Java ArrayList sample code direction exception methods

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.

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 list) method to sort a Java ArrayList in natural order defined by Comparable interface and can use Collections.sort(List list, Comparator c) method to sort your Java ArrayList based upon provided Comparator. You can also see this post to sort ArrayList into descending order in Java

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.