Covert List To Array And Other Collections In Java

This Tutorial Explains how to Convert Java List to Array and Other Collections. It Includes Examples to Convert List to Array, String, Set, and vice-versa:

In our earlier tutorials, we discussed the details of the list collection. We have seen the various operations on the list, iterators, etc. Given a list, we can convert this list into any other collections or data structures like strings, arrays, ArrayList, set, etc.

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Convert Java List to Array and other Collections

List Conversions

Here, what we actually do is we transfer the elements from one collection/structure to another and in doing so, we are actually changing the layout of the elements as each collection or data structure has its own layout.

In this tutorial, we will discuss a few conversions from the list to other data structures and vice-versa.

Convert List To String

You can easily convert the list of elements into its string representation. Mostly you will want to convert a list of strings or characters to a string representation.

There are various methods of converting the list to a string. The most popular ones are discussed below.

#1) Using The toString Method

This is the simplest method to convert list to string. In this method, you simply use the ‘toString’ method on a list that converts the list into a string representation.

The program below shows the implementation of the toString method.

import java.util.Arrays; 
import java.util.List; 

// Convert List of Characters to String in Java 
class Main { 
public static void main(String[] args)   { 
        // create and initialize a character list
        List<Character> strList = Arrays.asList('H', 'e', 'l', 'l', 'o'); 
          //convert list to string using toString method
         System.out.println("List converted to string representation:\n" + strList.toString());
        //replace comma between characters with blanks
        String string = strList.toString()
                            .substring(1, 3 * strList.size() - 1) 
                            .replaceAll(", ", ""); 
          // print string 
       System.out.println("String representation by removing delimiters:\n" + string); 
    } 
}

Output:

Using the toString method - Output

In this program, you can see that after converting the character list into a string representation, the string is refined further by removing the delimiting characters from it and representing it as a single word.

#2) Using Collectors Class

From Java 8 onwards, you can make use of stream APIs with the ‘Collectors’ class to convert the list into a string.

Example:

import java.util.stream.Collectors; 
import java.util.Arrays; 
import java.util.List; 

class Main { 
public static void main(String[] args) 
    { 
        // create and initialize list
        List<Character> strList = Arrays.asList('W', 'o', 'r', 'l','d'); 

        // convert list to string using collect and joining() method 
        String string=  strList.stream().map(String::valueOf).collect(Collectors.joining()); 

        // print string 
       System.out.println("List converted to string:" + string); 
    } 
}

Output:

Using Collectors class

In the above program, we make use of stream APIs, map the values and then using joining() method of Collectors class, and convert the list to string.

#3) Using StringBuilder Class

The third method of converting the list to string is by using a StringBuilder object. Here, each element in the list is appended to the StringBuilder object using a loop. Then the StringBuilder object is converted to its string representation using the toString method.

Check the following program for the demonstration.

import java.util.Arrays; 
import java.util.List; 

class Main { 
   public static void main(String[] args) 
    { 
        // create and initialize the list  
        List<Character> strList =  
               Arrays.asList('I', 'n', 'd', 'i', 'a'); 

        // object of stringBuilder
        StringBuilder sb = new StringBuilder(); 

        // use append method to append list element to StringBuilder object  
        for (Character ch : strList) { 
            sb.append(ch); 
        } 

        // convert StringBuilder object to string using toString()
        String string = sb.toString(); 

        // print string 
        System.out.println("List converted to string: " + string); 
    } 
}

Output:

Using StringBuilder class

The above program shows the usage of the StringBuilder object to which the elements in the list are appended. Then the object is converted to a string.

Converting List To An Array

Given a list of elements, you need to convert this list into an array. To do this, you can use any of the methods described below.

#1) Using toArray

The simplest way to convert a list to an array is to use the ‘toArray’ method of the list.

