Java Traps: Big Decimal

Author: Kasper B. Graversen, 22/06/08 Keywords: Java, Java traps trail, Numbers, BigDecimal Abstract: This article documents the traps and pitfalls around using the BigDecimal class

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Java Traps: Big Decimal

// Bill Gates has 100,000,000,000 in his savings account. His bank's
// statement is printed using:
#include <stdio.h>
int main(void) {
	float y = 1e11F;
	printf("Your balance is %.2f\n", y);
	return 0;
}

// The bank's computer prints the result as:
Your balance is 99999997952.00

Printing

double d1 = 2.2;
double d2 = 2.3;
System.out.println(d1 + d2);

>> 4.5

double d1 = 2.2;
double d2 = 2.3;
System.out.println(new BigDecimal(d1));
System.out.println(new BigDecimal(d2));

>> 2.20000000000000017763568394002504646778106689453125
>> 2.29999999999999982236431605997495353221893310546875

System.out.println(new BigDecimal(d1).add(new BigDecimal(d2)));

>> 4.50000000000000000000000000000000000000000000000000

Solution

• Always use strings when instantiating BigDecimal
• Or, don't rely on toString() call doubleValue() instead.

System.out.println(new BigDecimal("" + d1));
System.out.println(new BigDecimal("" + d2));
System.out.println(new BigDecimal("" + d1).add(new BigDecimal("" + d2)));
System.out.println(new BigDecimal(d1).add(new BigDecimal(d2)).doubleValue());

>> 2.2
>> 2.3
>> 4.5
>> 4.5

Math

BigDecimal bd = new BigDecimal("2.3");
bd.add( new BigDecimal("2.3") );
System.out.println(bd);

>> 2.3

Solution

BigDecimal bd = new BigDecimal("2.3");
bd = bd.add( new BigDecimal("2.3") );
System.out.println(bd);

>> 4.6

JDK1.4.2 and 1.5 differences

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