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.
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);
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());
}
and nobody would be using it]. It means Strings were immutable that’s why java is still around in the game.