The following program shows this implementation of the toArray method.

import java.util.*; 

public class Main { 
     public static void main(String[] args) 
    { 
        //create and initialize list
        List<String> strList = new LinkedList<String>(); 
        strList.add("This"); 
        strList.add("is"); 
        strList.add("Java"); 
        strList.add("Tutorial");
        System.out.println("The given list:" + strList);

        //use toArray method to convert list to array  
       String[] strArray = strList.toArray(new String[0]); 

        //print the array
        System.out.println("The Array from List: ");
        for (String val : strArray) 
              System.out.print(val + " "); 
    } 
}

Output:

Using toArray

In the above program, we have converted a list of strings into a string array using the ‘toArray’ method.

#2) Using Java 8 Stream

The next method to convert a list to an array is by using the stream APIs of Java 8. In this, the list is first converted to stream and then using stream().toArray, it is converted to an array.

The following Java program shows this conversion.

import java.util.*;

class Main
{
  public static void main (String[]args)
  {
    //create and initialize the list
    List < String > l_list = new LinkedList< String > ();
    l_list.add ("Software");
    l_list.add ("Testing");
    l_list.add ("Help");
    l_list.add ("Tutorial");
    System.out.println("The given list:" + l_list);

    //Convert list to array using stream and toArray methods
    String[] str_array = l_list.stream ().toArray (String[]::new);
    //print the array
    System.out.println("The Array converted from list : ");
    for (String val:str_array)
          System.out.print (val + " ");
  }
}

Output:

Using Java 8 stream

The above program uses stream APIs of Java 8 and converts the list to an array. The array is then printed using for each loop.

#3) Using get Method

This is another method of converting the list to an array. In this, we use the get () of the list that can be used to retrieve individual elements of the list.

The program to convert the list to an array using get () method is shown below.

import java.io.*; 
import java.util.List; 
import java.util.LinkedList; 

class Main 
{ 
    public static void main (String[] args) 
    { 
        List<String> colors_list = new LinkedList<String>(); 
        colors_list.add("Red"); 
        colors_list.add("Green"); 
        colors_list.add("Blue"); 
        colors_list.add("Yellow");

        System.out.println("Given list: " + colors_list);

        //define the array
        String[] colors_array = new String[colors_list.size()]; 

        // get list elements into the array 
        for (int i =0; i <colors_list.size(); i++) 
              colors_array[i] = colors_list.get(i); 
        //print the list
        System.out.println("Array from list: ");
        for (String val : colors_array) 
             System.out.print(val + " "); 
    } 
}

Output:

Using get method

In the above program, we created an array of the same size as a list. Then in a loop, the list elements are retrieved using the get () method and assigned to the array.

Convert Array To List

As you can convert a list to an array, you can also convert an array to a list in Java. Given below are some of the methods using which you can convert an array to a list.

#1) Using plain Java Code

This is the traditional method of converting an array to a list. Here you add each array element to the list one by one using a loop. For this add method of the list is used.

The following program implements the traditional method to convert array to a list.

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

class Main { 
    public static void main(String args[]) 
    { 
        // Create and initialize an Array 
        String strArray[] = { "Java", "Python", "C++", "SQL", "Perl" }; 

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

         // Create a List 
        List<String> strlist = new ArrayList<>(); 

        // Iterate through the array and add each element to the list 
        for (String val : strArray) { 
             strlist.add(val); 
        } 

        // Print the List 
       System.out.println("\nList obtained from array: " + strlist); 
    } 
}

Output:

Using plain Java code

#2) Using asList() Method

The next method of converting an array to list is by using the asList () method of the Arrays class. Here, you can pass Array as an argument to asList () method and it returns the list obtained.

The following program shows this.

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

class Main { 
public static void main(String args[]) 
    { 
        //create and intialize array
        Integer[] oddArray = { 1,3,5,7,9,11 };
        //declare a list and use asList method to assign the array to list
        List<Integer> oddList = Arrays.asList(oddArray);

        // Print the List 
        System.out.println("List from array: " + oddList); 
    } 
}

Output:

Using asList() method

In the above program, we have an array of odd numbers. Then we create a list of Integers and assign it to the output of the asList method which is a list.

