ArrayList


ArrayList
Unlike ordinary arrays, the   ArrayList from the java.util package is an automatically expanding array. When you create an object of type ArrayList, you do not need to specify its dimension. For example, ArrayList list = new ArrayList();
Advantages ArrayList compared to a regular array:
  • fast access to elements by index in constant time O(1);
  • access elements by value in linear time O(n);
  • allows you to store any value, including null.

Weaknesses ArrayList:
Inserting/removing an element  causes overwriting of all elements placed "to the right" in the list one position to the left, that is, it works in linear time O(n);

Remember that lists can only contain objects, so if you want to store numbers use Integer instead of int. Integer is the name of a class, and an object of this class can contain an integer value. You can read more about classes and objects here: introduction to object-oriented programming.

Training problem

Creating an ArrayList

There are two options for adding an element to an ArrayList:
add(value);  - adding a value to the end of the ArrayList
add(index, value); - adding a value to the right place by index.

For example:
arr.add(10);
arr.add(5,10);
 

Training problem

Adding an element

To get the value of an element from an ArrayList, use the  get(index)
method For example:
int a = arr. get(0);

To find out the number of elements in an array, you can use the size() method
For example:
int count = arr. size();

Training problem

Getting an element

To change the value by index in the list, use name.set(index,value); 
  
arr.set(0,10);

Training problem

Pairwise permutation

There are two options for adding an element to an ArrayList:
add(value);  - adding a value to the end of the ArrayList
add(index, value); - adding a value to the right place by index.

For example:
arr.add(10);
arr.add(5,10);
 

Training problem

Insert by condition

You can remove elements in an ArrayList using the remove method in two ways:
  • by index remove(index)
  • by value remove(value)
For example:
  
arr.remove(0); //removes the first element
arr.remove(< strong>new Integer(10)); //deletes element with value 10

Training problem

Removing elements

Reverse ArrayList
The Collections  interface also provides methods for bulk operations that work on the entire collection:
containsAll - returns the boolean value true if this collection contains all the elements in the called collection, otherwise it returns false.
addAll - adds all of the specified elements to the specified collection. The elements to be added can be specified individually or as an array.
removeAll -  is used to remove all elements from the list that are contained in the specified collection.
clear - remove all elements
reverse - array reverse
etc.

Example
Using removeAll. ArrayList<Integer> first = new ArrayList<Integer>(); first.add(1); first.add(2); first.add(3); ArrayList<Integer> second = new ArrayList<Integer>(); second.add(1); second.add(2); // removes all elements from the second list, // if they exist in the first list second.removeAll(first);

Training problem

Reverse ArrayList