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1.5 Echo in Reverse
Example 1-5 is a lot
like the Echo program of Example 1-4, except that it prints out the command-line
arguments in reverse order, and it prints out the characters of each
argument backwards. Thus, the Reverse program can
be invoked as follows, with the following output:
% java je3.basics.Reverse this is a test
tset a si siht
This
program is interesting because its nested for
loops count backward instead of forward. It is also interesting
because it manipulates String objects by invoking
methods of those objects and the syntax starts to get a little
complicated. For example, consider the expression at the heart of
this example:
args[i].charAt(j)
This expression first extracts the ith element of
the args[ ] array. We know from the declaration of
the array in the signature of the main( ) method
that it is a String array; that is, it contains
String objects. (Strings are not a primitive type,
like integers and boolean values in Java: they are full-fledged
objects.) Once you extract the ith
String from the array, you invoke the
charAt( ) method of that object, passing the
argument j. (The . character in
the expression refers to a method or a field of an object.) As you
can surmise from the name (and verify, if you want, in a reference
manual), this method extracts the specified character from the
String object. Thus, this expression extracts the
jth character from the ith
command-line argument. Armed with this understanding, you should be
able to make sense of the rest of Example 1-5.
Example 1-5. Reverse.java
package je3.basics;
/**
* This program echos the command-line arguments backwards.
**/
public class Reverse {
public static void main(String[ ] args) {
// Loop backwards through the array of arguments
for(int i = args.length-1; i >= 0; i--) {
// Loop backwards through the characters in each argument
for(int j=args[i].length( )-1; j>=0; j--) {
// Print out character j of argument i.
System.out.print(args[i].charAt(j));
}
System.out.print(" "); // Add a space at the end of each argument.
}
System.out.println( ); // And terminate the line when we're done.
}
}
