ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 502

With Number defined as the superclass for the numeric classes, we can define methods to
perform common operations for numbers. Listing 13.5 gives a program that finds the largest
number in a list of Number objects.

L ISTING 13.5

LargestNumbers.java
1 import java.util.ArrayList;
2 import java.math.*;
3
4 public class LargestNumbers {
5 public static void main(String[] args) {
6 ArrayList<Number> list = new ArrayList<>();
7 list.add( 45 ); // Add an integer
8 list.add( 3445 . 53 ); // Add a double
9 // Add a BigInteger
10 list.add( new BigInteger( "3432323234344343101" ));
11 // Add a BigDecimal
12 list.add( new BigDecimal( "2.0909090989091343433344343" ));
13
14 System.out.println( "The largest number is " +
15 getLargestNumber(list));
16 }
17
18

create an array list
add number to list

invoke getLargestNumber

public static Number getLargestNumber(ArrayList<Number> list) {

19 if (list == null || list.size() == 0 )

20 return null ;

21
22 Number number = list.get( 0 );
23 for ( int i = 1 ; i < list.size(); i++)
24 if (number.doubleValue() < list.get(i).doubleValue())
25 number = list.get(i);
26
27

doubleValue

return number;

28 }
29 }

The largest number is 3432323234344343101

The program creates an ArrayList of Number objects (line 6). It adds an Integer object,
a Double object, a BigInteger object, and a BigDecimal object to the list (lines 7-12).
Note that 45 is automatically converted into an Integer object and added to the list in line
7 and that 3445.53 is automatically converted into a Double object and added to the list in
line 8 using autoboxing.
Invoking the getLargestNumber method returns the largest number in the list (line 15).
The getLargestNumber method returns null if the list is null or the list size is 0 (lines
19-20). To find the largest number in the list, the numbers are compared by invoking their
doubleValue() method (line 24). The doubleValue() method is defined in the Number
class and implemented in the concrete subclass of Number . If a number is an Integer
object, the Integer 's doubleValue() is invoked. If a number is a BigDecimal object, the
BigDecimal 's doubleValue() is invoked.
If the doubleValue() method were not defined in the Number class, you will not be able
to find the largest number among different types of numbers using the Number class.

13.4

✓ ✓

Why do the following two lines of code compile but cause a runtime error?

Check

Point

Number numberRef = new Integer( 0 );
Double doubleRef = (Double)numberRef;