All classes which are child of List interface can also manage single set of elements like Set but unlike List allows duplicate elements. List is an ordered collections as it stores the elements in a sequential order. List is a Collection and hence inherits the methods from the Collection interface and allows to add, remove or traverse on the elements.

• ArrayList : ArrayList is a dynamic array which store the objects in a sequential manner and in the contiguous memory locations. Internally uses an array to store the elements. ArrayList is the Most commonly used List implementation class. It can grow dynamically and can have duplicate elements. Like Array, ArrayList is also a random access data structure which can access the elements using index in O(1). Insertion and deletion worst time complexity is also like Array – O(n) as elements needs to be shifted.
• LinkedList: Like ArrayList, LinkedList also stores elements sequentially but elements are stored in the nodes that are scattered in the memory and may not be contiguous but are linked through references. LinkedList is not a random access data structure. To find an element, whole LinkedList need to traversed that makes the time complexity as O(n). Insertion and deletion operations are easier to perform than ArrayList. Java LinkedList can also be used as a Queue as it provides Queue data structure operations. LinkedList supports below operations for queue :
  add() - To add at the front of the LinkedList same as enqueue in Queue.
  poll() - To remove from the rear of the LinkedList same as the dequeue as in Queue.

Out of the above two List implementations. ArrayList is usually considered as the better performing implementation but there are situations in which LinkedList can provided better performance. So which one to use depends on the type of the operations which we are going to perform on the list frequently.

List Interface Basic Operations

• size()- returns the cardinality(number of elements in the List) .
• isEmpty()- returns¬¬ true if it is empty.
• add() - adds the specified element if it is not present in the List.
• remove()- removes the passed element from the List.
• Iterator() - returns an Iterator for the List.

Apart from the operations provided from the Collection interface, List interface also provides the additional operations. Some of them are below:
• Positional access or Index based access — List provides operations like get, set, add, addAll, and remove which can operate on the basis of the position provided in the. i.e can add element at a particular position.
• Search — Search for the particular object in the list and in return get the index position of the element.
• Iteration — get the list specific iterator and traverse over the list.
• Range-view — The sublist method performs arbitrary range operations on the list.

Following program creates a List of Strings using ArrayList implementation class and perform basic operations add(), remove(), isEmpty(), size() and iterates over it using an iterator.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class ListTest {

	public static void main(String[] args) {
		
		List<String> strList=new ArrayList<String>();
		
		System.out.println("As of now List is empty so result for isEmpty() is : "+strList.isEmpty());
		System.out.println("Size of the List is Zero so result for size() is : "+strList.size());
		
		strList.add("Ram");
		strList.add("Ajit");
		strList.add("Bhanu");
		strList.add("Ram");//Adding duplicate element
		
		System.out.println();
		System.out.println("Added elements");
		System.out.println();
		
		System.out.println("After adding elements, result for isEmpty() is : "+strList.isEmpty());
		System.out.println("After adding elements, result for size() is : "+strList.size());

		System.out.println();
		System.out.println("Printing elements :");
		
		Iterator<String> itr = strList.iterator();
		//Printing all the List elements
		while (itr.hasNext()) {
			String string = (String) itr.next();
			System.out.println(string);
			if (string.equals("Ajit")) {
				itr.remove();//Removing "Ajit" from the list
			}
		}

		System.out.println();
		System.out.println("Printing the below strings from List after removal of \"Ajit\" :");
		//After removal of the string printing the List strings using for-each construct
		for (String string : strList) {
			System.out.println(string);
		}
	}
	
}

We can traverse a List using following three ways -

• for-each construct
• Iterators.
• Aggregate Operations

For-each allows to traverse an array or List using a loop. Using for-each we can go through each element and perform some operation like printing the value for the element as shown in the below code snippet. In the below code, a collection to hold string has been prepared which uses an array list and using for-each construct traversing and printing the array list values.

