HashTable

Hi guys,
I was asked this question in the interview. I just wanted to know a better answer.
Q. What are hashtables? How does it work internally.
If you have 1 million values in Hashtables, being a key-pair value, does it generate 1 million hashcodes. How does it work.
My answer was:
I told Hashtables are data structures that help us perform lookup. They store key-value pair and are synchronized. The vales are stored by calculating the hashcode. (after that i was asked the 1 million values ques)
I replied that it does not generate 1 million hashcodes. There might be many hash codes that are same. In that case the values with the same hash code are stored using linked lists one after the another.
Can anyone gimme some better answer and temme if I was right. I have always been asked this 1 million values in hashtable questions. If anyone has some other related questions, it would be very helpful if they could post them here.
Thanks!
P

Hey,
Thank you for that link. It was indeed useful !
P

Similar Messages

How can i get the all values from the Property file to Hashtable?

how can i get the all values from the Property file to Hashtable?
ok,consider my property file name is pro.PROPERTIES
and it contain
8326=sun developer
4306=sun java developer
3943=java developer
how can i get the all keys & values from the pro.PROPERTIES to hashtable
plz help guys..............

The Properties class is already a subclass of Hashtable. So if you have a Properties object, you already have a Hashtable. So all you need to do is the first part of that:Properties props = new Properties();
InputStream is = new FileInputStream("tivoli.properties");
props.load(is);

Unable to reference items in a hashtable

HI,
I'm having a bad PowerShell day today!
My sample script to read in a text file and put the data into a hash table works just fine:
$InputFile=Get-Content C:\Out\ktf06011.REQ
$CurrentUserHashTable = @{}
ForEach ($Line in $InputFile)
$Data=$Line.Split("=")
$Label=$Data[0].Trim()
$Label=$Label.Insert(($Label.Length),"`"")
$Label=$Label.Insert(0,"`"")
$Value=$Data[1].Trim()
$Value=$Value.Insert(($Value.Length),"`"")
$Value=$Value.Insert(0,"`"")
$CurrentUserHashTable.Add($Label,$Value)
$CurrentUserHashTable
However, when I use this within a script that uses a file system watcher event to discover the presence of the text file, I cannot get anything from the hash table. I can see my variables being created, ready to pass on to the .add statement. I have found
a workaround for now - not using a hash table - but would love to know why I see this behaviour and suggestions for getting at the data.
Here is the code that I'm having trouble with.
$folder = 'c:\In'
$OutFolder = 'c:\Out'
$filter = '*.*'
$CurrentUserHashTable = $NULL
$FileSystemWatcher = New-Object IO.FileSystemWatcher $folder, $filter -Property @{IncludeSubdirectories = $false;NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'}
Register-ObjectEvent $FileSystemWatcher Created -SourceIdentifier FileCreated -Action {
$name = $Event.SourceEventArgs.Name
$InputFile=Get-Content $folder\$name
ForEach ($Line in $InputFile)
#Write-Host $Line
$CurrentUserHashTable = @{}
$Data=$Line.Split("=")
#Write-Host "Data: " $Data
$Label=$Data[0].Trim()
$Label=$Label.Insert(($Label.Length),"`"")
$Label=$Label.Insert(0,"`"")
#Write-Host "Label: " $Label
$Value=$Data[1].Trim()
$Value=$Value.Insert(($Value.Length),"`"")
$Value=$Value.Insert(0,"`"")
#Write-Host "Value: " $Value
$CurrentUserHashTable.Add($Label,$Value)
$CurrentUserHashTable
Move-Item $folder\$name $OutFolder -Force
#Label: "ID"
#Value: "abc00001"
#Label: "PID"
#Value: "00001"
#Label: "Title"
#Value: "Mr"
#Label: "Initials"
#Value: "XY"
#Label: "Forename"
#Value: "Mickey"
#Label: "Surname"
#Value: "Mouse"
#Label: "Siteabbrev"
#Value: "UK"
#Label: "Status"
#Value: "STAFF"
#Label: "BUabbrev"
#Value: "NOBU"
#Label: "Department"
#Value: "Finance"
Note the commented bits at the bottom are the output of
Write-Host "Value: " $Value
and
Write-Host "Label: " $Label
Also, if the following are placed within the loop, I see all data items being returned, so I know that they are added to the hash table. If they are added outside the loop, I just get the final entry "Department"
Write-Host "Keys:" $CurrentUserHashTable.Keys
Write-Host "Values:" $CurrentUserHashTable.Values
Many thanks,
Jon

