In this example we will show how to use ArrayList in Java. The class
java.util.ArrayList provides resizable-array, which means that items can be added and removed from the list. It implements the
A major question related to arraylists is about when to use arraylists instead of arrays and vice versa. An
ArrayList is a dynamic data structure so it can be used when there is no upper bound on the number of elements.
From the other side, a simple
Array in java is a static data structure, because the initial size of array cannot be changed, so it can be used only when the data has a known number of elements.
1. ArrayList Constructors
ArrayList class supports three constructors.
This constructor builds an empty list.
ArrayList(Collection<? extends E> c)
This constructor creates a list containing the elements of the specified collection. Note that E is the notation for the type of an element in a collection.
This constructor creates an empty list with the specified initial capacity.
For example, if you want to create an empty array list of Strings then you would do the following:
ArrayList<String> list = new ArrayList<String>();
If you want to create an array list with initial capacity, then you should do the following:
ArrayList<Integer> list = new ArrayList<Integer>(7);
ArrayList class supports only object types and not primitive types.
2. ArrayList common methods
Here are some of the most useful
- Adding elements to the list
boolean add(Element e)
Adds the specified element to the end of this list.
void add(int index, Element e)
Adds the specified element at the specified position in the list.
- Removing elements from the list
Removes all the elements from the list.
E remove(int index)
Removes the element at the specified position in the list.
protected void removeRange(int start, int end)
Removes from the list all the elements starting from index
start(included) until index
- Getting elements from the list
E get(int index)
Returns the element at the specified position.
Returns an array containing all the elements of the list in proper sequence.
- Setting an element
E set(int index, E element)
Replaces the element at the specified position with the specified element.
- Searching elements
boolean contains(Object o)
Returns true if the specified element is found in the list.
int indexOf(Object o)
Returns the index of the first occurrence of the specified element in the list. If this element is not in the list, the method returns -1.
int lastIndexOf(Object o)
Returns the index of the last occurrence of the specified element in the list. If this element is not in the list, the method returns -1.
- Iterating the arraylist
Returns an iterator over the elements in the list.
Returns a list iterator over the elements in this list.
- Checking if the list is empty
Returns true if the list does not contain any element.
- Getting the size of the list
Returns the length of the list (the number of elements contained in the list).
Those were the most commonly used methods of
java.util.ArrayList. For further details for each method or for other methods that are not mentioned in this section, you can have a look at official java api.
3. Examples of using ArrayList in Java
Create a java class named
ArrayListExamples.java with the following code:
In the above code, we can see that many
ArrayList usage cases are covered. Adding elements to the list using 2 different methods, removing elements, getting the size of the list, checking if the list is empty, checking if a specific element is contained to the list. Also, 3 different ways are presented for retrieving the elements of a list. Finally, we show how to convert an
The arraylist contains the following elements: [Item1, Item2, Item3, Item4] The index of Item2 is: 1 Checking if the arraylist is empty: false The size of the list is: 4 Checking if the arraylist contains the object Item5: false The item is the index 0 is: Item1 Retrieving items with loop using index and size list Index: 0 - Item: Item1 Index: 1 - Item: Item2 Index: 2 - Item: Item3 Index: 3 - Item: Item4 Retrieving items using foreach loop Item is: Item1 Item is: Item2 Item is: Item3 Item is: Item4 Retrieving items using iterator Item is: Item1 Item is: Item2 Item is: Item3 Item is: Item4 The arraylist after the replacement is: [Item1, NewItem, Item3, Item4] The final contents of the arraylist are: [NewItem, Item4] The array created after the conversion of our arraylist is: [NewItem, Item4]
As we see in the output, the results are complied with what we described in the previous section.
4. Download the source code
This was an example of using ArrayList in Java. Download the source code here: ArrayListExample.zip