Algorithms

1.2 ■ Data Abstraction
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O b je c t s a s a r g u m e n t s . You can pass objects as arguments to methods. This ability typically
simplifies client code. For example, when we use a Counter as an argument, we are
essentially passing both a name and a tally, but need only specify one variable. When
we call a method with arguments, the effect in Java is as if each argument value were
to appear on the right-hand side of an assignment statement with the corresponding
argument name on the left. That is, Java passes a copy of the argument value from the
calling program to the method. This arrangement is known as pass by value (see page
24). One important consequence is that the method cannot change the value of a caller’s
variable. For primitive types, this policy is what we expect (the two variables are independent),
but each time that we use a reference type as a method argument we create
an alias, so we must be cautious. In other words, the convention is to pass the reference
by value (make a copy of it) but to pass the object by reference. For example, if we pass
a reference to an object of type Counter, the method cannot change the original reference
(make it point to a different Counter), but it can change the value of the object,
for example by using the reference to call increment().
O b j e c t s a s re t u r n va l u e s . Naturally, you can also use an object as a return value from
a method. The method might return an object passed to it as an argument, as in the
example below, or it might create an object and return a reference to it. This capability
is important because
Java methods allow only one
return value—using objects
enables us to write code that,
in effect, returns multiple
values.
public class FlipsMax
{
public static Counter max(Counter x, Counter y)
{
if (x.tally() > y.tally()) return x;
else return y;
}
% java FlipsMax 1000000
500281 tails wins
}
public static void main(String[] args)
{
int T = Integer.parseInt(args[0]);
Counter heads = new Counter("heads");
Counter tails = new Counter("tails");
for (int t = 0; t < T; t++)
if (StdRandom.bernoulli(0.5))
heads.increment();
else tails.increment();
}
if (heads.tally() == tails.tally())
StdOut.println("Tie");
else StdOut.println(max(heads, tails) + " wins");
Example of a static method with object arguments and return values