Rhys,
Thanks that was so obvious, I couldn't see it!
I knew the answer would be simple.
Made the amendment to my script and can now get at the data in the hashtable, via (e.g.)
$Department=$CurrentUserHashTable.'"Department"'
Thanks again.
Jon

Difference Between HashMap and HashTable

Difference Between HashMap and HashTable
Please explain with an example

I have a situation in Java Collection and i am not
able to figure a good solution. I am scared about the
performance and memory that wil be used
I have 5 List objects with thousands and thousands of
records in it. The List is populated by a database
query using jdbcTemplate which return like 200,000
records
Each record is identified by POLICY_ID. They may be
List with multiple records for a POLICY_ID
I want to extract each POLICY_ID from all the 5 List
and make a single List object for each POLICY_ID and
for each List and pass it to a print job which will
print the data for a POLICY_ID
Example
Let say we have POLICY_ID = 15432
List1 has one record for 15432
List2 has one record for 15432
List3 has one record for 15432
List4 has three record for 15432
List5 has three record for 15432
From the 200,000 records in List1 i want to generate
a seperate list with 1 record for policy id 15432 and
let name is Listperpolicy
after this logic we have
Listperpolicy1
Listperpolicy2
Listperpolicy3
Listperpolicy4
Listperpolicy5
call print job ( Listperpolicy1, Listperpolicy2,
Listperpolicy3, Listperpolicy4, Listperpolicy5)
Please let me know
Thanks a Lotttttttttdon't worry about performance until you've got a working application. second-guessing what the performance bottlenecks will be is futile

How can I get the total "values" in a hashtable ?

i know that i can get the total values in a hashtable by hash.elements() method. It returns an
enumeration with all the values present in this hashtable. this is fine upto here.
Now the preoblem is:
According to what rule this enumeration will be returned. I mean..
If i added in key A with value a,
then key B with value b;
then key C with value c;
then key D with value d;
(They all are objects of type String)
now i call ... hash.elements(); Suppose it returns Enumeration enum;.
Now in what order they all are present in this hashtable.
Meaning is that if i move arond this enum in what sequence they all will be returned.
option A ) In the same order as they were inserted in hashtable.
option B ) According to LIFO;
option C) There is no fix rules , simply it return all the elements and u cannot judge that the first element in enum was really the first element inserted in the hashtable and the second element of enum was really the second element inserted in the hashtable.
What do u think..which option is correct ?
Ny idea will highly appreciated.
Thanks in advance.
Sanjeev Dhiman

hi, i am again..
boss ! this is not true..u just change the order and or change the keys and something like ...
"Sanjeev", "hello"
"Dhiman", "hi"
"Technosys" ,"Services"
u will find that its not LIFO..really i was thinking before coding my project that option A is correct and with knowledge i wrote 3 - 4 classes but when i run the programm its starts throwing errors.
so, i posted this question. I think "apppu" is right.
I think , firstly hash is calculated for each value and that value is returned which can be received in a fastest way and hence not necessarily in LIFO and FIFO..
Thanks to u also as u gave ur precious time for me.
Once again.
Thank you very much.
Sanjeev Dhiman

Problem in Jtree while retrieving values from a hashtable

Hi
I am trying to show some values in a Jtree.I am receiving the values in a hashtable.The hashtable contains unique key but duplicate values.i want to show that values of that hashtable in a tree(but unique values)and under each value show all the key of it.can you please tell me how to do that.
thanks

If i understand right, you want to do something like this:
               Hashtable values = yourhashtable;
          DefaultMutableTreeNode root = new DefaultMutableTreeNode("Values");
          HashMap<Object, DefaultMutableTreeNode> nodes = new HashMap<Object, DefaultMutableTreeNode>();
          Iterator ii = values.keySet().iterator();
          while (ii.hasNext()) {
               Object key = ii.next();
               Object value = values.get(key);
               if (nodes.containsKey(value)) {
                    DefaultMutableTreeNode node = nodes.get(value);
                    DefaultMutableTreeNode child = new DefaultMutableTreeNode(key);
                    node.add(child);
               } else {
                    DefaultMutableTreeNode node = new DefaultMutableTreeNode(value);
                    DefaultMutableTreeNode child = new DefaultMutableTreeNode(key);
                    node.add(child);
                    nodes.put(value, node);
                    root.add(node);
          JTree tree = new JTree(root);Hope this helps,
Alex.

