Array has a length property, which provides the length of the Array or Array object. It is the total space allocated in memory during array initialization. The array is static, so when we create an array of size n, n blocks of array type will be created, and the JVM will initialize each block with a default value. Let us see this in the image below.

On the other hand, Java ArrayList has no length property. Java ArrayList has a size() method for ArrayList which provides the total number of objects available in the collection.

In Java, an array is a fixed-size data structure that holds a collection of elements of the same type. The length of an array is the number of elements it can hold. Once the array is created, its length cannot be changed. You can access an element in an array using its index, which is a zero-based integer that represents the position of the element in the array.

On the other hand, an ArrayList is a dynamic data structure that can grow or shrink as needed. It is part of the Java Collections Framework and provides more flexibility than arrays. The size of an ArrayList is the number of elements it currently holds, and it can change as elements are added or removed from the list.

Unlike arrays, an ArrayList provides methods for adding, removing, and accessing elements in a more convenient way than using array indexes. For example, you can use the add() method to add an element to the end of the list, or the remove() method to remove an element from the list by specifying its index or value. You can also use the get() method to access an element by its index.

In summary, the main difference between the length of an array and the size of an ArrayList in Java is that the length of an array is fixed and cannot change, while the size of an ArrayList can change dynamically as elements are added or removed.