Java String toLowerCase() & toUpperCase(): Convert Text Case with Locale Awareness

1. toLowerCase()

The toLowerCase() method transforms every character of a string into its lower‑case equivalent using the default locale’s rules. Because it is locale‑sensitive, results can differ for characters like the German "ß" or Turkish "İ" when the default locale is not appropriate.

For situations that require a specific language, you can call the overloaded variant:

String lower = original.toLowerCase(Locale.forLanguageTag("de"));

Key points:

• Returns a new String; the original remains unchanged.
• No parameters in the no‑arg form.
• Useful for case‑insensitive comparisons and normalizing user input.

Example 1:

public class Guru99 {
    public static void main(String[] args) {
        String s1 = new String("UPPERCASE CONVERTED TO LOWERCASE");
        System.out.println(s1.toLowerCase());
    }
}

Output: uppercase converted to lowercase

2. toUpperCase()

The toUpperCase() method converts all characters of a string to upper‑case using the default locale. As with toLowerCase(), it is locale‑sensitive, so be cautious when handling strings that contain locale‑specific characters.

For explicit control, use the locale‑aware overload:

String upper = original.toUpperCase(Locale.forLanguageTag("tr"));

Key points:

• Produces a new String; the source string is unchanged.
• No parameters in the no‑arg form.
• Commonly used for formatting output or preparing keys for case‑insensitive storage.

Example 2:

public class Guru99 {
    public static void main(String[] args) {
        String s1 = new String("lowercase converted to uppercase");
        System.out.println(s1.toUpperCase());
    }
}

Output: LOWERCASE CONVERTED TO UPPERCASE