Why arraylist implements list

The ArrayList class is not synchronized. If multiple threads try to modify an ArrayList at the same time then the final result becomes not-deterministic because one thread might override the changes done by another thread.

The following example shows what happens when multiple threads try to modify an ArrayList at the same time. But if you run the program, it will produce different output every time it is run -. Try running the above program multiple times and see how it produces different outputs. To learn more about such issues in multi-threaded programs, check out my article on Java Concurrency Issues and Thread Synchronization.

All right! The following example shows the synchronized version of the previous example. Unlike the previous program, the output of this program is deterministic and will always be the same. The above example uses Collections. Moreover, the modifications to the ArrayList inside the incrementArrayList method is wrapped inside a synchronized block. This ensures that no two threads can increment ArrayList elements at the same time.

It is a thread-safe version of the ArrayList class. It implements all the mutating operations by making a fresh copy of the ArrayList. In this article, you learned what is an ArrayList, how to create an ArrayList, how to add, modify and remove elements from an ArrayList, how to iterate over an ArrayList, how to sort an ArrayList, and how to synchronize access to an ArrayList.

Following are few key points to note about ArrayList in Java - An ArrayList is a re-sizable array, also called a dynamic array. Java ArrayList allows duplicate and null values. Java ArrayList is an ordered collection. It maintains the insertion order of the elements.

It creates an Array with the default capacity, which is It invokes the default constructor of the ArrayList class. It uses an empty array instance to create the new object, and the following code is executed by the Java compiler:. From the above code, we can see an empty ArrayList is internally created with the default capacity, which is As we can see from the above line of code, from Java 8, the private keyword has been removed for providing access to nested classes such as Itr, ListItr, SubList.

We can also define the List with the specific capacity. When we provide an initial capacity, the ArrayList constructor is invoked internally to specify the Array internally. For example, if we define an array list with the capacity of 20, we have to define the ArrayList as follows:.

We can also create an object of ArrayList for a specific collection by executing the below line of code:. The above code will create a non-empty list having the objects of LinkedList. When we add a new object to the ArrayList, it will check for the initial size of the ArrayList whether it has space or not.

If there is enough space for the new object, it will add simply using the add method. If there is not enough space to add a new object, it will dynamically increase the size of the Array by that much capacity. The size, isEmpty, get, set, iterator, and listIterator tasks run in a constant time of O 1. JavaTpoint offers too many high quality services.

Mail us on [email protected] , to get more information about given services. Please mail your requirement at [email protected] Duration: 1 week to 2 week. Java Training Java Tutorial. Abstract class Interface Abstract vs Interface. Package Access Modifiers Encapsulation. Reinforcement Learning. R Programming. React Native. Python Design Patterns. Since the lead JCF designer J. Bloch does not say why it's like this in "Effective Java" it seems we will never know why. Yes, it is, but they did it just to clarify the code, to be easy to see that the class implements List interface.

As you could see, that was redundant, you don't need to re-declare the implementation of List interface if AbstractList already declares that implementation. The ArrayList implemented in that code is not the same ArrayList of java. ArrayList, they just share the same name, but they are not the same code.

So you got this compile time error. ClassCastException: java. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Why does arraylist class implement List as well as extend AbstractList?

Ask Question. Asked 8 years, 2 months ago. Active 3 years, 4 months ago. Viewed 18k times. Improve this question. Sergey Kalinichenko k 71 71 gold badges silver badges bronze badges. Ashwin Ashwin 11k 30 30 gold badges silver badges bronze badges. The answer to your first question is yes but java is just like that — tom.

Arrays asList does not return an ArrayList. MarounMaroun : please see the edit. In fact, there is a sound reason for explicitly implementing List. See Pshemo's answer. Show 2 more comments. Active Oldest Votes. Improve this answer. Sergey Kalinichenko Sergey Kalinichenko k 71 71 gold badges silver badges bronze badges. In Java would it be safe to say that there are three main type of lists.

List 2. Set 3. Map — Doug Hauf.