#3) Using Collection.addAll() Method

You can also use the addAll () method of Collections class as the array and list are both parts of the collection framework in Java.

The following program shows the use of the Collections.addAll () method to convert array to list.

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

class Main { 
   public static void main(String args[])   { 
        // Create and initialize an Array 
        String strArray[] = { "Java", "C++", "C", "Perl", "Python" }; 

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

        // create a string list
        List<String> myList = new ArrayList<>(); 
          // Add array to list using Collections addAll method 
         Collections.addAll(myList, strArray); 

        // Print the List 
       System.out.println("List obtained from array: " + myList); 
    } 
}

Output:

Using Collection.addAll() method

Here we have initialized an array. We created an empty list. Then the Collections.addAll () method is used by passing lists and array as an argument. Successful execution of this method will have a list populated with the array elements.

#4) Using Java 8 Streams

The next approach to convert array to list is by using Java 8 Stream API and Collectors class. Here the array is first converted to stream and then the elements are collected into a list using stream. Collect method. The list is returned finally.

The following program shows the implementation that converts the array to list using Stream API.

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

class Main { 
   public static void main(String args[]) 
    { 
        // Create and initialize an Array 
        String colorsArray[] = { "Red", "Green", "Blue", "Yellow", "Magenta" }; 

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

        // convert the Array to List using stream () and Collectors class 
        List<String>
             colorslist = Arrays 
            .stream(colorsArray) 
            .collect(Collectors.toList());

        // Print the List 
       System.out.println("List from Array: " + colorslist); 
    } 
}

Output:

Using Java 8 Streams

In the above program, an array of colors is created. An empty list is created next and then the list obtained from the array by using stream API is assigned to the list.

Convert List To Set

A set is an unordered collection of elements that does not allow duplicate values. Set is part of the Java Collections framework. You can convert a list to set and vice-versa if need be.

In this section let us see some of the methods that are used to convert a list to a set.

#1) Using The Traditional Method

You can convert the list to set using traditional java code. In this, you can create a HashSet or treeSet. And then using add method, add each list element to the set in a loop.

This implementation is shown below.

import java.util.*; 

class Main { 
    public static void main(String[] args) 
    { 
        // Creating a list of strings 
        List<String> strList = Arrays.asList("Java", "Perl", "Python", "C++", "C"); 
        //print the list
        System.out.println("The list : " + strList);
        //create a set
        Set<String> hashSet = new HashSet<String>(); 
        //add list elements to hashset
        for (String ele : strList) 
            hashSet.add(ele); 
        //print the set  
        System.out.println("HashSet from list:"); 
        for (String val : hashSet) 
             System.out.print(val + " "); 

       } 
}

Output:

Using the traditional method

In the above program, you can see we have created a HashSet object of type string. Then using enhanced for loop, each element of the list is added to the set. Finally, the set is printed.

#2) Using HashSet or treeset Constructor

The next method is using a set constructor. Here we create a set constructor (treeSet or HashSet). A list is passed to this constructor as an argument. Using this list, the set is constructed.

Check the program below for this implementation.

import java.util.*; 

class Main { 
    public static void main(String[] args) 
    { 
        // Create a list of strings 
        List<String> strList = Arrays.asList("Red", "Green", "Blue", "Yellow", "Cyan", "Magenta");
        System.out.println("Original List:" + strList);
        // Creating a hash set using constructor and pass list to the constructor 
        Set<String> hashSet = new HashSet<String>(strList); 

        System.out.println("\nHashSet created from list:"); 
        //print the hashSet
        for (String val : hashSet) 
             System.out.print(val + " "); 

        //Create a treeset using constructor and pass list to the constructor
        Set<String> treeSet = new TreeSet<String>(strList); 
        System.out.println("\n\nTreeSet from list: "); 
        //print the treeset
       for (String x : treeSet) 
          System.out.print(x + " "); 
    } 
}

Output:

Using HashSet or treeset constructor

In the above program, we create both HashSet and treeSet by passing the list as the argument. Finally, the contents of both HashSet and treeset are displayed.

