Double vs. BigDecimal?

user_399104 0
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I have to calculate some floating point variables and my colleague suggest me to use BigDecimal instead of double since it will be more precise. But I want to know what it is and how to make most out of BigDecimal?

bigdecimal floating-point double java
7 Answers

If you are dealing with calculation, there are laws on how you should calculate and what precision you should use. If you fail that you will be doing something illegal. The only real reason is that the bit representation of decimal cases are not precise. As Basil simply put, an example is the best explanation. Just to complement his example, here's what happens: 

static void theDoubleProblem1() {
    double d1 = 0.3;
    double d2 = 0.2;
    System.out.println("Double:\t 0,3 - 0,2 = " + (d1 - d2));

    float f1 = 0.3f;
    float f2 = 0.2f;
    System.out.println("Float:\t 0,3 - 0,2 = " + (f1 - f2));

    BigDecimal bd1 = new BigDecimal("0.3");
    BigDecimal bd2 = new BigDecimal("0.2");
    System.out.println("BigDec:\t 0,3 - 0,2 = " + (bd1.subtract(bd2)));
}

Output: 

Double:  0,3 - 0,2 = 0.09999999999999998
Float:   0,3 - 0,2 = 0.10000001
BigDec:  0,3 - 0,2 = 0.1

Also we have that: 

static void theDoubleProblem2() {
    double d1 = 10;
    double d2 = 3;
    System.out.println("Double:\t 10 / 3 = " + (d1 / d2));

    float f1 = 10f;
    float f2 = 3f;
    System.out.println("Float:\t 10 / 3 = " + (f1 / f2));

    // Exception! 
    BigDecimal bd3 = new BigDecimal("10");
    BigDecimal bd4 = new BigDecimal("3");
    System.out.println("BigDec:\t 10 / 3 = " + (bd3.divide(bd4)));
}

Gives us the output: 

Double:  10 / 3 = 3.3333333333333335
Float:   10 / 3 = 3.3333333
Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion

But: 

static void theDoubleProblem2() {
    BigDecimal bd3 = new BigDecimal("10");
    BigDecimal bd4 = new BigDecimal("3");
    System.out.println("BigDec:\t 10 / 3 = " + (bd3.divide(bd4, 4, BigDecimal.ROUND_HALF_UP)));
}

Has the output: 

BigDec:  10 / 3 = 3.3333
user_18509300

If you need to use division in your arithmetic, you need to use double instead of BigDecimal. Division (divide(BigDecimal) method) in BigDecimal is pretty useless as BigDecimal can't handle repeating decimal rational numbers (division where divisors are and will throw java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

Just try BigDecimal.ONE.divide(new BigDecimal("3"));

Double, on the other hand, will handle division fine (with the understood precision which is roughly 15 significant digits)

user_195059070
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