List Interface in Java

List Interface in Java | In this post we will discuss the List interface in Java. What are the different List methods in Java? Program to demonstrate List and its methods.

List is the child interface of Collection(I). If we want to represent a group of individual objects as a single entity where duplicates are allowed & insertion order must be preserved then we should go for List.

We can preserve insertion order with index & we can differentiate duplicate objects by using index hence index will play a very important role in List. Similar to the array its index starts from 0. We can use its index to iterate to the List.

List	A	B	C	D	E	A
Index	0	1	2	3	4	5

List interface declaration:-

public interface List<E> extends Collection<E>

List Interface Implementation Classes

Implementation classes of List interface in Java,

• Collection(I)
  • List(I)
    • ArrayList
    • LinkedList
    • Vector 
      • Stack 

ArrayList, LinkedList, and Vector classes implement List(I). Stack class extends from Vector class. The Vector and Stack classes are considered legacy classes because they were introduced in the 1.0 version. The remaining classes ArrayList, & LinkedList were introduced in the 1.2 version. In Java 1.2 version.

Class/Interface	Introduced in Version
Collection(I), List(I), ArrayList, LinkedList	Java 1.2
Vector and Stack	Java 1.0

List Methods in Java

List interface implements Collection interface, therefore all the 15 methods of Collection interface is also available to the List interface. The 15 methods given in Collection interface are as follows:- add(E e), addAll(Collection<? extends E> c), remove(Object o), removeAll(Collection<?> c), retainAll(Collection<?> c), isEmpty(), size(), clear(), contains(Object o), containsAll(Collection<?> c), Object[ ] toArray(), <T> T[ ] toArray(T[ ] a), iterator(), equals(Object o), hashCode()

List(I) also defines the following 10 specific methods:- add(int index, E element), addAll(int index, Collection<? extends E> c), get(int index), remove(int index), set(int index, E element), indexOf(Object o), lastIndexOf(Object o), listIterator(), listIterator(int index), subList(int fromIndex, int toIndex)

Hence there is a total of 25 public methods in the List interface. Let us see these methods by arranging into some groups for easy understanding purposes.

List Methods in Java to add elements to the list,

1. boolean add(E e):- Add a specific element to the list.
2. void add(int index, E element):- Insert an element at the given index.
3. boolean addAll(Collection<? extends E> c):- Add all elements present in the collection “c”.
4. boolean addAll(int index, Collection<? extends E> c):- Insert a group of elements at given index.

List Methods in Java to remove elements from the list,

5. boolean remove(Object o):- Remove the first occurrence of specific element from the list.
6. E remove(int index):- Remove/delete element at given index.
7. boolean removeAll(Collection<?> c):- Remove all elements present in collection “c”.
8. boolean retainAll(Collection<?> c):- Remove all elements from the list except those are present in collection “c”.

List methods in Java to get/set or find the elements,

9. E get(int index):- Fetch element at given index.
10. E set(int index, E element):- Replace the element present at the specified index with the provided object and return the old object.
11. int indexOf(Object o):- Returns index of first occurrence of element “o”.
12. int lastIndexOf(Object o):- Returns index of last occurrence of element “o”.
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). 

To check the list contains a given object or collection or not,

14. boolean contains(Object o):- Check whether a specific element is available or not?
15. boolean containsAll(Collection<?> c):- Check whether all elements of collection “c” is available or not.

List methods in Java to iterator over the list,

16. Iterator iterator():- Returns an iterator over the elements in this list.
17. ListIterator<E> listIterator():- Returns a list iterator over the elements in this list (in proper sequence).
18. ListIterator<E> listIterator(int index):- Returns a list iterator over the elements in this list, starting at the specified position in the list.

Java List interface methods for query operations,

19. boolean isEmpty():- Check whether the list is empty or not. 
20. int size():- Find the number of elements in the list.
21. void clear():- Remove all elements from the list. 

