Equals operator for zeros (BigDecimal / Double) in Java

Question

A few interesting observations w.r.t equals operator on 0 and 0.0

1. new Double(0.0).equals(0) returns false, while new Double(0.0).equals(0.0) returns true.

2. BigDecimal.ZERO.equals(BigDecimal.valueOf(0.0)) returns false, while BigDecimal.ZERO.equals(BigDecimal.valueOf(0)) returns true.

Looks like the string comparison is being done in both the cases. Could anyone throw some light on this.

Thanks.

Answer 1

BigDecimal 'equals' compares the value and the scale. If you only want to compare values (0 == 0.0) you should use compareTo:

BigDecimal.ZERO.compareTo(BigDecimal.valueOf(0.0)) == 0 //true
BigDecimal.ZERO.compareTo(BigDecimal.valueOf(0)) == 0 //true

See the javadoc.

As for the Double comparison, as explained by other answers, you are comparing a Double with an Integer in new Double(0.0).equals(0), which returns false because the objects have different types. For reference, the code for the equals method in JDK 7 is:

public boolean equals(Object obj) {
    return (obj instanceof Double)
           && (doubleToLongBits(((Double)obj).value) ==
                  doubleToLongBits(value));
}

In your case, (obj instanceof Double) is false.