import java.util.ArrayList;
import java.util.List;

public class TraversalTestForEach {

	public static void main(String[] args) {
		List<String> strList=new ArrayList<String>();
		
		strList.add("Ram");
		strList.add("Ajit");
		strList.add("Bhanu");
		strList.add("Ram");
		
		for (String string : strList) {
			System.out.println(string);
		}	
	}
	
}

An Iterator object enables us to traverse through a List. Using iterator we can also remove the elements from the List. Using iterator() method from collection we can get the iterator for the collection and we can traverse through the collection.

public interface Iterator {
    boolean hasNext();
    E next();
    void remove(); //optional
}

Interator interface has got the following methods:

1. boolean hasNext() – returns true if iterator has more elements to be traversed
2. next() – returns the next element in the iteration
3. remove() – removes the last element from the List that was returned by the next. It can be called ony once after the next() has been called otherwise throws exception. Interator is the only safe way to modify the List(or any other collection) during iteration. In any other case unexpected events can happen.

Following is the code snippet to traverse List using iterator.

import java.util.Iterator;
import java.util.ArrayList;
import java.util.List;

public class TraversalTestIterator {
	public static void main(String[] args) {
		List<String> strList = new ArrayList<String>();
		strList.add("Ram");
		strList.add("Ajit");
		strList.add("Bhanu");
		strList.add("Ram");

		Iterator<String> itr = strList.iterator();

		while (itr.hasNext()) {
			String string = (String) itr.next();
			System.out.println(string);
		}
	}
}

JDK 8 release has introduced a new method which is also preferred method to traverse that is to obtain a stream over the List/collection and perform aggregate operations on it. Some of the aggregate operations are provided with the capability to use lambda expressions with them. Lambda expressions provides the capability to pass the code as parameters in the form of functional interfaces. Lambda expressions make the code short(lesser lines of code) and expressive.
The following code iterates through the List and prints the values:

import java.util.stream.Collectors;
import java.util.ArrayList;
import java.util.List;

public class TraversalUsingAggregateFun {

	public static void main(String[] args) {

		List<String> strList = new ArrayList<String>();

		strList.add("Ram");
		strList.add("Ajit");
		strList.add("Bhanu");
		strList.add("Ram");

		/*
		 * JDK 8 release has introduced a new method which is also preferred
		 * method to traverse that is to obtain a stream over the collection and
		 * perform aggregate operations on it. Aggregate operations are provided
		 * with the capability to use lambda expressions with them. Lambda
		 * expressions provides the capability to pass the code as param,eters
		 * in the form of functional interfaces. Lambda expressions make the
		 * code short(lesser lines of code) and expressive. The following code
		 * iterates through the collection and prints the values:
		 */

		/*
		 * In the below code, first we are getting the stream on the collection
		 * and traversing using the forEach() function and within the function
		 * paasing a Lambda Expression to print each element.
		 */

		System.out.println("Printing using stream along with a filter operation :");
		strList.stream()
		.filter(e -> e.length() == 4)
		.forEach(e -> System.out.println(e));

		System.out.println();
		System.out.println("Printing all the elements using stream :");
		strList.stream()
		.forEach(s -> System.out.println(s));

		/*
		 * Stream API also provides a parallel stream which will work
		 * efficiently with a large collection if computer system got multiple
		 * cores.
		 */

		System.out.println();
		System.out.println("Printing using parallel stream along with a filter operation :");
		strList.parallelStream()
		.filter(e -> e.length() == 4)
		.forEach(e -> System.out.println(e));

		System.out.println();
		System.out.println("Printing all the elements using parallel stream :");
		strList.parallelStream()
		.forEach(e -> System.out.println(e));

		/*
		 * There are other methods provided as part of the stream API using
		 * which we can collect data and perform operations. For example, having
		 * a collection of string and want to join them using comma as shown in
		 * the below code
		 */

		System.out.println();
		System.out.println("Performing an aggregate operation over stream : ");
		String joined = strList.stream()
				.collect(Collectors.joining(", "));
		System.out.println("Joined String --> " + joined);

	}

}