CS151 lecture notes, Fall 1999
Week 3, Wednesday

For Friday please read 4.1 through 4.7

Void methods continued
----------------------

Methods that take arguments:

  public static void printTwice (String phil) {
    System.out.println (phil);
    System.out.println (phil);
  }

   public static void main (String[] args) {
    printTwice ("Don't make me say this twice!");
  }

The expression you provide when you invoke the method is
called the argument.

The name the method uses to refer to the incoming information
is called the parameter.  Analogy: when you take a VCR in to
get repaired, the repair person puts a tag on it with a number.
You as the client don't know or care what the number is, and
the repair person doesn't care what you call your VCR.

Nevertheless, the two are so closely-related that it is easy to
confuse them, and they are sometimes used interchangably.

The important differences are:

1) An argument can be any kind of expression, as long as it
   has the right type.

2) The parameter is just a name that is used by the method.  The
   usual rules apply regarding variable names.

3) When you provide an argument, you don't have to specify the
   type.  Java can figure it out because Java can always figure
   out the type of an expression.

4) When you name a parameter, you have to specify the type.
   Parameter lists look something like variable declarations.


VERY COMMON ERROR:

     String s = "bamboo";
     printTwice (String bamboo);        // WRONG


More than one parameter
-----------------------

So far, all the methods we have looked at have taken either
zero or one parameters, so we have not had a chance to see
a parameter list.

  public static void printTime (int hour, int minute, int second) {
    System.out.println (hour + ":" + minute + ":" + second);
  }

The list of parameters is similar to a variable declaration,
in the sense that the types come before the names.

One difference is that you can create multiple variables with
the same type:

    int x, y, z;

But you have to give the types of all the parameters separately.

    int hour, int minute, int second

Of course, not all the parameters have to have the same type:

  public static void printDate (String day, int date, String month) {

  }


Matching arguments and parameters
---------------------------------

When you invoke a method, you have to provide arguments that
match the method's parameters in type and number (and order).

If you get it wrong, the error message can be confusing.


Modulus operator
----------------

Operator that works on integers, denoted by %

Yields the remainder when the operands are divided.

7%3 = 1

Checking for divisibility... if x%y = 0, then
x is divisible by y.

Extracting digits:  x%10 yields the rightmost digit of
x if x were written as a base 10 number.

x%100 yields the two rightmost digits.