Java List Interface

In Java, the List interface is a part of the Java Collections Framework that represents an ordered collection of elements. It allows duplicate values and null elements, and provides index-based operations to store, access, update, and manipulate data efficiently.

In this chapter, we will learn about the Java List interface, its features, methods, implementations such as ArrayList and LinkedList, and how it is used to manage collections of elements effectively.

What is Java List Interface?

The List interface is available in the java.util package and extends the Collection interface. It is designed to store elements in a sequential (ordered) manner and provides methods to perform operations like insertion, deletion, updating, and searching using indexes.

A List allows duplicate elements and also supports storing null values. It maintains the insertion order, meaning elements are retrieved in the same order as they were added.

Additionally, the List interface provides a ListIterator that allows traversal of elements in both forward and backward directions. Some commonly used implementation classes of the List interface include ArrayList, LinkedList, Stack, and Vector (although Vector is considered legacy and less commonly used today).

List Interface Declaration

The List interface is declared in the java.util package and extends the Collection interface.

Here is the syntax:

Constructors of List Interface

The List interface does not have constructors because it is an interface. However, objects of List can be created using its implementation classes like ArrayList and LinkedList.

Here are the syntaxes:

Java List Class Methods

The List interface provides various methods to perform operations such as adding, removing, updating, and accessing elements in an ordered collection.

Creating a List

The ArrayList and LinkedList classes provide the implementation of the List interface, allowing us to create lists of different data types.

Example

The following example demonstrates how to create List objects using ArrayList and LinkedList with different data types.

In short, we can create a List of any data type. The ArrayListand LinkedListclasses are used to specify the type, where T denotes the type of elements.

Common List Operations

Here are some essential operations that can be performed on Java Lists.

1. Adding Elements

We can add elements to a List using the add() method. It allows adding elements at the end or at a specific index.

Example:

2. Removing Elements

Elements can be removed from a List either by specifying the object or by providing the index position.

Example:

3. Accessing Elements

We can access elements using index-based methods like get() and also find positions using indexOf().

Example:

4. Iterate Over List

We can iterate through the elements of a List using loops such as the enhanced for loop.

Example:

Java List Example

Let's see a simple example of List where we are using the ArrayList class as the implementation.

Output:

Mango
Apple
Banana
Grapes

Converting Array to List

We can convert an array to a List by adding elements one by one using the add() method or by using built-in methods like Arrays.asList() and the ArrayList constructor.

Example

The following example demonstrates how to convert an array into a List by traversing each element.

Output:

Printing Array: [Java, Python, PHP, C++]
Printing List: [Java, Python, PHP, C++]

Converting List to Array

We can convert a List to an array using the toArray() method provided by the List interface.

Example

The following example demonstrates how to convert a List into an array.

Output:

Printing Array: [Mango, Banana, Apple, Strawberry]
Printing List: [Mango, Banana, Apple, Strawberry]

Getting and Setting Elements in List

The get() method is used to retrieve an element at a specific index, whereas the set() method is used to replace an element at a given index.

Example

The following example demonstrates how to access and update elements in a List.

Output:

Returning element: Apple
Mango
Dates
Banana
Grapes

Sorting a List

We can sort a List using the Collections.sort() method or the List.sort() method introduced in Java 8.

Example

The following example demonstrates how to sort a List of strings and numbers.

Output:

Apple
Banana
Grapes
Mango
Sorting numbers...
1
11
21
51

Java ListIterator Interface

The ListIterator interface is used to traverse the elements of a List in both forward and backward directions.

It provides additional methods to modify the elements during iteration, such as adding, removing, and updating elements.

ListIterator Interface declaration

Syntax to declare ListIterator Interface is as follows:

Methods of Java ListIterator Interface

The following table contains the commonly used methods of the ListIterator interface along with their descriptions.

Example of ListIterator Interface

The following example demonstrates how to use the ListIterator interface to traverse elements in both forward and backward directions.

Output:

Traversing elements in forward direction
index:0 value:Amit
index:1 value:Sachin
index:2 value:Vijay
index:3 value:Kumar
Traversing elements in backward direction
index:3 value:Kumar
index:2 value:Vijay
index:1 value:Sachin
index:0 value:Amit

Complete Example of List Interface

The following example demonstrates how to use the List interface to store and manage custom objects (Book) in Java.

Output:

101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications and Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6