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Java ArrayList Class

  • ArrayList class is sub class of collection interface which implements to List interface.
  • ArrayList class provides resizable-array so it can grow automatically as per requirement which resolves fixed Array limitation where you have to pre-define the size of array.
  • It can hold null elements too.
  • It is not synchronized implementation so if multiple threads accessing it concurrently and any one from them modifies the entry then it must be synchronized externally.
  • Also it can hold duplicate elements.
Java ArrayList Class hierarchy

Important methods of ArrayList class are as below.
  • boolean add(E e) : It will append given element at the end of this ArrayList.
  • void add(int index, E element) : It will add specified element at the given index in list.
  • boolean addAll(Collection<? extends E> c) : It will append all the elements of given collection at the end of array list. Appending order will be as per returned by collection's iterator.
  • boolean addAll(int index, Collection<? extends E> c) : It will insert all elements of specified collection in ArrayList starting from given index.
  • void clear() : It will clear List by removing all elements from it.
  • Object clone() : It will return shallow copy of ArrayList.
  • boolean contains(Object o) : It will return true if given element is present in list.
  • void ensureCapacity(int minCapacity) : It will increases the capacity of array list if required to make sure that it can store the elements as per given minCapacity argument.
  • E get(int index) : It will get element from specified inced from array list.
  • int indexOf(Object o) : It will return the index of specified object(first occurrence) from array list.
  • boolean isEmpty() : It will check and return true if array list is empty.
  • Iterator<E> iterator() : It will return an iterator over the elements from array list.
  • int lastIndexOf(Object o) : It will return index of last occurrence of specified element.
  • ListIterator<E> listIterator() : It will return the list iterator over the elements of array list.
  • ListIterator<E> listIterator(int index) : It will return the list iterator over the elements of array list starting from specified index.
  • E remove(int index) : It will remove element from array list which is stored at specified index.
  • boolean remove(Object o) : It will remove specified element(first occurrence) from array list.
  • boolean removeAll(Collection<?> c) : It will remove all elements from list which are specified in given collection.
  • protected void removeRange(int fromIndex, int toIndex) : It will remove elements from list starting from fromIndex to toIndex.
  • boolean retainAll(Collection<?> c) : It will retain only those elements in array list which are specified in given collection.
  • E set(int index, E element) : It will replace given element in array list at specified position.
  • int size() : It will return size of array list.
  • List<E> subList(int fromIndex, int toIndex) : It will return sub list of elements from this list starting from fromIndex(inclusive) to toIndex(exclusive).
  • Object[] toArray() :It will return proper sequential array of all array list elements.
  • void trimToSize() :It will trim size of array list as per it's current size.
Below given example will show you usage of ArrayList class's different methods.

ArrayListExample
package JAVAExamples;

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

public class ArrayListClassExample {

 public static void main(String[] args) {
  // Create array list.
  ArrayList<String> a = new ArrayList<String>();
  // Add elements in array list.
  a.add("one");
  a.add("two");
  a.add("three");
  a.add("four");
  a.add("one");
  a.add("six");
  a.add("seven");
  a.add("eight");

  //Iterating over the arraylist elements.
  Iterator itr = a.iterator();
  System.out.println("ArrayList items are : ");
  while (itr.hasNext()) {
   System.out.println(itr.next());
  }
  
  //Add item at specified index.
  a.add(5, "nine");
  System.out.println("Now ArrayList items are : "+a);
  
  //Get item from specified index.
  System.out.println("Item at index 6 is : "+a.get(6));
  
  //Remove item from array list.
  a.remove("three");
  System.out.println("Now ArrayList items are : "+a);
  
  //Get index of element from array list.
  System.out.println("Index of one is : "+a.indexOf("one"));
  
  //Get last index of element from array list.
  System.out.println("Last Index of one is : "+a.lastIndexOf("one"));
 }
}

Output :
ArrayList items are : 
one
two
three
four
one
six
seven
eight
Now ArrayList items are : [one, two, three, four, one, nine, six, seven, eight]
Item at index 6 is : six
Now ArrayList items are : [one, two, four, one, nine, six, seven, eight]
Index of one is : 0
Last Index of one is : 3

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