Reverse a String in Java

Reversing a string

Reversing a string means writing its characters back to front — "Java" becomes "avaJ". Because Java strings are immutable, you always build a new reversed string. We will do it with a loop (to learn the logic) and with StringBuilder (the easy way).

Reversing with a loop

public class Reverse {
    public static void main(String[] args) {
        String str = "Java";
        String rev = "";
        for (int i = str.length() - 1; i >= 0; i--) {
            rev += str.charAt(i);    // add chars from the end
        }
        System.out.println(rev);
    }
}

We start at the last index (length() - 1) and move down to 0, reading each character with charAt(i) and adding it to rev. Reading back-to-front gives the reverse.

Dry run

Using StringBuilder

String str = "Java";
String rev = new StringBuilder(str).reverse().toString();
System.out.println(rev);

StringBuilder has a built-in reverse() method. This one line is the simplest and most efficient way in real code — no loop needed.

Which method to use

• StringBuilder.reverse() — best for real code: short and fast.
• Loop with charAt — best for learning how reversal works.

Both produce the same result. Use the loop to understand, and StringBuilder in practice.

Common mistakes

• Starting the loop at length(). The last index is length() - 1.
• Trying to change the string in place. Strings are immutable — build a new one.
• Looping upward. To reverse, go from the last index down to 0.
• Slow += for very long strings. Use StringBuilder for efficiency.

Summary

• Reversing writes a string's characters back to front.
• Strings are immutable, so you build a new reversed string.
• A loop reads from length()-1 down to 0 with charAt.
• new StringBuilder(str).reverse().toString() is the easy way.
• Use the loop to learn, StringBuilder in real code.

Next, use this in a palindrome check, or learn text tools in String vs StringBuilder.