Convert list of strings to uppercase and lowercase using Lambda expression in Java

Last update on May 09 2025 12:54:59 (UTC/GMT +8 hours)

3. Convert strings to upper/lowercase using lambda

Write a Java program to implement a lambda expression to convert a list of strings to uppercase and lowercase.

Sample Solution:

Java Code:

// Main.java
import java.util.Arrays;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        // Create a list of strings
        List stringList = Arrays.asList("Red", "Green", "Blue", "PINK");

        // Print the original strings
        System.out.println("\nOriginal strings:");
        for (String str : stringList) {
            System.out.println(str);
        } 

        // Convert strings to lowercase using lambda expression
        stringList.replaceAll(str -> str.toLowerCase());

        // Print the list of lowercase strings
        System.out.println("\nLowercase strings:");
        for (String str : stringList) {
            System.out.println(str);
        }
		
		// Convert strings to uppercase using lambda expression
        stringList.replaceAll(str -> str.toUpperCase());

        // Print the list of uppercase strings
        System.out.println("\nUppercase strings:");
        for (String str : stringList) {
            System.out.println(str);
        }
    }
}

Sample Output:

Original strings:
Red
Green
Blue
PINK

Lowercase strings:
red
green
blue
pink

Uppercase strings:
RED
GREEN
BLUE
PINK

Explanation:

In the above exercise, we start by creating a list of strings stringList.

The replaceAll() method applies a lambda expression that converts each string in the list to uppercase using the toUpperCase() method. This lambda expression is (str -> str.toUpperCase()). After that, the replaceAll() method applies a lambda expression that converts each string in the list to lowercase using the toLowerCase method. This lambda expression is (str -> str.toLowerCase()).

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to implement a lambda expression that converts a list of strings to title case.
• Write a Java program to create a lambda that simultaneously returns two lists: one with uppercase and one with lowercase versions.
• Write a Java program to implement a lambda that maps a list of mixed-case strings to a list of alternating uppercase and lowercase letters.
• Write a Java program to chain lambda expressions to convert a list of strings first to uppercase and then sort them alphabetically.

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