Convert bitstring to byte in java

Question

I need to set each bit in 1 byte in Java.

bit7 -  1,
bit6 -  1,
bit5 - 1,
bit4 -  0,
bit3 – 0,
bit2 – 0,
bit1 – 0,
bit0 – 0

I've written:

byte extra_dop = 0b00000111;

but got the following error:

binary literals are not supported in -source 1.5 (use -source 7 or higher to enable binary literals)

Solution 1

Binary literal were introduced in Java7.

Use following for older version:

byte b = Byte.parseByte("00000111", 2);

Solution 2

As the error message says, the 0b... syntax did not exist yet in Java 5 (which is what you seem to be using); it was introduced with Java 7. If you are using Java 7, make sure your compiler settings (in your IDE or build file) are set so that it accepts Java 7 syntax.

Bits are normally counted from the right to the left, so if you say bit 7 is 1, bit 6 is 1, etc. then I would expect the binary number to be 11100000 instead of 00000111.

To write this in source code in a Java version older than Java 7, you could simply write it as a hexadecimal or decimal number:

// Hexadecimal
byte extra_dop = (byte)0xE0; // or did you mean 0x07?

// Decimal
byte extra_dop = (byte)224; // or did you mean 7?

You could also use Integer.parseInt() with radix 2:

byte extra_dop = (byte)Integer.parseInt("11100000", 2);

(Note, you could also use Byte.parseByte but it will not accept 11100000 since it exceeds the range of the signed byte type).

Comments

[Jon Skeet]

Well what version of Java are you using, and is using Java 7 an option?

[Jon Skeet]

"That's not the way we do things in Java." That's far too general a statement. While BitSet is an option, using a single byte value is a perfectly legitimate option too.

[Sean Patrick Floyd]

I agree that it's technically valid, but "Effective Java Item 47: Know and use the libraries" suggests that I should use existing solutions instead of reinventing them.

[Sean Patrick Floyd]

since he wants primitive bytes, not Byte Objects, he should use parseByte instead of valueOf.

[Jon Skeet]

It depends on the context. Sometimes it's appropriate to use BitSet, other times it isn't. Knowing the libraries exist allows you to make that decision. Suppose we're storing a million entries like this - creating a million BitSet objects may be prohibitively expensive compared with just creating a million-byte array. The OP apparently only wants to store 8 flags, so the "it can support more flags" isn't a benefit in this case. I think it's reasonable to suggest looking into it - but I'd have done that as a comment rather than an answer.

[Amit G]

With autoboxing never noticed difference. Thanks for pointing, updated answer.

[Robert]

Byte.parseByte("11111111", 2); -> java.lang.NumberFormatException: Value out of range