Can you please clarify my doubt about HashTable.

the locking will happen only for adding/removing the object or every operation (reading also )

Reply

84. Ganesh said on 08/01/2013,

Nice article Joe, helps understand the basics very well !

Reply

85. abhishek said on 08/01/2013,

Hi Joe,

It was nice article

Thanks

Reply

86. Narpath said on 16/01/2013,
http://javapapers.com/core-java/java-hashtable/#comment-21907http://javapapers.com/core-java/java-hashtable/?replytocom=21398#respondhttp://javapapers.com/core-java/java-hashtable/#comment-21398http://javapapers.com/core-java/java-hashtable/?replytocom=21394#respondhttp://javapapers.com/core-java/java-hashtable/#comment-21394http://javapapers.com/core-java/java-hashtable/?replytocom=21336#respondhttp://javapapers.com/core-java/java-hashtable/#comment-21336http://javapapers.com/core-java/java-hashtable/?replytocom=21181#respondhttp://javapapers.com/core-java/java-hashtable/#comment-21181http://javapapers.com/core-java/java-hashtable/?replytocom=20427#respondhttp://javapapers.com/core-java/java-hashtable/#comment-20427http://javapapers.com/core-java/java-hashtable/?replytocom=20425#respondhttp://javapapers.com/core-java/java-hashtable/#comment-20425http://javapapers.com/core-java/java-hashtable/?replytocom=19647#respondhttp://javapapers.com/core-java/java-hashtable/#comment-19647http://javapapers.com/core-java/java-hashtable/?replytocom=18934#respond


17/20



Hi joe,

Please provide How will we handle the error code like 500 internal server error in application level ?

Reply

87. Kushagra Thapar said on 24/01/2013,

Hey Joe!

It is a very nice article, it helped me a lot.But it would be very nice if you explain about how buckets are created and handled in hashtables.

Reply


It was nice article.

Thanks,

Subbu

Reply

89. Gangadhar siraveni said on 06/02/2013,

Hi Good Morning Sir,

I am getting confusion in ovberriding equals and hasshcode methods will please provide in detail

information

Reply

90. Ashok said on 17/02/2013,

Dear Joe,

Please explain how this hashCode() returns hash number and why attributes i and j are converted into

String

int i,j;

public int hashCode()

{

String s1 += Integer.toString(i);

String s2 += Integer.toString(j);

return hash;}

Reply

Sanjay said on 17/02/2013,

Dear Ashok i think u missing some codes how it will return hash whn u not defined

Reply

91. Joseph said on 26/02/2013,
http://javapapers.com/core-java/java-hashtable/#comment-24131http://javapapers.com/core-java/java-hashtable/?replytocom=23597#respondhttp://javapapers.com/core-java/java-hashtable/#comment-23597http://javapapers.com/core-java/java-hashtable/?replytocom=23596#respondhttp://javapapers.com/core-java/java-hashtable/#comment-23596http://javapapers.com/core-java/java-hashtable/?replytocom=23077#respondhttp://javapapers.com/core-java/java-hashtable/#comment-23077http://javapapers/http://javapapers.com/core-java/java-hashtable/?replytocom=22672#respondhttp://javapapers.com/core-java/java-hashtable/#comment-22672http://javapapers.com/core-java/java-hashtable/?replytocom=22331#respondhttp://javapapers.com/core-java/java-hashtable/#comment-22331http://javapapers.com/core-java/java-hashtable/?replytocom=21907#respond


18/20



Really fantastic article, thanks, Joe

Reply

92. Chitta said on 11/03/2013,

Beautiful article

Reply

93. kavitha said on 17/03/2013,

hi joe,

can you tell me the formula to calculate hash function?

Reply

94. vamsee said on 09/04/2013,

Hi Joe,

i have been following your Articles in your websiteThey are just awesome

Can u please come up with Struts explanation Saying what happens inside the ActionServlet and how

RequestProcessor Handles

Reply

95. r.emmanuel angelo said on 11/04/2013,

iam new to java can anyone explain why hash map is used for?

Reply96. pandu said on 22/04/2013,

good explaination

Reply

97. Rajkumar Chaudhary said on 26/04/2013,

Fantastic explanation,You are really Astute person.

Reply

98. Sowmya said on 06/05/2013,

Very Good Explanation

Reply

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