String.charAt(int index)
...evaluates to the character
that is at location index
in the String.
The beginning character in a String is at location 0.
This program reverses the characters in a String. It does this by copying characters one at a time from right to left from the input String to the end of a reversed String.
public class ReverseTester
{
  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;
  }
  public static void main ( String[] args )
  {
    System.out.println( reverse( "Hello" ) );
  }
}
Here is a sample run of this program:
C:\>java ReverseTester olleH