ArrayList in Java
ArrayList is a part of collection framework and is present in java.util package. It provides us with dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed. This class is found in java.util package.
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Illustration:
Example: The following implementation demonstrates how to create and use an ArrayList.
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// Java program to demonstrate the
// working of ArrayList in Java
import java.io.*;
import java.util.*;
class ArrayListExample {
public static void main[String[] args]
{
// Size of the
// ArrayList
int n = 5;
// Declaring the ArrayList with
// initial size n
ArrayList arrli
= new ArrayList[n];
// Appending new elements at
// the end of the list
for [int i = 1; i c] |
Retains only the elements in this list that are contained in the specified collection. |
| set?[int index, E element] | Replaces the element at the specified position in this list with the specified element. |
| size?[] | Returns the number of elements in this list. |
| spliterator?[] | Creates a late-binding and fail-fast Spliterator over the elements in this list. |
| subList?[int fromIndex, int toIndex] | Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. |
| toArray[] | This method is used to return an array containing all of the elements in the list in the correct order. |
| toArray[Object[] O] | It is also used to return an array containing all of the elements in this list in the correct order same as the previous method. |
| trimToSize[] | This method is used to trim the capacity of the instance of the ArrayList to the lists current size. |
Note: You can also create a generic ArrayList:
// Creating generic integer ArrayList ArrayList arrli = new ArrayList[];
Lets see how to perform some basics operations on the ArrayList as listed which we are going to discuss further alongside implementing every operation.
- Adding element to List
- Changing elements
- Removing elements
- Iterating elements
Operation 1: Adding Elements
In order to add an element to an ArrayList, we can use the add[] method. This method is overloaded to perform multiple operations based on different parameters. They are as follows:
- add[Object]: This method is used to add an element at the end of the ArrayList.
- add[int index, Object]: This method is used to add an element at a specific index in the ArrayList.
Example:
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// Java Program to Add elements to An ArrayList
// Importing all utility classes
import java.util.*;
// Main class
class GFG {
// Main driver method
public static void main[String args[]]
{
// Creating an Array of string type
ArrayList al = new ArrayList[];
// Adding elements to ArrayList
// Cutom inpus
al.add["Geeks"];
al.add["Geeks"];
// Here we are mentioning the index
// at which it is to be added
al.add[1, "For"];
// Printing all the elements in an ArrayList
System.out.println[al];
}
}
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Operation 2: Changing Elements
After adding the elements, if we wish to change the element, it can be done using the set[] method. Since an ArrayList is indexed, the element which we wish to change is referenced by the index of the element. Therefore, this method takes an index and the updated element which needs to be inserted at that index.
Example
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// Java Program to Change elements in ArrayList
// Importing all utility classes
import java.util.*;
// main class
class GFG {
// Main driver method
public static void main[String args[]]
{
// Creating an Arratlist object of string type
ArrayList al = new ArrayList[];
// Adding elements to Arraylist
// Custom input elements
al.add["Geeks"];
al.add["Geeks"];
// Adding specifying the index to be added
al.add[1, "Geeks"];
// Printing the Arraylist elements
System.out.println["Initial ArrayList " + al];
// Setting element at 1st index
al.set[1, "For"];
// Printing the updated Arraylist
System.out.println["Updated ArrayList " + al];
}
}
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Operation 3: Removing Elements
In order to remove an element from an ArrayList, we can use the remove[] method. This method is overloaded to perform multiple operations based on different parameters. They are as follows:
- remove[Object]: This method is used to simply remove an object from the ArrayList. If there are multiple such objects, then the first occurrence of the object is removed.
- remove[int index]: Since an ArrayList is indexed, this method takes an integer value which simply removes the element present at that specific index in the ArrayList. After removing the element, all the elements are moved to the left to fill the space and the indices of the objects are updated.
Example
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// Java program to Remove Elements in ArrayList
// Importing all utility classes
import java.util.*;
// Main class
class GFG {
// Main driver method
public static void main[String args[]]
{
// Creating an object of arraylist class
ArrayList al = new ArrayList[];
// Adding elements to ArrayList
// Custom addition
al.add["Geeks"];
al.add["Geeks"];
// Adding element at specific index
al.add[1, "For"];
// Printing all elements of ArrayList
System.out.println["Initial ArrayList " + al];
// Removing element from above ArrayList
al.remove[1];
// Printing the updated Arraylist elements
System.out.println["After the Index Removal " + al];
// Removing this word element in ArrayList
al.remove["Geeks"];
// Now printing updated ArrayList
System.out.println["After the Object Removal "
+ al];
}
}
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Operation 4: Iterating the ArrayList
There are multiple ways to iterate through the ArrayList. The most famous ways are by using the basic for loop in combination with a get[] method to get the element at a specific index and the advanced for loop.
Example
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// Java program to Iterate the elements
// in an ArrayList
// Importing all utility classes
import java.util.*;
// Main class
class GFG {
// Main driver method
public static void main[String args[]]
{
// Creating an Arraylist of string type
ArrayList al = new ArrayList[];
// Adding elements to ArrayList
// using standard add[] method
al.add["Geeks"];
al.add["Geeks"];
al.add[1, "For"];
// Using the Get method and the
// for loop
for [int i = 0; i < al.size[]; i++] {
System.out.print[al.get[i] + " "];
}
System.out.println[];
// Using the for each loop
for [String str : al]
System.out.print[str + " "];
}
}
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Must Read: Array vs ArrayList in Java