Question

How can I get single bits (or an entire variable) of a double or a float?

For example if I have
float a = 0.5;

I would expect a String equal to:
"00111111000000000000000000000000"

or in hex:
"F000000"

Answer 1

For Java, a float is on 32 bits like a int, and a double is on 64 bits like a long. Finding the binary representation of these numbers, you need to:

float a = 0.5f;
int bits = Float.floatToIntBits(a);
String.format("%32s", Integer.toBinaryString(bits)).replace(" ", "0");
String.format("%8s", Integer.toHexString(bits)).replace(" ", "0");

"00111111000000000000000000000000" bit representation (32 bits)
"3f000000" hexadecimal representation (32 bits)

double a = 0.5d;
long bits = Double.doubleToLongBits(a);
String.format("%64s", Long.toBinaryString(bits)).replace(" ", "0");
String.format("%16s", Long.toHexString(bits)).replace(" ", "0");

"0011111111100000000000000000000000000000000000000000000000000000" bit representation (64 bits)
"3fe0000000000000" hexadecimal representation (64 bits)

You can check that it works by doing the reverse operation:

int bits = Integer.parseInt("00111111000000000000000000000000", 2);
Float.intBitsToFloat(bits);

0.5 (float)

long bits = Long.parseLong("0011111111100000000000000000000000000000000000000000000000000000", 2);
Double.longBitsToDouble(bits);

0.5 (double)

For the hexadecimal representation, you can use respectively Integer.parseInt("...", 16); and Long.parseLong("...", 16);