Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 161

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Puzzle 56: Big Problem
As a warm-up, test your knowledge of BigInteger . What does this program print?
import java.math.BigInteger;
public class BigProblem {
public static void main(String[] args) {
BigInteger fiveThousand = new BigInteger("5000");
BigInteger fiftyThousand = new BigInteger("50000");
BigInteger fiveHundredThousand
= new BigInteger("500000");
BigInteger total = BigInteger.ZERO;
total.add(fiveThousand);
total.add(fiftyThousand);
total.add(fiveHundredThousand);
System.out.println(total);
}
}
Solution 56: Big Problem
You might think that this program prints 555000 . After all, it sets total to the BigInteger
representation for 0 and then adds 5,000, 50,000, and 500,000. If you ran the program, you found
that it doesn't print 555000 but 0 . Apparently all that addition has no effect on total .
There is a good reason for this: BigInteger instances are immutable. So are instances of String ,
BigDecimal , and the wrapper types: Integer , Long , Short , Byte , Character , Boolean , Float , and
Double . You can't change their values. Instead of modifying existing instances, operations on these
types return new instances. At first, immutable types might seem unnatural, but they have many
advantages over their mutable counterparts. Immutable types are easier to design, implement, and