ArrayList in Java is most frequently used collection class after HashMap in Java. Java ArrayList represents an automatic re-sizeable array and used in place of array. Since we can not modify size of an array after creating it, we prefer to use ArrayList in Java which re-size itself automatically once it gets full. ArrayList in Java implements List interface and allow null. Java ArrayList also maintains insertion order of elements and allows duplicates opposite to any Set implementation which doesn't allow duplicates. ArrayList supports both Iterator and ListIterator for iteration but it’s recommended to use ListIterator as it allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the Iterator's current position in the list. But while using ListIterator you need to be little careful because ListIterator has no current element; its cursor position always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next(). In this Java ArrayList tutorial we will see how to create Java ArrayList and perform various operations on Java ArrayList. This collection class is also favourites on many core Java interviews with questions like Difference between ArrayList and Vector or LinkedList vs ArrayList.
ArrayList has been modified in Java 5 (Tiger) to support Generics which makes Java ArrayList even more powerful because of enhanced type-safety. Before Java5 since there was no generics no type checking at compile time which means there is chance of storing different type of element in an ArrayList which is meant for something and ultimately results in ClassCastException during runtime. with generics you can create Java ArrayList which accepts only type of object specified during creation time and results in compilation error if someone tries to insert any other object into ArrayList in Java; for example if you create an ArrayList of String object you can not store Integer on it because add() method of ArrayList will check Type before adding object into ArrayList in Java opposite to add() method of Java 1.4 which accepts any object.
Java ArrayList with Generics in JDK 1.5
It’s also important to remember that ArrayList is not synchronized and should not be shared between multiple threads. If multiple threads access a Java ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (As per Java doc a structural modification is any operation that adds or deletes one or more elements, or explicitly re-sizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList() method. It’s recommended to synchronize the list at the creation time to avoid any accidental non synchronized access to the list. Another better option is to use CopyOnWriteArrayList which is added from Java 5 and optimized for multiple concurrent read. In CopyOnWriteArrayList all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array and that's why it is called as "CopyOnWrite"
Example of ArrayList in Java
Let's see some example of creating ArrayList in Java and using them, I have tried to provide as much example as possible to illustrate different operations possible on Java ArrayList. Please let me know if you need any other Java ArrayList examples and I will add them here.
1) Creating an ArrayList
You can use ArrayList in Java with or without Generics both are permitted by generics version is recommended because of enhanced type-safety.
In this example we will create an ArrayList of String in Java. This Java ArrayList will only allow String and will throw compilation error if we try to any other object than String.
ArrayList<String> stringList = new ArrayList<String>(); //Generic ArrayList to Store only String objects
2) Putting an Item into ArrayList
Second line will result in compilation error because this Java ArrayList will only allow String elements.
stringList.add("Item"); //no error because we are storing String
stringList.add(new Integer(2)); //compilation error
3) Checking size of ArrayList
Size of an ArrayList in Java is total number of elements currently stored in ArrayList.
int size = stringList.size();
4) Checking Index of an Item in Java ArrayList
You can use indexOf() method of ArrayList in Java to find out index of a particular object.
int index = stringList.indexOf("Item"); //location of Item object in List
5) Retrieving Item from ArrayList in a loop
Many a times we need to traverse on Java ArrayList and perform some operations on each retrieved item. Here are two ways of doing it without using Iterator. We will see use of Iterator in next section.
for (int i = 0; i < stringList.size(); i++)
String item = stringList.get(i);
System.out.println("Item " + i + " : " + item);
}
From Java 5 onwards you can use foreach loop as well
for(String item: stringList){
System.out.println("retrieved element: " + item);
}
6) Checking ArrayList for an Item
Sometimes we need to check whether an element exists in ArrayList in Java or not for this purpose we can use contains() method of Java. contains() method takes type of object defined in ArrayList creation and returns true if this list contains the specified element. Alternatively you can also use Collections.binarySearch() method to see if an object is present inside List or not. ArrayList, Vector, CopyOnWriteArrayList and Stack implements RandomAccess interface, they can be used for performing binary search. To see which approach is better, see this article.
7) Checking if ArrayList is Empty
We can use isEmpty() method of Java ArrayList to check whether ArrayList is empty. isEmpty() method returns true if this ArrayList contains no elements. You can also use size() method of List to check if List is empty
boolean result = stringList.isEmpty(); //isEmpty() will return true if List is empty
if(stringList.size() == 0){
System.out.println("ArrayList is empty");
}
8) Removing an Item from ArrayList
There are two ways to remove any elements from ArrayList in Java. You can either remove an element based on its index or by providing object itself. Remove remove (int index) and remove (Object o) method is used to remove any element from ArrayList in Java. Since ArrayList allows duplicate its worth noting that remove (Object o) removes the first occurrence of the specified element from this list, if it is present. In below code first call will remove first element from ArrayList while second call will remove first occurrence of item from ArrayList in Java.
stringList.remove(0);
stringList.remove(item);
9) Copying data from one ArrayList to another ArrayList in Java
Many a times you need to create a copy of ArrayList for this purpose you can use addAll(Collection c) method of ArrayList in Java to copy all elements from on ArrayList to another ArrayList in Java. Below code will add all elements of stringList to newly created copyOfStringList.
