Example: String reverse

The following program reverses a string in a very straightforward, but rather inefficient way. When you learn about StringBuilder (or the equivalent StringBuffer), you can do this more efficiently. But the purpose of this is to see how looping over a string works.

Nested loops. There are two nested loops in the is program, the outer while loop reads more input. The inner for loop gets every character from the input string starting at character 0.

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// File   : loops/reverse/Reverse.java
// Purpose: Reverse a string using a loop.
// Author : Fred Swartz
// Date   : Oct 23 2005
// Comments: Building a String one character at a time
//           is very inefficient because it requires
//           creating a new object for each concatenation.
//           StringBuilder is better, and it already
//           has a reverse() method!

import javax.swing.*;

public class Reverse {
    public static void main(String[] args) {

        String input;    // Used for the input string.
        String reversed; // Reversed form or the input string.

        while (true) {
            input = JOptionPane.showInputDialog(null, "Enter a string");
            if (input == null) break;

            reversed = "";
            for (int i=0; i<input.length(); i++) {
                reversed = input.substring(i, i+1) + reversed;
            }

            JOptionPane.showMessageDialog(null, "Reversed:\n" + reversed);
        }
    }
}

While loop vs For loop

Counting. A for loop is preferred to a while loop when counting through a series of numbers -- in this case all character positions in a string.

Equivalent. A for loop has the same condition as the equivalent while loop, but also incorporates an initialization, which would be before the while statement, and the increment, which would be at the end of the while body. You can write the loop either way, but putting the initialization, condition, and increment in one statement increases the readability.

for (int i=0; i<input.length(); i++) {
    reversed = input.substring(i, i+1) + reversed;
}

int i = 0;
while (i<input.length()) {
    reversed = input.substring(i, i+1) + reversed;
    i++;
}

A single character - String or char?

This program uses substring(...) to get a single character. It would be more efficient to use charAt(...), which returns a single primitive char value.

for (int i=0; i<input.length(); i++) {
    reversed = input.charAt(i) + reversed;
}