Program to convert List of String to List of Integer in Java

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.

1. Using Java 8 Stream API:

A Stream is a sequence of objects that supports various methods that can be pipelined to produce the desired result. Java 8 Stream API can be used to convert List to List.

Algorithm:

• Get the list of String.
• Convert List of String to Stream of String. This is done using List.stream().
• Convert Stream of String to Stream of Integer. This is done using Stream.map() and passing Integer.parseInt() method as lambda expression.
• Collect Stream of Integer into List of Integer. This is done using Collectors.toList().
• Return/Print the list of String.

Program:

Java

// Java Program to convert
// List<String> to List<Integer> in Java 8
import java.util.*;
import java.util.stream.*;

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

        // Create a list of String
        List<String> listOfString = Arrays.asList("1", "2", "3", "4", "5");

        // Print the list of String
        System.out.println("List of String: " + listOfString);

        // Convert List of String to List of Integer
        List<Integer> listOfInteger = listOfString.stream()
                            .map(Integer::valueOf)
                            .collect(Collectors.toList());

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

Output

List of String: [1, 2, 3, 4, 5]
List of Integer: [1, 2, 3, 4, 5]

2. Using Guava’s List.transform():

Algorithm:

• Get the list of String.
• Convert List of String to List of Integer using Lists.transform(). This is done using passing Integer.parseInt() method as lambda expression for transformation.
• Return/Print the list of String.

Program:

Java

// Java Program to convert
// List<String> to List<Integer> in Java 8
// Using Guava's List.transform()
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

class GFG {

    // Generic function to convert List of
    // String to List of Integer
    public static List<Integer> convertStringListToIntList(List<String> listOfString,
                                                           Function<String, Integer> function) {
        return listOfString.stream()
                .map(function)
                .collect(Collectors.toList());
    }

    public static void main(String args[]) {

        // Create a list of String
        List<String> listOfString = Arrays.asList("1", "2", "3", "4", "5");

        // Print the list of String
        System.out.println("List of String: " + listOfString);

        // Convert List of String to list of Integer
        List<Integer> listOfInteger = convertStringListToIntList(listOfString, Integer::parseInt);

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

Output

List of String: [1, 2, 3, 4, 5]
List of Integer: [1, 2, 3, 4, 5]