The Weird BigInteger.mod Modulus Method

Question

In Java, we know if we want to compare two reference objects, we usually need to use equals, but here I am so confused about the output of the following:

System.out.println(new BigInteger("0") == BigInteger.ZERO);                     // false
System.out.println(new BigInteger("0").mod(BigInteger.ONE) == BigInteger.ZERO); // true

Why is the second statement true?

Answer 1

Took a while, but following the logical paths of the execution takes us to:

MutableBigInteger#toBigInteger(int sign)

Which has the following statement:

BigInteger toBigInteger(int sign) {
    if (intLen == 0 || sign == 0)
        return BigInteger.ZERO;
    return new BigInteger(getMagnitudeArray(), sign);
}

So in this case, the constant BigInteger.ZERO is returned, so the statement is true.

Stack trace:

BigInteger#mod(BigInteger)
BigInteger#remainder(BigInteger)
BigInteger#remainderKnuth(BigInteger)
MutableBigInteger#toBigInteger(int)