Reading Values from a Hashtable.Please guideu

Chaps,
I am in a strange situation
I am having a hashtable with the flwg values
Entertainment Games
Entertainment Video
PIM Calculator
Entertainment Jump
Settings Set
Entertainment Poker
Now,I need to search this hashtable for the
Object 'Entertainment' and whatever is the
value of this,is put in an Array
for this I am using:
String s[];
Enumeration e;
for(int i=0; i<ht.size(); i++){
String s = (String)ht.get("Entertainment");
// I am stuck after this? Can some one please help?

Object value = ht.get("Entertainment");
But if you are doing this (I can't tell if this is what you're doing from your first post, though):
ht.put("Entertainment", onething);
ht.put("Entertainment", anotherthing);
ht.put("Entertainment", athirdthing);
Then there will only be athirdthing in the table, cuz it'll overwrite the previous values.

Need help in using hashtable as a property of a form bean

Hi,
Is it possible to use a hashtable as a property of the bean.
Well this is the problem i have
I am using a hashtable as property and i want it to store from(/retrive into} form as ints/strings
I have a
JSP Page
SampleAction --Action
SampleForm --ActionForm
Sample(bean)
=============
JSP Page
(within the html:form tag)
<html:text property="sample.number" />
====================
struts-config links to
SampleAction
SampleForm
==============
SampleForm
Sample sample = new Sample();
reset(){ ...impl...}
validate(){.... impl.....}
=================
Sample.java
private Hashtable prop = new Hashtable();
public int getNumber()
return ((Integer)prop.get("number")).intValue();
public void setNumber(int number)
prop.put("number",new Integer(number));
====================================
this setup understandbly gives me a error like this
no getter method for property sample.number
PLEASE DO remember that i have a large number of mixed types in my form that needs to be populated into the bean and i detest using that many variables(in the bean)
I NEED TO A KNOW A SOLN. PLEASE
Thanx in advance
cheers
Ash

Hi,
I think the solution for your problem is
put a form tag in your jsp page.
name it as your form name defined in the struts-config.xml.
in your case
it is 'sample'.
And make changes in the html:text tag like,
<html:text name="sample" property="number" />
Hope it will work.. have fun !!!

How to put data into a hashtable from a text file

I need some help to put the words in a text file to a hashtable.The text file
consist of words and the following set of punctuation marks {, . ; : } and spaces.
thanks in advance.

Navy_Coder wrote:
6 zebras.But you must marry his eldest daughter as well, for that dowry.

Need to run a prog on 1.4 which is running on 1.5. .....using two hashtable

hello,
I've developed a program which is prefectly working on java 1.5.0 version.
but when run on java1.4.0. it gives run time error in this code.
first i'm getting error in second hashtable. put function where i'm using key as an integer.this is running on 1.5 version but have to change key as string on 1.4. but after that it is still not working properly.
I want that when i clicked on open then it must show both file
1. that have hdr extension
2. that have same name as the corrseponding .hdr file but without extension.
this program show desired result on 1.5 but on 1.4 it display only .hdr file.
pls. help me regarding this .
// class hdr file filter
import java.io.File;
import java.util.Hashtable;
import java.util.Enumeration;
import javax.swing.*;
import javax.swing.filechooser.*;
public class hdrFileFilter extends FileFilter {
private Hashtable filters = null;
private Hashtable noextensionfilter = null;
private String noextensionname=null;
private String extensionname = null;
private String description = null;
private String fullDescription = null;
private boolean useExtensionsInDescription = true;
private String fname = null;
public File noextensionarray[];
int count=0;
int counthdr=0;
int i=0;
     //all files are accepted.
public hdrFileFilter()
     this.filters = new Hashtable();
     //Creates a file filter that accepts files with the given extension.
public hdrFileFilter(String extension)
     this(extension,null);
     //Creates a file filter that accepts the given file type.
public hdrFileFilter(String extension, String description)
     this();
     if(extension!=null)
     addExtension(extension);
     if(description!=null) setDescription(description);
     //Return true if this file should be shown in the directory pane,false if it shouldn't.
public boolean accept(File f) {
     if(f != null) {
     if(f.isDirectory()) {
          return true;
     String extension = getExtension(f);
     if(extension != null && filters.get(getExtension(f)) != null)
          fname=f.getName();
          extensionname=fname.substring(0,fname.lastIndexOf('.'));
          counthdr+=1;
          return true;
     if(extension == null)
          noextensionfilter=new Hashtable(20);
          noextensionname=f.getName();
          noextensionfilter.put(count,noextensionname);
          count=count + 1;
          if(noextensionfilter.get(count) == extensionname)
               noextensionarray=new File[20];
               noextensionarray=f
               System.out.println("file:" +noextensionarray[i]);
               i++;
               return true;
     return false;
     //Return the extension portion of the file's name .
public String getExtension(File f)
          if(f != null)
               String filename = f.getName();
               int i = filename.lastIndexOf('.');
               if(i>0 && i<filename.length()-1)
                    return filename.substring(i+1).toLowerCase();
          return null;
     //Adds a filetype "dot" extension to filter against.
public void addExtension(String extension)
     if(filters == null)
     filters = new Hashtable(20);
     filters.put(extension.toLowerCase(), this);
     fullDescription = null;
public String getDescription()
          if(fullDescription == null)
               if(description == null || isExtensionListInDescription())
                    fullDescription = description==null ? "(" : description + " (";
                    // build the description from the extension list
                    Enumeration extensions = filters.keys();
                    if(extensions != null)
                         Object nn=extensions.nextElement();
                         fullDescription += "." + nn;
                         while (extensions.hasMoreElements())
                              fullDescription += ", ." + nn;
                    fullDescription += ")";
               else
                    fullDescription = description;
          return fullDescription;
public void setDescription(String description)
     this.description = description;
     fullDescription = null;
public void setExtensionListInDescription(boolean b) {
     useExtensionsInDescription = b;
     fullDescription = null;
          public boolean isExtensionListInDescription()
          return useExtensionsInDescription;

