public static String reverse( String data )
  {
    String rev = new String();

    for ( int j=data.length()-1; j >= 0; j-- )
      rev += data.charAt(j);

    return rev;
  }

One New Object per Character

When the input is "Hello" six Strings are constructed (including the first, empty String) because each time the following statement executes, a new String is created:

rev += data.charAt(j);

As the statement starts to execute, rev refers to a String. The concatenation operator "+" creates a new String with one more character on the end. The new String reference is assigned to rev (the old String is now garbage).

This is fine for programs that do a moderate amount of character manipulation. But such abundant construction of objects will slow down a program that does a great deal of character manipulation. Such programs are word processors, compilers, assemblers, data base programs, and many others.