Java Program to Convert an Array into a List

Given an array. Your task is to convert the given array into a list in Java.

Examples:

Input: Array: [Geeks, forGeeks, A computer Portal] 
Output: List: [Geeks, forGeeks, A computer Portal]

Input: Array: [1, 2, 3, 4, 5] 
Output: List: [1, 2, 3, 4, 5] 

Arrays: An array is a group of like-typed variables that are referred to by a common name. An array can contain primitives data types as well as objects of a class depending on the definition of the array. In the case of primitives data types, the actual values are stored in contiguous memory locations. In the case of objects of a class, the actual objects are stored in a heap segment.

List: The Java.util.List is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements. List Interface is implemented by ArrayList, LinkedList, Vector, and Stack classes.

There are numerous approaches to do the conversion of an array to a list in Java. A few of them are listed below.

• Brute Force or Naive Method
• Using Arrays.asList() Method
• Using Collections.addAll() Method
• Using Java 8 Stream API
• Using Guava Lists.newArrayList()

1. Brute Force or Naive Method

In this method, an empty List is created and all elements present in the Array are added to it one by one.

Algorithm:

1. Get the Array to be converted.
2. Create an empty List
3. Iterate through the items in the Array.
4. For each item, add it to the List
5. Return the formed List

Java

// Java Program to convert
// Array to List

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

class GFG {

    // Generic function to convert an Array to List
    public static <T> List<T> convertArrayToList(T array[])
    {

        // Create an empty List
        List<T> list = new ArrayList<>();

        // Iterate through the array
        for (T t : array) {
            // Add each element into the list
            list.add(t);
        }

        // Return the converted List
        return list;
    }

    public static void main(String args[])
    {
        // Create an Array
        String array[]
            = { "Geeks", "forGeeks", "A Computer Portal" };

        // Print the Array
        System.out.println("Array: "
                           + Arrays.toString(array));

        // convert the Array to List
        List<String> list = convertArrayToList(array);

        // Print the List
        System.out.println("List: " + list);
    }
}

Output

Array: [Geeks, forGeeks, A Computer Portal]
List: [Geeks, forGeeks, A Computer Portal]

2. Using Arrays.asList() Method

In this method, the Array is passed as the parameter into the List constructor with the help of the Arrays.asList() method.

Algorithm: 

1. Get the Array to be converted.
2. Create the List bypassing the Array as a parameter in the constructor of the List with the help of Arrays.asList() method
3. Return the formed List

Java

// Java Program to convert
// Array to List

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

class GFG {

    // Generic function to convert an Array to List
    public static <T> List<T> convertArrayToList(T array[])
    {

        // Create the List by passing the Array
        // as parameter in the constructor
        List<T> list = Arrays.asList(array);

        // Return the converted List
        return list;
    }

    public static void main(String args[])
    {
        // Create an Array
        String array[]
            = { "Geeks", "forGeeks", "A computer Portal" };

        // Print the Array
        System.out.println("Array: "
                           + Arrays.toString(array));

        // convert the Array to List
        List<String> list = convertArrayToList(array);

        // Print the List
        System.out.println("List: " + list);
    }
}

Output

Array: [Geeks, forGeeks, A computer Portal]
List: [Geeks, forGeeks, A computer Portal]

3. Using Collections.addAll() 

Since List is a part of the Collection package in Java. Therefore the Array can be converted into the List with the help of the Collections.addAll() method.

Algorithm: 

1. Get the Array to be converted.
2. Create an empty List.
3. Add the array into the List by passing it as the parameter to the Collections.addAll() method.
4. Return the formed List

Java

// Java Program to convert
// Array to List

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

class GFG {

    // Generic function to convert an Array to List
    public static <T> List<T> convertArrayToList(T array[])
    {

        // Create the List by passing the Array
        // as parameter in the constructor
        List<T> list = new ArrayList<>();

        // Add the array to list
        Collections.addAll(list, array);

        // Return the converted List
        return list;
    }

    public static void main(String args[])
    {
        // Create an Array
        String array[]
            = { "Geeks", "forGeeks", "A computer Portal" };

        // Print the Array
        System.out.println("Array: "
                           + Arrays.toString(array));

        // convert the Array to List
        List<String> list = convertArrayToList(array);

        // Print the List
        System.out.println("List: " + list);
    }
}

Output

Array: [Geeks, forGeeks, A computer Portal]
List: [Geeks, forGeeks, A computer Portal]

4. Using Java 8 Stream API 

An ArrayList constructor can take another collection object to construct a new list containing the elements of the specified array.

Algorithm: 

1. Get the Array to be converted.
2. Convert the array to Stream
3. Convert the Stream to List using Collectors.toList()
4. Collect the formed list using the collect() method
5. Return the formed List

Java

// Java Program to convert
// Array to List in Java 8

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

class GFG {

    // Generic function to convert array to list
    public static <T> List<T> convertArrayToList(T array[])
    {
        // create a list from the Array
        return Arrays.stream(array).collect(
            Collectors.toList());
    }

    public static void main(String args[])
    {
        // Create an Array
        String array[]
            = { "Geeks", "forGeeks", "A computer Portal" };

        // Print the Array
        System.out.println("Array: "
                           + Arrays.toString(array));

        // convert the Array to List
        List<String> list = convertArrayToList(array);

        // Print the List
        System.out.println("List: " + list);
    }
}

Output

Array: [Geeks, forGeeks, A computer Portal]
List: [Geeks, forGeeks, A computer Portal]

5. Using Guava Lists.newArrayList() 

Lists.newArrayList() creates a mutable ArrayList instance containing the elements of the specified array. 

Algorithm: 

1. Get the Array to be converted.
2. Create an empty List.
3. Add the array into the List by passing it as the parameter to the Lists.newArrayList() method.
4. Return the formed List

Java

// Java Program to convert
// Array to List in Java 8

import static com.google.common.collect.Lists.*;

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

class GFG {

    // Generic function to convert array to list
    public static <T> List<T> convertArrayToList(T array[])
    {
        // create a list from the Array
        return Lists.newArrayList(array);
    }

    public static void main(String args[])
    {
        // Create an Array
        String array[]
            = { "Geeks", "forGeeks", "A computer Portal" };

        // Print the Array
        System.out.println("Array: "
                           + Arrays.toString(array));

        // convert the Array to List
        List<String> list = convertArrayToList(array);

        // Print the List
        System.out.println("List: " + list);
    }
}

Output

Array: [Geeks, forGeeks, A computer Portal]
List: [Geeks, forGeeks, A computer Portal]