Java List - List in Java

Java List interface extends Collection interface. Collection interface externs Iterable interface. Some of the most used List implementation classes are ArrayList, LinkedList, Vector, Stack, CopyOnWriteArrayList. AbstractList provides a skeletal implementation of the List interface to reduce the effort in implementing List.

Some of the useful Java List methods are;

1. int size(): to get the number of elements in the list.
2. boolean isEmpty(): to check if list is empty or not.
3. boolean contains(Object o): Returns true if this list contains the specified element.
4. Iterator<E> iterator(): Returns an iterator over the elements in this list in proper sequence.
5. Object[] toArray(): Returns an array containing all of the elements in this list in proper sequence
6. boolean add(E e): Appends the specified element to the end of this list.
7. boolean remove(Object o): Removes the first occurrence of the specified element from this list.
8. boolean retainAll(Collection<?> c): Retains only the elements in this list that are contained in the specified collection.
9. void clear(): Removes all the elements from the list.
10. E get(int index): Returns the element at the specified position in the list.
11. E set(int index, E element): Replaces the element at the specified position in the list with the specified element.
12. ListIterator<E> listIterator(): Returns a list iterator over the elements in the list.
13. List<E> subList(int fromIndex, int toIndex): Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

Some of the default methods added to List in Java 8 are;

1. default void replaceAll(UnaryOperator<E> operator): Replaces each element of this list with the result of applying the operator to that element.
2. default void sort(Comparator<super E> c): Sorts this list according to the order induced by the specified Comparator.
3. default Spliterator<E> spliterator(): Creates a Spliterator over the elements in this list.

We can use Arrays class to get the view of array as list. However we won’t be able to do any structural modification to the list, it will throw java.lang.UnsupportedOperationException. So the best way is to use for loop for creating list by iterating over the array. Below is a simple example showing how to convert java array to list properly.

package com.journaldev.examples;

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

public class ArrayToList {

	public static void main(String[] args) {
		
		String[] vowels = {"a","e","i","o","u"};
		
		List<String> vowelsList = Arrays.asList(vowels);
		System.out.println(vowelsList);
		
		/**
		 * List is backed by array, we can't do structural modification
		 * Both of the below statements will throw java.lang.UnsupportedOperationException
		 */
		//vowelsList.remove("e");
		//vowelsList.clear();
				
		//using for loop to copy elements from array to list, safe for modification of list
		List<String> myList = new ArrayList<>();
		for(String s : vowels){
			myList.add(s);
		}
		System.out.println(myList);
		myList.clear();
	}

}

Choose any of the above methods based on your project requirements.

A simple example showing the correct way to convert a list to array.

package com.journaldev.examples;

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

public class ListToArray {

	public static void main(String[] args) {

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

		// add example
		letters.add("A");
		letters.add("B");
		letters.add("C");
		
		//convert list to array
		String[] strArray = new String[letters.size()];
		strArray = letters.toArray(strArray);
		System.out.println(Arrays.toString(strArray)); //will print "[A, B, C]"
	}

}

There are two ways to sort a list. We can use Collections class for natural sorting or we can use List sort() method and use our own Comparator for sorting. Below is a simple example for java list sorting.

package com.journaldev.examples;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;

public class ListSortExample {

	public static void main(String[] args) {
		
		List<Integer> ints = new ArrayList<>();
		Random random = new Random();
		for (int i = 0; i < 10; i++) ints.add(random.nextInt(1000));
		
		//natural sorting using Collections class
		Collections.sort(ints);
		System.out.println("Natural Sorting: "+ints);
		
		//My custom sorting, reverse order
		ints.sort((o1,o2) -> {return (o2-o1);});
		System.out.println("Reverse Sorting: "+ints);
	}
}

A sample output is given below. Since I am using Random for generating list elements, it will be different every time.

Natural Sorting: [119, 273, 388, 450, 519, 672, 687, 801, 812, 939]
Reverse Sorting: [939, 812, 801, 687, 672, 519, 450, 388, 273, 119]

Most common operations performed on java list are add, remove, set, clear, size etc. Below is a simple java list example showing common method usage.

package com.journaldev.examples;

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

public class ListExample {

	public static void main(String args[]) {
		List<String> vowels= new ArrayList<String>();
		
		//add example
		vowels.add("A");
		vowels.add("I");
		
		//let's insert E between A and I
		vowels.add(1,"E");
		System.out.println(vowels);
		
		List<String> list = new ArrayList<String>();
		list.add("O");list.add("U");
		
		//appending list elements to letters
		vowels.addAll(list);
		System.out.println(vowels);
		
		//clear example to empty the list
		list.clear();
		
		//size example
		System.out.println("letters list size = "+vowels.size());
		
		//set example
		vowels.set(2, "E");
		System.out.println(vowels);
		
		//subList example
		vowels.clear();vowels.add("E"); vowels.add("E");vowels.add("I"); vowels.add("O");
		list = vowels.subList(0, 2);
		System.out.println("letters = "+vowels+", list = "+list);
		vowels.set(0, "A");
		System.out.println("letters = "+vowels+", list = "+list);
		list.add("U");
		System.out.println("letters = "+vowels+", list = "+list);
		
		
	}

}

Output of above java list example program is;

[A, E, I]
[A, E, I, O, U]
letters list size = 5
[A, E, E, O, U]
letters = [E, E, I, O], list = [E, E]
letters = [A, E, I, O], list = [A, E]
letters = [A, E, U, I, O], list = [A, E, U]

Below is a simple example showing how to iterate over list in java.

package com.journaldev.examples;

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

public class ListIteratorExample {

	public static void main(String[] args) {

		List<Integer> list = new ArrayList<>();
		for(int i=0; i<5; i++) list.add(i);
		
		Iterator<Integer> iterator = list.iterator();
		
		//simple iteration
		while(iterator.hasNext()){
			int i = (int) iterator.next();
			System.out.print(i + ", ");
		}
		System.out.println("\n"+list);
		
		//modification of list using iterator
		iterator = list.iterator();
		while(iterator.hasNext()){
			int x = (int) iterator.next();
			if(x%2 ==0) iterator.remove();
		}
		System.out.println(list);
		
		//changing list structure while iterating
		iterator = list.iterator();
		while(iterator.hasNext()){
			int x = (int) iterator.next(); //ConcurrentModificationException here
			if(x==1) list.add(10);
		}
	}

}

Output of above java list iterator program is;

0, 1, 2, 3, 4, 
[0, 1, 2, 3, 4]
[1, 3]
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
	at java.util.ArrayList$Itr.next(ArrayList.java:851)
	at com.journaldev.examples.ListIteratorExample.main(ListIteratorExample.java:34)

That’s all of a quick roundup on List in Java. I hope these Java List examples will help you in getting started with List collection programming.