ArrayList

1. Automatic-resizable array and used in place of array
2. Once size is set can’t be modified
3. Implement List and allow null
4. Maintains Insertion order
5. Allows duplicates
6. ListIterator and Iterator both can be used. Prefer listiterator since it can traverse in both direction and modify the list as well
7. ArrayList is not synchronized so should not be shared between multiple threads.

a.       If multiple threads access concurrently, and one thread modifies the list it MUST be synchronized externally

b.      Else it should be wrapped using Collections.synchronizedList() method. Should be synchronized at the creation time to avoid any unsynchronized access

c.       Using CopyOnWriteArrayList which is creates a fresh copy of underlying array for all operations like add, remove etc,

8. Creating ArrayList

ArrayList<String> arrayList=new ArrayList<>();

ArrayList<String> arrayList1=new ArrayList<String>();

9. Adding Elements

arrayList.add(new Integer(1)); -- Compilation Error

arrayList.add("A");

arrayList.add("B");

arrayList.add("A");

Output: [A,B,A]

10. Size: size() Arraylist size could be different from the backing array size

11. indexOf(obj): returns the first index of object if found else -1. Internally uses .equals() method to find object so either implement hashcode() and .equals() method or Object class’s by default will be used.

12. boolean contains() : search an object in arraylist or we can use Collections.binarySearch().

13. isEmpty(): to check if it is empty

14. remove(): can remove by passing the object or index. Else through iterator also we can remove.

Iterator iterator=arrayList.iterator();

            while (iterator.hasNext()) {

                  String str=iterator.next().toString();

                  if(str.equals("B")){

                  arrayList.remove(1);-- ConcurrentModificationException

                        iterator.remove(); -- B will be removed

                  }

            }

15. boolean addAll(ArrayList): if we want to add copy of existing list or add a list to already existing

ArrayList<String> copyList1=new ArrayList<>();

         copyList1.addAll(arrayList); -- true

         System.out.println(copyList1); -- [arraylist…]

         ArrayList<String> addList1=new ArrayList<>();

         addList1.add("D");

         addList1.addAll(arrayList); -- true

         System.out.println(addList1);-- [D,arraylist …]

16. set(int Index, element): replace
17. clear() : remove all elements from arraylist. It can be used again.
18. Converting arraylist to array

a.       Array list can hold both primitives and objects while arrayList can hold only objects

b.      List<String> arrayList=Arrays.asList(String[])

c.       List<String> arrayList= new ArrayList();
String[] A={“A”,”B”};

Collections.addAll(arrayList, A);

19. mk

Array v/s ArrayList

1. ArrayList is a class and has methods. Array is an object so no methods we can get only length
2.