Java Collections #2

Hi Guys,

Welcome back, this is second episode on Java collections. In this episode we are going to take a look at a program based on the collections. :-

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Listing #1
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package dev.data.workSpace.projects.scjp.section9;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Vector;

public class ListExampleTester {

/**
* Omkar
* 15 Jun, 2007
* 9:43:56 AM
* ListExampleTester
* main
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

ListExample listExample = new ListExample();

Vector vector = new Vector(10,3);
LinkedList linkedList = new LinkedList();
vector.add(new Integer(2));
vector.add(new Integer(23));
vector.add(new Integer(-2121));
vector.add(new Integer(92));
vector.add(new Integer(0));
vector.add(new Integer(-654));
vector.add(new Integer(13));
vector.add(new Integer(9876));

System.out.println("The Vector now contains the following elements :-");
System.out.println("vector "+vector);
System.out.println("Initially the list is empty :-");
System.out.println("linkedList "+linkedList);

System.out.println("Now the Vector elements are added into the empty list using the \"addAll()\" function");
linkedList.addAll(vector);
System.out.println("After adding all the elements to the linked list the contents of the list are :-");
System.out.println("linkedList "+linkedList);

System.out.println("Clearing the vector ...");
vector.clear();

Collections.sort(linkedList);
System.out.println("Now the linked list looks something like this after sorting");
System.out.println("linkedList "+linkedList);

System.out.println("Now filling the vector again from the sortelist similary by using the \"addAll()\" function.");
vector.addAll(linkedList);
System.out.println("now the vector looks like this :-");

System.out.println("Now the vector looks like this ");
System.out.println("vector "+vector);

System.out.println("Creating an unmodfiable list from the vector");
LinkedList linkedList2 = new LinkedList();

linkedList2.addAll(Collections.unmodifiableCollection(vector));
System.out.println("The new list looks something like this ");
System.out.println(linkedList2);

linkedList2.add(0,new Integer(010));
System.out.println("Adding one element to the unmodifiable list, this list looks something like this ");
System.out.println(linkedList2);

List list = Collections.unmodifiableList(vector);
System.out.println("Above list was not unmodifiable,..But following list is unmodifiable list ");
System.out.println(list);


// Following line will throw a java.lang.UnsupportedOperationException since we cannot modify a un-modifiable list.
// list.add(new Integer(9999));
// System.out.println(list);
// System.out.println("Following line show what will removeAll() method return when it is called on an unmodifiable list.");
// System.out.println("list.removeAll()"+list.removeAll(list));



System.out.println("linkedList2.removeAll() "+linkedList2.removeAll(linkedList2)); // Will print "true"
System.out.println("linkedList2.removeAll() "+linkedList2.removeAll(linkedList2)); // Will print "false"
// Following line will throw a Exception
//linkedList2.add("Omkar");
//linkedList2.add("Patkar");
//linkedList2.add("is ");
//linkedList2.add("gr8");
//linkedList2.add("guy !");
//linkedList2.addAll(Collections.unmodifiableCollection(vector));
//System.out.println("Unsorted "+linkedList2);
//Collections.sort(linkedList2);
//System.out.println("Sorted "+linkedList2);


}

}

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The output of this program is as follows :-
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The Vector now contains the following elements :-
vector [2, 23, -2121, 92, 0, -654, 13, 9876]
Initially the list is empty :-
linkedList []
Now the Vector elements are added into the empty list using the "addAll()" function
After adding all the elements to the linked list the contents of the list are :-
linkedList [2, 23, -2121, 92, 0, -654, 13, 9876]
Clearing the vector ...
Now the linked list looks something like this after sorting
linkedList [-2121, -654, 0, 2, 13, 23, 92, 9876]
Now filling the vector again from the sortelist similary by using the "addAll()" function.
now the vector looks like this :-
vector [-2121, -654, 0, 2, 13, 23, 92, 9876]
Creating an unmodfiable list from the vector
The new list looks something like this
[-2121, -654, 0, 2, 13, 23, 92, 9876]
Adding one element to the unmodifiable list, this list looks something like this
[8, -2121, -654, 0, 2, 13, 23, 92, 9876]
Above list was not unmodifiable,..But following list is unmodifiable list
[-2121, -654, 0, 2, 13, 23, 92, 9876]
linkedList2.removeAll() true
linkedList2.removeAll() false
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This program demonstrates following observations :-
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1. linkedList2.addAll(Collections.unmodifiableCollection(vector)); will not make the linkedList2 as the unmodifiable list because, the linkedList2 was created as LinkedList linkedList2 = new LinkedList(); and here by default list is MODIFIABLE only the elements inserted into this by default modifiable list were obtained from Collections.unmodifiableList()...but this does not make the list unmodifiable.
2. Once the Collection obtained is unmodifiable, then the collection can only be read, and things like updates, inserts, deletes will not work.
3. Consider a modifiable List object. It contains few elements. When removeAll() is called on this list object for the first time then the removeAll() method returns a "true". An immediate call of "removeAll()" on the same empty list will return "false".
   This is because, the removeAll() method returns false when called on an empty collection.
4. The difference between the "clear()" and the "removeAll()" method, is that former has return type as void and the later has the return type as boolean indicating if the removeAll operation was done properly or not.
5. Adding of Heterogenous objects that is objects of different types, is possible for a given collection. But performing operations like sorting is not possible, since the objects belonging to different types have nothing in common to compare with while sorting.
   

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As you can see i have used a utitlity class available in this Collection framework. The name of the class is "Collections". This class provides many, utitlity methods like sorting....that i have used in the example.

So this was just about one part of the collections, i.e., the List and its subclasses. In my next post we will try to explore the other part of Collections framework.

Thanks and Regards
Omkar Patkar.