Question

Here is a main method that reverses a String. How can programmatically test that the reversed string is correct ?

public static void main(String args[]){

    String reverseStatement = "reverse this statement";
    String reversed = "";

    for(int count = reverseStatement.length() - 1; count >= 0; --count){
        reversed += String.valueOf(reverseStatement.charAt(count));
    }

    System.out.println("Reversed is - "+reversed);
}

Answer 1

Move your reverse code into a separate method, like so:

private static String reverse(String string) {
    return new StringBuilder(string).reverse().toString()
}

Be aware that adding a lot of Strings directly (like in your original code) can be very slow. I suggest you google for java.lang.StringBuilder for further examples.

After that, you can easily test and verify that your method does the correct thing.

public static void main(String args[]) {
    String statement = "reverse this statement";
    String reversed = reverse(statement);

    if (!reversed.equals("tnemetats siht esrever")) {
        throw new RuntimeException("the string was not correctly reversed");
    }

    // reversing twice should yield the original string
    if (!statement.equals(reverse(reverse(statement)))) {
        throw new RuntimeException("reversing a string twice should yield the original, but it doesn't");
    }
}