Conversion of Array To ArrayList in Java

Following methods can be used for converting Array To ArrayList:

Method 1: Using Arrays.asList() method

 Syntax: public static List asList(T... a)
// Returns a fixed-size List as of size of given array. 
// Element Type of List is of same as type of array element type.
// It returns an List containing all of the elements in this 
// array in the same order. 
// T represents generics

Note that the there is an array parameter and List return value.

Returns a fixed-size list backed by the specified array.
The returned list is serializable and implements RandomAccess.
Since returned List is fixed-size therefore we can’t add more element in it, but we can replace existing element with new element using set(index, new Element) method defined in ArrayList class.

// Java program to demonstrate conversion of 
// Array to ArrayList of fixed-size. 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh", 
                          "Shubham", "Neelam"}; 
  
        // Conversion of array to ArrayList 
        // using Arrays.asList 
        List al = Arrays.asList(geeks); 
  
        System.out.println(al); 
    } 
} 

Output:

[Rahul, Utkarsh, Shubham, Neelam]

What if we add more elements to the converted list?
Since returned List is fixed-size List, we can’t add more element(s). An attempt of adding more elements would cause UnsupportedOperationException.

Consider the following example.

// Java program to demonstrate error 
// if we add more element(s) to 
// a fixed-size List. 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
  
        String[] geeks = {"Rahul", "Utkarsh", 
                          "Shubham", "Neelam"}; 
  
        // Conversion of array to ArrayList 
        // using Arrays.asList 
        List al = Arrays.asList(geeks); 
        System.out.println(al); 
  
        // Adding some more values to the List. 
        al.add("Shashank"); 
        al.add("Nishant"); 
  
        System.out.println(al); 
    } 
} 

Output:

[Rahul, Utkarsh, Shubham, Neelam]

Runtime error

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.AbstractList.add(AbstractList.java:148)
    at java.util.AbstractList.add(AbstractList.java:108)
    at GFG.main(File.java:16)

It is therefore recommended to create new ArrayList and pass Arrays.asList(array reference) as an argument to it (i.e. as an constructor argument of ArrayList).

Consider the following example:

// Java program to demonstrate how to add 
// one or more element(s) to returned 
// resizeable List. 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh", 
                          "Shubham", "Neelam"}; 
  
        List<String> al = 
            new ArrayList<String>(Arrays.asList(geeks)); 
        System.out.println(al); 
  
        // Adding some more values to the List. 
        al.add("Shashank"); 
        al.add("Nishant"); 
  
        System.out.println("\nArrayList After adding two" + 
                           " more Geeks: "); 
        System.out.println(al); 
    } 
}

[Rahul, Utkarsh, Shubham, Neelam]

ArrayList After adding two more Geeks: 
[Rahul, Utkarsh, Shubham, Neelam, Nishant, Shashank]

&nbap;

Method 2: Using Collections.addAll() method

Syntax: public static  boolean addAll(Collection c, T... a)
// Adds all of the specified elements to the specified collection.
// Elements to be added may be specified individually or as an array.
// T is generics

Note that there is a collection parameter c into which elements to be inserted and array parameter a contains the elements to insert into c.
Return type is boolean type. It returns true if the collection changed as a result of the call.

It throws UnsupportedOperationException if collection c does not support add method and throws IllegalArgumentException if some aspect of a value in elements(or elements of array) prevents it from being added to collection c.

Consider the following example:

// Java program to demonstrate how to 
// add all elements of array to arrayList. 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh", 
                         "Shubham", "Neelam"}; 
  
        List<String> al = new ArrayList<String>(); 
  
        // adding elements of array to arrayList. 
        Collections.addAll(al, geeks); 
  
        System.out.println(al); 
    } 
} 

Output :

[Rahul, Utkarsh, Shubham, Neelam]

Adding null to the list
Note : If the specified collection or specified array is null then it throw NullpointerException.

// Adding null to a list 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh",  
                         "Shubham", "Neelam"}; 
  
        List<String> al = new ArrayList<String>(); 
        Collections.addAll(null, geeks); 
  
        System.out.println(al); 
    } 
} 

Adding null to the end of list

import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh",  
                          "Shubham", "Neelam"}; 
  
        List<String> al = new ArrayList<String>(); 
        Collections.addAll(al, null); 
  
        System.out.println(al); 
    } 
} 

RunTime Error

Exception in thread "main" java.lang.NullPointerException
    at java.util.Collections.addAll(Collections.java:5401)
    at GFG.main(File.java:11)

Method 3: Using Manual method to convert Array using add() method

We can use this method if we don’t want to use java in built method(s). This is a manual method of adding all array’s elements to List.

Syntax: public boolean add(Object obj)
// Appends the specified element to the end of this list.
// Returns true.

// Java program to convert a ArrayList to 
// an array using add() in a loop. 
import java.util.*; 
  
class GFG 
{ 
    public static void main (String[] args) 
    { 
        String[] geeks = {"Rahul", "Utkarsh", 
                          "Shubham", "Neelam"}; 
  
        List<String> al = new ArrayList<String>(); 
  
        // Array to ArrayList Conversion 
        for (String geek : geeks) 
            al.add(geek); 
  
        System.out.println(al); 
    } 
} 

Output:

[Rahul, Utkarsh, Shubham, Neelam]

Related Article: ArrayList to Array Conversion

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