How does Float.intBitsToFloat work?

Question

Can anyone explain me or link some helpful resource in order to understand the algorithm behind the Java method Float.intBitsToFloat(int)?

Answer 1

Java uses IEEE 754 floating point. Float.intBitsToFloat(int) works by interpreting the 32 bits of its argument as if they specified a 32-bit float in the format described here.

Double.longBitsToDouble(long) works similarly for 64-bit floats as described here.

In C you might achieve the same effect like so:

#include <stdint.h>

union int_float_bits {
    int32_t int_bits;
    float float_bits;
};

float intBitsToFloat(int32_t x)
{
    union int_float_bits bits;
    bits.int_bits = x;
    return bits.float_bits;
}

(Although technically this would be undefined behavior, in fact it virtually always works as expected.)

Answer 2

The JDK6 docs are quite good, and the source itself is pretty enlightening (it just uses a C union):

JNIEXPORT jfloat JNICALL
Java_java_lang_Float_intBitsToFloat(JNIEnv *env, jclass unused, jint v)
{
    union {
        int i;
        float f;
    } u;
    u.i = (long)v;
    return (jfloat)u.f;
}