Why use Float.floatToIntBits() in Java float comparisons?

Question

In JBox2d, there exists the following code for `Vec2.equals()`: ``` @Override public boolean equals(Object obj) { //automatically generated by Eclipse if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Vec2 other = (Vec2) obj; if (Float.floatToIntBits(x) != Float.floatToIntBits(other.x)) return false; if (Float.floatToIntBits(y) != Float.floatToIntBits(other.y)) return false; return true; } ``` I am wondering what purpose the float<->int bit conversions functions serve, here. Does this provide a way to get around Java's float comparison inaccuracy problem (if such is even possible)? Or is it something else altogether? I am wondering if it is an alternative to the epsilon approach: ``` if (Math.abs(floatVal1 - floatVal2) < epsilon) ``` PS. for the sake of completeness and interest, here is `Vec2.hashCode()`: ``` @Override public int hashCode() { //automatically generated by Eclipse final int prime = 31; int result = 1; result =…

Accepted Answer

The explanation can be found in Joshua Bloch's Effective Java: `float` and `Float` need special treatment because of the existence of `-0.0`, `NaN`, positive infinity, and negative infinity. That's why the Sun JVM's `Float.equals()` looks like this (6u21):

```
public boolean equals(Object obj)
{
    return (obj instanceof Float)
           && (floatToIntBits(((Float)obj).value) == floatToIntBits(value));
}
```

So, no, `Math.abs()` with an epsilon is not a good alternative. From the Javadoc:

If f1 and f2 both represent Float.NaN, then the equals method returns true, even though Float.NaN==Float.NaN has the value false. If f1 represents +0.0f while f2 represents -0.0f, or vice versa, the equal test has the value false, even though 0.0f==-0.0f has the value true.

That's why Eclipse's autogenerated code does that for you.

Why use Float.floatToIntBits() in Java float comparisons?

Solution

Problem