Objects and classes - Data Structures and Problem Solving Using Java - page 90

figure 3.13
BigRational add ,
subtract , multiply ,
divide

1 public BigRational add( BigRational other )
2 {
3 BigInteger newNumerator =
4 num.multiply( other.den ).add(
5 other.num.multiply( den ) );
6 BigInteger newDenominator = den.multiply( other.den );
7
8 return new BigRational( newNumerator, newDenominator );
9 }
10
11 public BigRational subtract( BigRational other )
12 {
13 return add( other.negate( ) );
14 }
15
16 public BigRational multiply( BigRational other )
17 {
18 BigInteger newNumer = num.multiply( other.num );
19 BigInteger newDenom = den.multiply( other.den );
20
21 return new BigRational( newNumer, newDenom );
22 }
23
24 public BigRational divide( BigRational other )
25 {
26 BigInteger newNumer = num.multiply( other.den );
27 BigInteger newDenom = den.multiply( other.num );
28
29 return new BigRational( newNumer, newDenom );
30 }

Observe that the BigRational class has no mutators: routines such as add
simply return a new BigRational that represents a sum. Thus BigRational is an
immutable type.

packages

3.8

Packages are used to organize similar classes. Each package consists of a set of
classes. Two classes in the same package have slightly fewer visibility restric-
tions among themselves than they would if they were in different packages.
Java provides several predefined packages, including java.io , java.lang ,
and java.util . The java.lang package includes the classes Integer , Math ,
String , and System , among others. Some of the classes in the java.util package

A package is used
to organize a col-
lection of classes.