Arraylist

ArrayList in Java is a fundamental and widely used data structure within the Java Collections Framework. It’s essentially a resizable array implementation of the List

for normal arrays, you have to decide the length of the array first (& cannot change it later)
add, get, remove, clear
Internally, it’s actually an array!
- It starts with default size 10
- There is a pointer that indicates how much of the array is occupied
- If increasing the arraylist, we’re just adding the default size to the arraylist

import java.util.ArrayList;  
  
public class Main {  
    public static void main(String[] args){  
        ArrayList nums = new ArrayList();  
        nums.add(true);  
        nums.add(4);  
        nums.add("hello");  
        System.out.println(nums);  //[true, 4, hello]
    }  
}

Raw type ArrayList:
- This means nums is a raw type, not ArrayList<String> or ArrayList<Integer>.
- In this case, the compiler treats all elements as Object types (the root class of everything in Java).
it just prints all the values!
- the ArrayList overrides the toString() method from Object

Generics

E - Element (used extensively by the Java Collections Framework, for example ArrayList, Set etc.)
K - Key (Used in Map)
N - Number
T - Type
V - Value (Used in Map)
S,U,V etc. - 2nd, 3rd, 4th types

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

ArrayList is of type E
- E can be anything - object, integers (class representations of the primitive type), any custom class
It extends AbstractList<E>
- it is an abstract class
it implements List<E>

It’s always to specify the type

Using generics you can specify the type you are working with

import java.util.ArrayList;  
import java.util.List;  
  
public class Main {  
    public static void main(String[] args){  
        ArrayList<Integer> nums = new ArrayList<>();  
        // u can remove the type here  
        ArrayList<Integer> nums1 = new ArrayList<>(); 
        // u can use the interface/abstract class
        List<Integer> nums2 = new ArrayList<>();  
    }  
}