Methods to convert List to Array,

22. Object[ ] toArray():- To convert list to object array type.
23. <T> T[ ] toArray(T[ ] a):- To convert list to specified array type.

Other Methods,

24. boolean equals(Object o):- Compares the specified object with this list for equality. It returns true if the specified object is equal to this list.
25. int hashCode():- Returns the hash code value for this list.

List in Java Example

Java List example to demonstrate add and remove elements from the List

import java.util.*;
public class ListTest1 {
   public static void main(String[] args) {

      List<String> l1 = new ArrayList<String>();
      l1.add("A");
      l1.add("B");
      l1.add("A");
      System.out.println("List 1: " + l1);
      
      // insert at 1 index
      l1.add(1, "Z");
      System.out.println("List 1: " + l1);
      
      List<String> l2 = new ArrayList<String>();
      l2.add("P");
      l2.add("Q");
      System.out.println("List 2: " + l2);
      
      // Add all element of l2 to l1
      l1.addAll(l2);
      System.out.println("List 1: " + l1);
      
      // remove from l1
      l1.remove("A");
      System.out.println("List 1: " + l1);
      l1.remove(3);
      System.out.println("List 1: " + l1);
      
      // remove all elements of l2 from l1
      l1.removeAll(l2);
      System.out.println("List 1: " + l1);
   }
}

Output:-

List 1: [A, B, A]
List 1: [A, Z, B, A]
List 2: [P, Q]
List 1: [A, Z, B, A, P, Q]
List 1: [Z, B, A, P, Q]
List 1: [Z, B, A, Q]
List 1: [Z, B, A]

List Example to Store Objects

List in Java Example with Product class:- Here we have a product class with fields:- PID (product id), name (product name), qty (product quantity), and price (product price). We have to store products into the List and display them.

class Product {

   private int pid;
   private String name;
   private int qty; // quantity
   private double price;

   public Product(int pid, String name, 
                  int qty, double price) {
      this.pid = pid;
      this.name = name;
      this.qty = qty;
      this.price = price;
   }

   @Override
   public String toString() {
      return "{" + pid + ", " + name 
         + ", " + qty + ", " + price + "}\n";
   }
}

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

public class Shop {
   public static void main(String[] args) {

      // ArrayList to store Products
      List<Product> al = new ArrayList<Product>();
      
      // Adding products to list
      al.add(new Product(123, "Laptop", 15, 2000));
      al.add(new Product(154, "Mobile", 30, 200));
      al.add(new Product(543, "Keyboard", 20, 30));
      // duplicate
      al.add(new Product(123, "Laptop", 15, 2000)); 

      // display list
      System.out.println(al);
   }
}

Output:-

[{123, Laptop, 15, 2000.0}
, {154, Mobile, 30, 200.0}
, {543, Keyboard, 20, 30.0}
, {123, Laptop, 15, 2000.0}
]

Java List example to get set elements

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

public class ListTest2 {
   public static void main(String[] args) {

      List<String> ls = new ArrayList<String>();
      ls.add("A");
      ls.add("B");
      ls.add("A");
      ls.add("C");
      ls.add("D");
      System.out.println(ls); // [A, B, A, C, D]
      
      // display list using get()
      for(int i=0; i<ls.size(); i++) {
         System.out.print(ls.get(i) + " ");
      }
      System.out.println(); // new line
      
      // get first occurrence of "A"
      System.out.println(ls.indexOf("A")); // 0
      // get last occurrence of "A"
      System.out.println(ls.lastIndexOf("A")); // 2
      
      // replace 3 index
      System.out.println(ls.set(3, "KP")); // C
      System.out.println(ls); // [A, B, A, KP, D]
      
      // sublist
      List<String> newlist = ls.subList(1, 3);
      System.out.println(newlist); // [B, A]
   }
}

Output:-

[A, B, A, C, D]
A B A C D
0
2
C
[A, B, A, KP, D]
[B, A]

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