ArrayList in java

ArrayList is a class that extends AbstractList class and implements List interface in java. 

It uses dynamic array for storing the elements thus gaining upper hand in comparison to Array (which is static in nature and has fixed length).

Key feature of ArrayList:

1. It implements List interface, RandomAccess interface, Collection interface, Iterable interface, Cloneable interface and Serializable interface.

2. It is a part of java.util package.

3. ArrayList can expand and shrink automatically as per the need because of its dynamic array storage capability.

4. ArrayList is not synchronized (else it would have been roughly equivalent to Vector).

5. It permits all element, including null.

6. ArrayList is generally slow while manipulating the elements as during insertion or deletion lot of shifting needs to be done.


Now let’s look at the ways in which you can create ArrayList in java. 

·         ArrayList<E> myList = new ArrayList <E>();     //<E> denotes generics

·         ArrayList<E> myList = new ArrayList <E>(Collection c);//<E> denotes generics

Simple Example:

package javaRadarArrayList;

import java.util.ArrayList;

public class SimpleArrayList {

      public static void main(String[] args) {

            //create Array List

            ArrayList<String> javaRadarList = new ArrayList<String>();

            //Add elements to ArrayList

            javaRadarList.add("Java");

            javaRadarList.add("Spring");

            javaRadarList.add("Hibernate");

            javaRadarList.add("EJB");

            System.out.println(javaRadarList);

      }

}

Now let’s look at different ways in which you can use the ArrayList that you created. 
Click on each link to read about them.

ArrayList operations

·         Sort the elements in ArrayList

·         Search element in ArrayList

·         Remove element from a particular index in ArrayList

Traverse ArrayList

·         Traverse ArrayList using Iterator

·         Traverse ArrayList using ListIterator

·         Traverse ArrayList using Enumeration

Convert

·       ArrayList to LinkedList

ArrayList or LinkedList. Which one to use?(Click to see the difference between two and decide)

How to synchronize ArrayList?

So far we have learned that ArrayList is non-synchronized but still what to do if we need to synchronize it.
So java provides a solution to synchronize your ArrayList. Use synchronizedList() method provided by Collections utility class.

                Collections.synchronizedList(List list)

Can we call run() directly on thread without calling start() ?

Yes, we can.

But if we do so then we must be aware that run( ) will not execute in separate call stack in this case. Rather it executes onto the current call stack.

Lets have a look at the below example:

Separate call stack not allotted to thread1 and thread2 on directly calling run()

In the above example we can see that even though there is sleep() called but context switching doesn't happen. Because both thread object thread1 and thread2 run in current call stack(i.e main stack). So on directly calling run(), thread1 and thread2 are treated as normal object and not the thread object. That is why, first thread1 completes its execution(0,1,2) is printed and then run() of thread2 gets chance to execute.

Now call start() on both the thread and see the difference. You will notice both thread object running in separate call stack and also going through context-switching. Look at below example:

Calling start() on thread internally calls run()

Here we can also see that we don't need to call run() on thread instance as we did in our first example. start() does this task internally.


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