#3) Using The addAll Method

This is the same as the addAll method of Collections we saw before. Here the addAll method is used to copy the list contents to the set.

The following program shows the usage of the addAll method.

import java.util.*; 

class Main { 
    public static void main(String[] args) 
    { 

        // Creating a list of strings 
        List<Integer> intList = Arrays.asList(1,3,5,7,9,11,13); 
        System.out.println("Original List: " + intList);

        //create a hashset
        Set<Integer> hashSet = new HashSet<Integer>(); 

        //add elements of list to hashSet using addAll method
       hashSet.addAll(intList); 
        System.out.println("HashSet created from list: "); 

        //print the hashSet
        for (Integer val : hashSet) 
            System.out.print(val + " "); 
    } 
}

Output:

Using the addAll method

This program creates a HashSet object. Then the addAll method is invoked on the HashSet object with the list as a parameter. This method copies the list contents to the set.

#4) Using Java 8 Streams

As already seen, you can also use Java 8 streams to convert the list to any other collection including set. You have to use the stream().collect method to do this.

The following program shows this implementation.

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

class Main { 
    public static void main(String[] args) 
    { 

        // Create a list of strings 
        List<String> colorsList = Arrays.asList("Red", "Green", 
                    "Blue", "Cyan", "Magenta", "Yellow"); 
        System.out.println("Original List:" + colorsList);
        // Convert to set using stream and Collectors class toSet() method 
        Set<String> colorsSet = colorsList.stream().collect(Collectors.toSet()); 
        System.out.println("The set from list:");
        //print the set
       for (String x : colorsSet) 
            System.out.print(x + " "); 
    } 

Output:

Using Java 8 streams_convert list to set

The above program is similar to that shown in the case of conversion from the list to an array. First, the list is converted to stream and then the collect method collects the stream elements and converts to set.

Now that we have seen various methods that perform the conversion from the list to set, let us see the methods that are used to convert set to the list.

Convert Set To List

Similar to the way in which, you convert list to set, you can also convert set to a list. You can use the same methods described above for this conversion. The only difference is that you switch the places of the list and set in the program.

Given below are examples of conversion from set to list for each method.

#1) Using plain Java

importjava.util.*; 

class Main { 
   public static void main(String[] args) { 
     // Create a set(hashset) and initialize it 
    Set<String> hashSet = new HashSet<String>(); 
    hashSet.add("Red"); 
    hashSet.add("Green");
    hashSet.add("Blue");
    hashSet.add("Cyan");
    hashSet.add("Magenta");
    hashSet.add("Yellow");

    //print the set
    System.out.println("The set elements:");
    for (String x : hashSet) 
        System.out.print(x + " ");

    //create a list (ArrayList)  
    List<String> strList = new ArrayList<String>(hashSet.size()); 
    //traverse the set and add its elements to the list
    for (String x : hashSet) 
         strList.add(x); 
    //print the list
    System.out.println("\nCreated ArrayList:" + strList); 

  } 
}

Output:

Using plain Java_convert set to list

The above program declares and initializes a set. Then it creates a list and adds each set element to the list. Finally, it prints the list.

#2) Using Constructors

import java.util.*; 

class Main { 
  public static void main(String[] args) { 

    // Create a set(hashset) of strings & initialize it 
    Set<String> hashSet = new HashSet<String>(); 
    hashSet.add("Delhi"); 
    hashSet.add("Mumbai");
    hashSet.add("Chennai");
    hashSet.add("Kolkata");
    //print the Set
   System.out.println("The set :");
   for(String str: hashSet)
        System.out.print(str + " ");

    //pass hashset to linkedlist constructor
    List<String> l_List = new LinkedList<String>(hashSet);
    //print the linked list
    System.out.println ("\n\nLinkedList from set: " + l_List);      
  } 
}

Output:

Using constructors

Here, you can use the list constructor with a set object as its argument. This copies all the set elements to the list object.

