Java Program to Change a Collection to an Array

The array is a linear data structure containing elements whose size is defined at the time of creation and can hold both object and primitive homogeneous data. The collection is a predefined class holding only heterogeneous object types but primitive. Method of java utility class can be used to change a collection to an array in java. Following methods can be used to convert Collection to arrays:

1. Using list.add() method
2. Using list.toArray() method 

Approach 1: Using list.add() method
list.add() is used to insert the specified element E at the specified position in the list.

Syntax:

public void add(int index, E element);

Parameters:

• Index: The index where the element is to be inserted.
• Element: The element is to be inserted.

Return Value: This method does not return anything.
Exception: 

• IndexOutOfBoundsException: When the index is out of the range.
  

Example:

Java

// Java program to change Collection to an array

import java.util.Arrays;
import java.util.ArrayList;
import java.util.Arrays;

// Or simply add all generic lava libraries
import java.util.*;

public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating arrayList list dynamically
        List<String> list = new ArrayList<String>();
        // List is created

        // Adding elements to the list
        list.add("Geeks ");
        list.add("for ");
        list.add("Geeks ");
        list.add("is ");
        list.add("the ");
        list.add("Best.");

        // Converting list to an array
        String[] str = list.toArray(new String[0]);

        // Iterating over elements of array
        for (int i = 0; i < str.length; i++) {
            String data = str[i];

            // Printing elements of an array
            System.out.print(data);
        }
    }
}

Output:

Geeks for Geeks is the Best

Approach 2: Using list.toArray() method

It is a method present there in the list interface that returns all the elements of the list in sequential order as an array.

Syntax:

 public Object[] toArray() ;

Parameters:

• It is specified by ‘toArray’ in interface Collection and interface List
• It overrides ‘toArray’ in class Abstract Collection
• It returns an array containing all the elements in this list in the correct order.

Return Type: Value

An array of sequential elements of the list

Implementation: Below is an example of a clear understanding of the method and its usage:

Java

// Java program to convert Collections into Array

// Importing generic Java libraries
import java.util.*;
import java.io.*;

public class GFG {

    // Main driver code
    public static void main(String[] args)
    {
        // Reading input from the user
        // via BufferedReader class
        BufferedReader in = new BufferedReader(
            new InputStreamReader(System.in));
        // 'in' is object created of this class

        // Creating object of Scanner class
        Scanner sc = new Scanner(System.in);

        // Creating ArrayList to store user input
        List<String> list = new ArrayList<String>();

        // Taking input from user
        // adding elements to the list
        while (sc.hasNext()) {
            String i = sc.nextLine();
            list.add(i);
        }

        // Converting list to an array
        String[] str = list.toArray(new String[0]);

        // Iteration over array
        for (int i = 0; i < str.length; i++) {
            String data = str[i];

            // Printing the elements
            System.out.print(data);
        }
    }
}

Output :

User Input : Geeks for Geeks
Output     : Geeks for Geeks