How to Check if a String Contains Only Lowercase Letters in Java?

A string is a data structure that contains a set of characters. It can be a word or can be a sentence. Also, it can be empty or can have a single letter. A string can contain Uppercase letters, Lowercase letters, or numbers of special characters.

In this article, we will learn how to check if a String contains only lowercase letters in Java.

Approaches to Check Whether a String Contains Only Lowercase Letters

The idea here is very simple. We will go to each letter and check whether it is a lowercase letter or not. We will see two methods for this.

• By Iterating over the String
• Using String.matches function

1. By iterating over the String

• Iterate on the string using a loop from the 0th index to the last index of the string.
• At each iteration check if it is a lowercase letter or not.
• To check it we can use the concept of ascii values. If the ASCII value of the current letter falls between a to z then it is a lowercase letter. Otherwise not.
• If any letter is not a lowercase letter then simply return false as all the string can’t contain lowercase letters now.
• If we iterate through all the characters successfully then return true.

Code Implementation:

Java

// Java program to check if a String contains only lowercase letters
import java.util.*;
public class LowercaseChecker
{
        public static boolean containsOnlyLowercase(String str)
    {
        for (char c : str.toCharArray())
        {
            // check if the character is not a lowercase letter
            if (!(c >= 'a' && c <= 'z'))
            {
                return false;
            }
        }
        // all characters are lowercase letters
        return true;
    }

    public static void main(String args[])
    {
        // test cases
        String example1 = "lowercaseonly";
        String example2 = "MixedCase123";

        // check if each example contains only lowercase letters
        System.out.println("Example 1 contains only lowercase letters: " + containsOnlyLowercase(example1));
        System.out.println("Example 2 contains only lowercase letters: " + containsOnlyLowercase(example2));
    }
}

Output

Example 1 contains only lowercase letters: true
Example 2 contains only lowercase letters: false

Explanation of the above Program:

• The containsOnlyLowercase method takes a string str as input and iterates over each character in the string.
• For each character, it checks if it falls within the lowercase letter range ('a' to 'z'). If any character is found that is not a lowercase letter, the method returns false.
• If all characters pass the lowercase check, the method returns true.
• The main method tests the containsOnlyLowercase method with two example strings and prints the results.

Complexity of the above method:

Time complexity: O(N)
Space complexity: O(1)

2. Using String.matches function

In this method, all the things will remain same the change here is instead of checking whether the ASCII value of character lies in between ASCII value of a and z we will use inbuilt function is Java called String.matches. Following is the method for the same.

Code Implementation:

Java

// Java program to check if a String contains
// Only lowercase letters using String.matches function
import java.util.*;

// Driver Class
public class LowercaseChecker {
    // Method to Check if a String
      // Contains only lowercase letters
    public static boolean containsOnlyLowercase(String str)
    {
          // True if the string contains
          // Only lowercase letters, otherwise false
        return str.matches("[a-z]+");
    }
    // Main Method to test the containsOnlyLowercase method.
    public static void main(String args[])
    {
        String example1 = "lowercaseonly";
        String example2 = "MixedCase123";

        System.out.println("Example 1 contains only lowercase letters: " + containsOnlyLowercase(example1));
        System.out.println("Example 2 contains only lowercase letters: " + containsOnlyLowercase(example2));
    }
}

Output

Example 1 contains only lowercase letters: true
Example 2 contains only lowercase letters: false

Explanation of the above Program:

• The containsOnlyLowercase method takes a string str as input and checks if it contains only lowercase letters using a regular expression [a-z]+.
• The main method tests the containsOnlyLowercase method with two examples:
  • example1 contains only lowercase letters, so it prints true.
  • example2 contains mixed cases and digits, so it prints false.

Complexity of the above method:

Time complexity: O(N)
Space complexity: O(1)