Since
2008
Since
2008
Since
2008
I do build crazy buildings using my collection of Lego blocks. My 11 months old kid Ben curiously stares at me build it. He always wishes to get hold of it. After I complete the building when I give that to his hand, you know what the first thing he does.
Modify the building blocks. Though I wish them to be intact forever Lego buildings are built to be modified.
But this is not the case in programming. You create a java collection and store objects in it. Then there are scenarios where you want them not be modified. Obsessed with file system terminology Java guys have named it as read only collections.
By default some of the languages like dot net provide read only collections. But in Java there are no such things. This is not a special type of collection it is an additional facility provided to change the usual collections as read only.
The Collections class provides six factory methods, one for each of Collection, List, Map, Set, SortedMap, and SortedSet.
You should set the collection with required values then pass it as value to the Collections’ respective method. Most important thing here is, just passing and setting it as unModifiableX is not enough. These methods will return you collection as read only. You need to overwrite your old collection with this new read only collection. If you don’t do that, using the reference of the old collection the values can be modified. Cool right!
The returned set will be serializable if the specified set is serializable. If you attempt to modify a read-only collection it will throw an UnsupportedOperationException.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
// example program to demonstrate the read-only collection in java
public class ReadOnlyCollections {
public static void main(String args[]) {
// creating a list
List godList = Arrays
.asList(new String[] { "Donald", "Dennis", "Ken" });
// making a read-only list
List list = new ArrayList(godList);
list = Collections.unmodifiableList(list);
// checking the reference in a read-only set
Set set = new HashSet(godList);
Collections.unmodifiableSet(set);
// the following statement allows to modify the above set as the
// reference is pointing to the original collection therefore it is not
// read-only
set.add("Alan");
// making a read-only map and try to modify it
Map godMap = new HashMap();
godMap.put("TAOCP", "Donald");
godMap.put("C", "Dennis");
godMap = Collections.unmodifiableMap(godMap);
try {
// modifying the read-only map to check what happens
godMap.put("Unix", "Ken");
} catch (UnsupportedOperationException e) {
System.out.println("You cannot modify a read only collection!");
}
}
}
Comments are closed for "Read Only Collections".
Thanks, you are exposing new feature of Collection framework.
Your second para was really super cool.
Gr8 catch Man.
Cheers, Mayank
very clear and simple document.easy to understand..thank u.
how to utilize collections?it would be more useful if u have used more examples.otherwise add examples as a new menu…
Thanks, you are exposing new feature of Collection framework. I really thanks to you
hi i am getting collection from entity bean
and them i am going to remove element from that collection it gives the error as below
javax.ejb.EJBException: This collection is a read-only snapshot
please help me regarding this.
Thanks
Thank you very much Joe…:)
Thanks for the article. A new thing which I studied today.
Joe ,i have a question- how does set check that a duplicate object is being added to it?
thank u Joe,its very nice collection and easy to understand;
thank u Joe,its very nice collection and easy to understand;
how to delete the duplicate element using collections.i want answer.
Thanks Joe… Your explanation is good…
Thanks for the article. A new thing which I studied today. Keep it up Joe!
Regards,
Snehal Masne
GOOD ONE
super good one thanks
Thank you.. can you get some OCJP dumps..
Thanks, useful information.
Correction neeeded : The Collections class provides six factory methods, one for each of Collection, List, Map, Set, SortedMap, and SortedSet.
Map is not a collection
This is one of the best para for me It is really helpful for me
Thanks
Please post Interview Question regarding these topics individually
concurrency
executors
collections
hashmap
internal implementation of each of these collections
blocking queue
core java design patterns
singleton
observer 1 case study 1 written test
linked list implementation
multithreading program
to implement join using thread join method
deadlock
mutithreading and concurrency
strings, inheritence, overriding
servlets
concurrency in servlets
hibernate – basics only
webservices – soap and rest – where to use
project related questions.
Hi Joe
Very good article. Can u come up with more collection article.
Thanks
Suganya