Java How to Convert String to Uppercase Easily

In Java, you can convert a string to uppercase by using the toUpperCase() method like this: String upper = original.toUpperCase();.

Examples

Inputhello

OutputHELLO

InputJava Programming

OutputJAVA PROGRAMMING

Input123abc!

Output123ABC!

How to Think About It

To convert a string to uppercase, think of changing every letter to its capital form. Java provides a built-in method toUpperCase() that does this for you by scanning each character and converting it if it is a lowercase letter.

Algorithm

1

Get the input string.

2

Call the <code>toUpperCase()</code> method on the string.

3

Store or return the new uppercase string.

Code

java

public class Main {
    public static void main(String[] args) {
        String original = "hello world";
        String upper = original.toUpperCase();
        System.out.println(upper);
    }
}

Output

HELLO WORLD

Dry Run

Let's trace the string "hello world" through the code

1

Original string

original = "hello world"

2

Convert to uppercase

upper = original.toUpperCase() -> "HELLO WORLD"

3

Print result

Output: HELLO WORLD

Step	String Value
Initial	hello world
After toUpperCase()	HELLO WORLD

Why This Works

Step 1: Using toUpperCase()

The toUpperCase() method creates a new string where all lowercase letters are changed to uppercase.

Step 2: Immutable strings

Strings in Java cannot be changed, so toUpperCase() returns a new string instead of modifying the original.

Step 3: Printing the result

The new uppercase string is printed to show the conversion result.

Alternative Approaches

Using Locale with toUpperCase()

java

import java.util.Locale;
public class Main {
    public static void main(String[] args) {
        String original = "hello world";
        String upper = original.toUpperCase(Locale.ENGLISH);
        System.out.println(upper);
    }
}

This approach is useful to handle locale-specific uppercase rules, ensuring consistent results across different languages.

Complexity: O(n) time, O(n) space

Time Complexity

The method processes each character once, so the time grows linearly with the string length.

Space Complexity

A new string is created to hold the uppercase result, so space also grows linearly with input size.

Which Approach is Fastest?

Using toUpperCase() with or without locale has similar performance; locale-aware calls add slight overhead but improve correctness.

Approach	Time	Space	Best For
toUpperCase()	O(n)	O(n)	Simple uppercase conversion
toUpperCase(Locale)	O(n)	O(n)	Locale-sensitive uppercase conversion