AbstractList in Java with Examples

AbstractList in Java is an abstract class in the Java Collections Framework that provides a skeletal implementation of the List interface. It helps developers create custom list implementations by only overriding a few essential methods.

• Provides default implementations for most List methods, reducing coding effort
• Requires overriding core methods like get() and size()
• Improves code reusability when creating custom list classes

Declaration of AbstractList

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>

import java.util.*;

public class AbstractListDemo {
    public static void main(String args[]) {
        // Creating an AbstractList using ArrayList
        AbstractList<String> list = new ArrayList<String>();

        // Use add() method to add elements
        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
        list.add("10");
        list.add("20");

        // Display the list
        System.out.println("AbstractList: " + list);
    }
}

AbstractList: [Geeks, for, Geeks, 10, 20]

Explanation:This program creates an AbstractList using its subclass ArrayList, adds elements to it using the add() method, and prints the list in insertion order. It demonstrates that AbstractList cannot be instantiated directly but can be used via a subclass.

Hierarchy of AbstractList  

Implementation Classes of AbstractList

• ArrayList : Resizable array implementation of List.
• LinkedList : Doubly-linked list implementation supporting sequential access.
• Stack: Last-in-first-out stack extending Vector.
• Vector: Synchronized, resizable array implementation.

Constructor Of AbstractList

protected AbstractList(): Being protected and abstract, you cannot create AbstractList objects directly.

AbstractList<Integer> al = new ArrayList<Integer>();

import java.util.*; 

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

        // Creating an empty AbstractList 
        AbstractList<String> 
            list = new LinkedList<String>(); 

        // Using add() method to add elements in the list 
        list.add("Geeks"); 
        list.add("for"); 
        list.add("Geeks"); 
        list.add("10"); 
        list.add("20"); 

        // Output the list 
        System.out.println("AbstractList: " + list); 

        // Remove the head using remove() 
        list.remove(3); 

        // Print the final list 
        System.out.println("Final AbstractList: " + list); 

        // getting the index of last occurrence 
        // using lastIndexOf() method 
        int lastindex = list.lastIndexOf("A"); 

        // printing the Index 
        System.out.println("Last index of A : "
                        + lastindex); 
    } 
} 

AbstractList: [Geeks, for, Geeks, 10, 20]
Final AbstractList: [Geeks, for, Geeks, 20]
Last index of A : -1

Methods in AbstractList

Methods declared in class java.util.AbstractCollection

Methods declared in interface java.util.Collection

Methods declared in interface java.util.List