How to Get ArrayList from Stream in Java 8?

Last Updated : 17 Jul, 2024

Given a Stream, the task is to convert this Stream into ArrayList in Java 8.

Examples:

Input: Stream: [1, 2, 3, 4, 5]
Output: ArrayList: [1, 2, 3, 4, 5]

Input: Stream: ['G', 'e', 'e', 'k', 's']
Output: ArrayList: ['G', 'e', 'e', 'k', 's']

1. Using Collectors.toList() method:

Approach:

  • Get the Stream to be converted.
  • Collect the stream as List using collect() and Collectors.toList() methods.
  • Convert this List into an ArrayList
  • Return/Print the ArrayList

Below is the implementation of the above approach.

Program: Java 

// Java program to convert Stream to ArrayList
// using Collectors.toList() method

import java.util.*;
import java.util.stream.*;

public class GFG {

   // Driver code
    public static void main(String args[])
    {

        Stream<Integer>
            stream = Stream.of(1, 2, 3, 4, 5);

        // Convert Stream to ArrayList in Java
        ArrayList<Integer>
            arrayList = new ArrayList<Integer>(stream.collect(Collectors.toList()));

        // Print the arraylist
        System.out.println("ArrayList: " + arrayList);
    }
}

Output
ArrayList: [1, 2, 3, 4, 5]

Explanation of the Program:

  • This Java program demonstrates converting a Stream to an ArrayList using the Collectors.toList method.
  • In the main method, a Stream of integers is created and then collected into a List, which is used to initialize an ArrayList.
  • The resulting ArrayList is then printed to the console.

2. Using Collectors.toCollection() method:

Approach:

  • Get the Stream to be converted.
  • Collect the stream as ArrayList using collect() and Collectors.toCollection() methods.
  • Return/Print the ArrayList

Below is the implementation of the above approach.

Program: Java 

// Java program to convert Stream to ArrayList
// using Collectors.toList() method

import java.util.*;
import java.util.stream.*;

public class GFG {

    // Function to get ArrayList from Stream
    public static <T> ArrayList<T>
    getArrayListFromStream(Stream<T> stream)
    {

        // Convert the Stream to ArrayList
        ArrayList<T>
            arrayList = stream
                            .collect(Collectors
                            .toCollection(ArrayList::new));

        // Return the ArrayList
        return arrayList;
    }

    // Driver code
    public static void main(String args[])
    {

        Stream<Integer>
            stream = Stream.of(1, 2, 3, 4, 5);

        // Convert Stream to ArrayList in Java
        ArrayList<Integer>
            arrayList = getArrayListFromStream(stream);

        // Print the arraylist
        System.out.println("ArrayList: "
                           + arrayList);
    }
}

Output
ArrayList: [1, 2, 3, 4, 5]

Explanation of the Program:

  • This Java program converts a Stream to an ArrayList using the Collectors.toCollection method.
  • The getArrayListFromStream function takes a Stream as input, collects its elements into an ArrayList, and returns it.
  • In the main method, a Stream of integers is converted into an ArrayList and printed.


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