String toLowerCase and toUpperCase Methods in Java

1. Overview

In this tutorial, we’ll cover the toUpperCase and toLowerCase methods included in the Java String class.

We’ll start by creating a String called name:

String name = "John Doe";

2. Convert to Uppercase

To create a new uppercase String based on name, we call the toUpperCase method:

String uppercaseName = name.toUpperCase();

This results in uppercaseName having the value “JOHN DOE”:

assertEquals("JOHN DOE", uppercaseName);

Note that Strings are immutable in Java and that calling toUpperCase creates a new String. In other words, name is unchanged when calling toUpperCase.

3. Convert to Lowercase

Similarly, we create a new lowercase String based on name by calling toLowerCase:

String lowercaseName = name.toLowerCase();

This results in lowercaseName having the value “john doe”:

assertEquals("john doe", lowercaseName);

Just as with toUpperCase, toLowerCase does not change the value of name.

4. Change Case Using Locales

Additionally, by supplying a Locale to the toUpperCase and toLowerCase methods, we can change the case of a String using locale-specific rules.

For example, we can supply a Locale to uppercase a Turkish i (Unicode 0069):

Locale TURKISH = new Locale("tr");
System.out.println("\u0069".toUpperCase());
System.out.println("\u0069".toUpperCase(TURKISH));

Accordingly, this results in an uppercase I and a dotted uppercase I:

I
İ

We can verify this using the following assertions:

assertEquals("\u0049", "\u0069".toUpperCase());
assertEquals("\u0130", "\u0069".toUpperCase(TURKISH));

Likewise, we can do the same for toLowerCase using the Turkish I (Unicode 0049):

System.out.println("\u0049".toLowerCase());
System.out.println("\u0049".toLowerCase(TURKISH));

Consequently, this results in a lowercase i and a lowercase dotless i:

i
ı

We can verify this using the following assertions:

assertEquals("\u0069", "\u0049".toLowerCase());
assertEquals("\u0131", "\u0049".toLowerCase(TURKISH));

5. Conclusion

In conclusion, the Java String class includes the toUpperCase and toLowerCase methods for changing the case of a String. If needed, a Locale can be supplied to provide locale-specific rules when changing the case of a String.

The source code for this article, including examples, can be found over on GitHub.