Might be fixable if you do something like
javac -target 1.4
Bytecodes are different in 1.5 and thus not backward compatible

How can I use active user session that's in an application scope Hashtable?

First of all, is it possible to use the same session after a user exits and then returns?
Second, if it is possible, then please tell me what I need to do. So far, this is what I have been doing...
1.) The user submits login credentials
2.) The user is authenticated.
3.) The user does not have an existing session, so a new one is created.
4.) The user closes the browser.
5.) The user returns to login page and submits login credentials.
6.) The user is authenticated.
7.) The user has an existing session, so it should be used.
This is where I'm having trouble. All active sessions for my application are stored in a Hashtable. I have been using <%@ page session="false" %> so that a new session is not automatically created. Each time before I create a new session, I check my Hashtable to see if that user has an existing session. If a session exists for that user, then I would like to continue using that session if possible. I have used the methods request.getSession() and request.getSession(false), but a new session is always returned.
I could create a new session, copy the attributes from the old session(which is stored in my Hashtable) to the new session, and then invalidate the old session, but I'd rather not do that.
Is there a way that I can use existing sessions that I have stored in my Hashtable?

First of all, is it possible to use the same session after a user exits and then returns?No, I don't think so. Let me explain why. When the server creates a session object for each client, it needs to know which client is making the request. Remember that HTTP is a stateless protocol. Every time a client makes a request, it sends some sort of session id to the server to let the server know who is trying to make the request. The server will then check to see if a session object exists for that particular client and if so, it will make sure that the max inactive interval (maximum time alloted for that client's session) has not been exceeded. If every thing is okay, then the client can access the session object to get values that were previously placed there. There are many ways that servers try to keep track of clients. One way is to have the clients write the session ID using cookies. But, many people like disallow cookies. So some servers do what is known as URL rewriting. That is, they will write the session ID on the end of the query string. This can also be accomplished programmatically, but it can be taxing to do. Anways, the point is that the client and the server have to have some sort of link between each other and that link is the session ID. So, if the browser is closed, the session ID is lost. That particular client will be forced to get a new session ID the next time the following code is executed:
//create a session object and set its values
HttpSession session = request.getSession(true);>
Second, if it is possible, then please tell me what I
need to do. So far, this is what I have been doing...
1.) The user submits login credentials
2.) The user is authenticated.
3.) The user does not have an existing session, so a
new one is created.
4.) The user closes the browser.
5.) The user returns to login page and submits login
credentials.
6.) The user is authenticated.
7.) The user has an existing session, so it should
be used.If you really want to do something like this, you could make up your own ID and store it as a cookie on the client. I've never tried anything like this before so you would have to do your own research. Are you sure you want to do something like this. There is a reason why it works the way it does. There is also a reason why you want to keep the session timeout value some what small. Let me give you an example of some craziness with sessions. A client we once had wanted to keep their sessions open for 4 hours because they said there clients simply did not like to log in all the time. I nearly gasped when I was told we needed to do this. When you make the session time out large (i.e. the maxInactiveInterval( )), then session objects stick around longer in the server. Let's say a client logs into the server and receives a session object. Then, the client makes a few requests. The server knows to keep the session alive as long as the time between requests has not exceeded 4 hours. Then the client closes the browser. How is the server suppose to know that the browser was closed. Well, it doesn't. It just knows to check times between requests. So, that particular session object won't be garbage collected until the session times out. What if a whole bunch of clients did this. Yucko. The server would have a whole bunch of session objects living in memory. What a waste. This is all above and beyond the typical security problems that can arise from having a session open for so long. To make a long story short, you really shouldn't do what you are trying to do unless it is the nature of the app.
>
This is where I'm having trouble. All active sessions
for my application are stored in a Hashtable. I have
been using <%@ page session="false" %> so that a new
session is not automatically created. Each time
before I create a new session, I check my Hashtable
to see if that user has an existing session. If a
session exists for that user, then I would like to
continue using that session if possible. I have used
the methods request.getSession() and
request.getSession(false), but a new session is
always returned.
I could create a new session, copy the attributes from
the old session(which is stored in my Hashtable) to
the new session, and then invalidate the old session,
but I'd rather not do that.
Is there a way that I can use existing sessions that I
have stored in my Hashtable?

