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Ruslan López
Asked: 2020-04-28 10:51:58 +0800 CST 2020-04-28 10:51:58 +0800 CST

Why doesn't -0.0 equal 0.0 to me?

  • 772

I have the following unit test and discovered something very interesting

public class MyTest {
    @Test
    public void BasicTest() {
     assertEquals("0.0!=-0.0",new Double(0.0D),new Double(-3.0D*0.0D));
    }
}

It gives me the error message:

0.0!=0.0 expected:<0.0> but was:<-0.0>

Why is 0.0 not equal to -0.0 in Java? What ways are there to get around this error?

java

1 Answers

  1. Best Answer

    Why is 0.0 not equal to -0.0 in Java?

    The answer lies in the bit representation of the number. Use this method:

     Long.toBinaryString(Double.doubleToRawLongBits(x));

    Where xis a type variable doubleto see its representation in bits. For the numbers 0.0 and -0.0 respectively, this is your result:

    double x = 0.0d;
double y = -0.0d;
System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(x)) );
System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(y)) );

    Departure:

    0
1000000000000000000000000000000000000000000000000000000000000000

    As you can see, the numbers 0.0 and -0.0 are different . And this makes sense since Java uses the IEEE floating point format for the representation of double, where the first bit is for the sign.

    What ways are there to get around this error?

    Add 0.0 to the negative and the problem is solved:

    double x2 = 0.0;
double y2 = -0.0;
System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(x2)) );
System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(y2 + 0.0)) );

    Departure:

    0
0
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