#3) Using The addAll Method

import java.util.*; 

class Main { 
  public static void main(String[] args) { 
    // Create a set(hashset) of strings & initialize it 
    Set<String> hashSet = new HashSet<String>(); 
    hashSet.add("Red"); 
    hashSet.add("Green"); 
    hashSet.add("Blue"); 
    hashSet.add("Cyan"); 
    hashSet.add("Magenta"); 
    hashSet.add("Yellow");

    //print the Set
    System.out.println("The set: ");
    for(String x:hashSet)
         System.out.print(x + " ");
    //create a list(ArrayList)        
    List<String> colorList = new ArrayList<String>(); 
    //use addAll method to add elements from set
    colorList.addAll(hashSet); 

    //print the list
    System.out.println("\n\nThe ArrayList from set: " + colorList);     
  } 
}

Output:

Using the addAll method_convert set to list

In this program, a list object is created. Then using the Collections.addAll() method, the elements of the set are added to the list.

#4) Using Java 8 Stream

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

class Main { 
   public static void main(String[] args) { 

    // Create a set(hashset) of strings & initialize the set 
    Set<String> hashSet = new HashSet<String>(); 
    hashSet.add("Yellow");
    hashSet.add("Magenta");
    hashSet.add("Cyan");
    hashSet.add("Red"); 
    hashSet.add("Green");
    hashSet.add("Blue");

    //print the Set
    System.out.println("The set:");
    for(String str : hashSet)
         System.out.print(str + " ");
    //create a list and assign it elements of set through stream and Collectors class
    List<String> strList = hashSet.stream().collect(Collectors.toList()); 

    //print the list
    System.out.println("\n\nList obtained from set: " + strList); 
  } 
}

Output:

Using Java 8 Stream_Convert set to list

You can also use Java 8 streams and the collect method to convert set into the list as shown in the above program.

Array Vs List

Let's discuss some of the differences between an array and a list.

ArrayList
The array is a basic data structure in Java.The list is an interfacethat is part of the collection framework in Java from which many of the classes can be extended like LinkedList, ArrayList etc.
Has fixed sizeList size is dynamic.
Array elements can be accessed using [].List members are accessed using methods.
The array can have primitive types as well as objects as its elements.Lists can contain only objects.
Arrays can use operators with its elements.Lists cannot use operators. Instead,it lists use methods.
Arrays cannot work with generics to ensure type safety.Lists can be generic.

Frequently Asked Questions

Q #1) How do you convert a list to an array in Java?

Answer: The basic method to convert a list to an array in Java is to use the ‘toArray()’ method of List interface.

The simple call below converts the list to an array.

Object[] array = list.toArray();

There are also other methods as discussed above to convert the list to an array.

Q #2) Is an array a list in Java?

Answer: No. An array is a basic structure in Java with a fixed length. It does not extend from the list interface. The structure that extends from the list interface and similar to array is ArrayList.

Q #3) How do you convert an array to a list in Java?

Answer: One of the basic methods to convert an array to a list in Java is to use the asList () method of the Arrays class.

List<String> aList = Arrays.asList (myarray);

Apart from this, there are more methods that convert an array to a list as discussed earlier in this tutorial.

Q #4) Which is a faster set or list in Java?

Answer: It depends on the data being stored. If the data is unique, then the list is better and faster. If you have a large data set, then go for sets. The set structure usually requires 10 times more memory than lists.

Q #5) What is the difference between an ArrayList and a Set?

Answer: The list is a collection of ordered elements while the set is a collection of unordered elements. The list can have duplicate elements but the set cannot have duplicate elements.

Conclusion

In this tutorial, we have seen various list conversions to the array, set and vice-versa. We have also seen the major differences between an array and a list.

In this next tutorial, we will discuss the list classes like ArrayList, LinkedList, etc. in detail.

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