Getting the value from a hashtable

Hi all,
Can I get the value from a Hashtable by passing vector.toString() , in the get method?
I am storing the key value pair using the key as vector.toString() as the key and a string value. When i am using the get method by passing vector.toString() I am getting an exception
found : Object
required : String.
Thanks and regards
Tanmoy

Exception? "found: X required: Y" sounds more like a compiler error to me.
You need to post the lines of code that cause the error so we can see what's going on.

Problem with doubleValue() method and hashtable

Hi,
I made a type double variable named tempNumber.
and I used this to put that number in a hashtable
CODE 1 : hash.put(key, new Double(tempNumber));
Then I wrote this code to retrieve it.
CODE 2: (hash.get(st.sval)).doubleValue();
Note: I am using streamtokenizer to get the nexttoken, so st.sval is a string representation of the token.
Now my problem is when I compile CODE 2, I get an error message saying that "method doubleValue() not found in java.lang.Object". Whats going on here??
I am using java 1.1 by the way.
Thanks in advance...

Hash table stored and returns objects. You have to typecast the return value of get() method to Double and then call doubleValue() on it.
Its something as follows
((Double)hash.get(key)).doubleValue()

Loading a text file on startup into a hashtable

I am trying to load to seperate textfiles into the same hashtable.
The textfiles are something like an employee and employee number everything goes in at the same time. How do i load these text files into the hashtable. Also any good tutorials or code samples on hashtables would be greatly appreciated.
Thanks In Advance

You read the text files, one line at a time, and break up each line in whatever way you need to. Then you choose the bits to put in the hashtable (key and value) for each line and put those bits into the hashtable.
http://java.sun.com/docs/books/tutorial/essential/index.html
http://java.sun.com/docs/books/tutorial/collections/index.html

How can I get keys from Hashtable in the same order?

Hello, everyone.
I have a Hashtable containing key-value pairs, I need to get the keys in the same order as I use
method put(key,value) to save the key-value pairs. But I can only find Hashtable.keys() to fetch the keys in form of Enumeration and when retrieve the keys from the Enumeration,they are not in the original order!
The following is my code:
Hashtable ht = new Hashtable();
ht.put("Name","Harry Bott");
ht.put("Gender","Male");
ht.put("Age","25");
String[] Items = new String[ht.size()];
Enumeration e = ht.keys();
int i = 0;
while(e.hasMoreElements()) {
Items[i++] =(String)(e.nextElement());
The Items contains the keys but they are not in the original order.
Does anyone know how to get the keys from a Hashtable in the same order when they're put?
Thank you!

yeah, another solution is to stored keys on a Collection at the same time you put them on the HashMap with its values.
Then when ur going to retrieve objects from the Map u iterate the Collection that has the ordered keys and use them.

Use of Hashtable in a JSP

I have a JSP page, displaying Employee details(5 attrbutes in total)
(code, name, address, telephone, email)
And this employee is coming from Employee table, which is having around 50 attributes, and I have one entity class for the employee doing the database operation.
Now since I have to show only 5 fields, using Employee class (which is having 50 attributes) will be very heavy.
Should I create a new bean class EmployeeView having only 5 attributes or shall i go for <b>hashtable</b> , having 5 entries, with attribute name as key and attribute value as key value.
Are there any disadvantage on using hashtable like this, or what can be better approach.
Please suggest.

Hi,
The best way for this is to create view for the table in the database.
Then you can access the view like how you access a table.
This will reduce the amount of data transfer from database to your application.
Hope this helps.
Good Luck.
Gayam.Srinivasa Reddy
Developer Technical Support
Sun Micro Sysytems
http://www.sun.com/developers/support/

HashTable

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