ArrayList<String> copyOfStringList = new ArrayList<String>();
copyOfStringList.addAll(stringList);
10) Replacing an element at a particular index
You can use set (int index, E element) method of Java ArrayList to replace any element from a particular index. Below code will replace first element of stringList from "Item" to "Item2".
stringList.set(0,"Item2");
11) Clearing all data from ArrayList
ArrayList in Java provides clear() method which removes all of the elements from this list. Below code will remote all elements from our stringList and make the list empty. You can reuse Java ArrayList after clearing it.
stingList.clear();
12) Converting from ArrayList to Array in Java
Java ArrayList provides you facility to get the array back from your ArrayList. You can use toArray(T[] a) method returns an array containing all of the elements in this list in proper sequence (from first to last element). "a" is the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
String[] itemArray = new String[stringList.size()];
String[] returnedArray = stringList.toArray(itemArray);
If you want to convert ArrayList back to Array than see 3 ways to convert array into arraylist in Java
13) Creating Synchronized ArrayList
Some times you need to synchronize your ArrayList in java to make it shareable between multiple threads you can use Collections utility class for this purpose as shown below.
List list = Collections.synchronizedList(new ArrayList(...));
14) Creating ArrayList from Array in Java
ArrayList in Java is amazing you can create even an ArrayList full of your element from an already existing array. You need to use Arrays.asList(T... a) method for this purpose which returns a fixed-size list backed by the specified array.
ArrayList stringList = Arrays.asList(new String[]{"One", "Two", "Three"); //this is not read only List you can still update value of existing elements
15) Traversing in ArrayList in Java
You can use either Iterator or ListIterator for traversing on Java ArrayList. ListIterator will allow you to traverse in both directions while both Iterator and ListIterator will allow you to remove elements from ArrayList in Java while traversing.
Iterator itr = stringList.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
ListIterator listItr = stringList.listIterator();
while(listItr.hasNext()){
System.out.println(itr.next());
}
see How to loop ArrayList in Java for more alternative ways of traversing a List
16) Sorting elements of ArrayList in Java
You can use Collections.sort(List
17) ArrayList to HashSet conversion
Most of Collection class provides a constructor which accepts a Collection object as argument. Which can be used to copy all elements of one Collection into another. HashSet also provide such constructors which can be used to copy all object from ArrayList to HashSet. But be careful since HashSet doesn't allow duplicates some of the objects will not be included which result in less number of objects. See How to convert ArrayList to HashSet in Java for step by step example.
Tips about ArrayList in Java
1) ArrayList is not a synchronized collection hence it is not suitable to be used between multiple threads concurrently. If you want to use ArrayList like data-structure in multi-threaded environment, then you need to either use new CopyonWriteArrayList or use Collections.synchronizedList() to create a synchronized List. Former is part of concurrent collection package and much more scalable than the second one, but only useful when there are many readers and only few writes. Since a new copy of ArrayList is created every time a write happens, it can be overkill if used in a write heavy environment. Second option is a strictly synchronized collection, much like Vector or Hashtable, but it's not scalable because once number of thread increases drastically, contention becomes a huge issue.
2) CopyOnWriteArrayList is recommended for concurrent multi-threading environment as it is optimized for multiple concurrent read and creates copy for write operation. This was added in Tiger, aka JDK 1.5. It's part of java.util.concurrent package, along with ConcurrentHashMap and BlockingQueue.
3) When ArrayList gets full it creates another array and uses System.arrayCopy() to copy all elements from one array to another array. This is where insertion takes a lot of time.
4) Iterator and ListIterator of Java ArrayList are fail-fast it means if ArrayList is structurally modified at any time after the Iterator is created, in any way except through the iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, that's why it’s called fail-fast.
5) ConcurrentModificationException is not guaranteed and it only thrown at best effort.
6) If you are creating Synchronized List it’s recommended to create while creating instance of underlying ArrayList to prevent accidental non synchronized access to the list.
7) An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity() operation. This may reduce the amount of incremental reallocation due to incremental filling of ArrayList.
8) The size(), isEmpty(), get(), set(), iterator(), and listIterator() operations run in constant time because ArrayList is based on Array but adding or removing an element is costly as compared to LinkedList.
9) ArrayList class is enhanced in Java 1.5 to support Generics which added extra type-safety on ArrayList. It’s recommended to use generics version of ArrayList to ensure that your ArrayList contains only specified type of element and avoid any ClassCastException.
10) Since ArrayList implements List interface it maintains insertion order of elements and also allow duplicates.
11) If we set ArrayList reference to null in Java, all the elements inside ArrayList becomes eligible to garbage collection in Java , provided there are no more strong reference exists for those objects.
12) Always use isEmpty() method to check if ArrayList is empty or not, instead of using size() == 0 check. Former one is much more readable, as shown below
if(!listOfItems.isEmpty(){
System.out.println("Starting order processing);
}
if(listOfOrders.size() != 0){
System.out.println("Order processing started);
}
13) From Java 5 Tiger, ArrayList was made parametrized and you should always use generic version of this class. This prevents classical error of insertion fish in list of fruits, or insertion dice in list of cards. When use use generics, those error will be caught by compiler. Consequently, it also prevents ClassCastException at runtime because compiler ensures right kind of object is stored and retrieved from Collection. It also removes need of manual cast, as Java compiler automatically adds implicit cast. For beginners, understanding generics is little bit tricky, but it's worth learning as no body use collection without generics now days.
When to use ArrayList in Java
Now, you know what is ArrayList, different methods of this class and how it works. It's time now to learn when to use ArrayList in Java. For a Java programmer, as much important is to know about Collection classes, equally important is develop ability to decide which collection to use in a particular scenario. Most of the time two factors drives your decision, performance and functionality. ArrayList is a index based data-structure which means it provides O(1) search performance if you know the index, similarly adding an element into ArrayList is also O(1) performance in best case, but if addition trigger resizing of list then it would be on level of O(n) because that much time will be spent on copying elements of old list into new ArrayList. Coming back to functionality, if you are fine with duplicate elements then only use this collection class. It is not thread-safe so don't use it on concurrent environment.
Hope this Java ArrayList tutorial will be helpful for beginners in Java.
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