How can I look for patterns in the first 3 bits of an int which is assigned from a FileInputStream.read() which returns an int between 0:255 or -1?

Question

I am reading bytes as follows:

        try(FileInputStream f = new FileInputStream("nameOfFile")){
            int readByte = 0;
            int mask =0b111;
            while((readByte=f.read())!=-1) {
                System.out.println(Integer.toBinaryString(readByte & mask));
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

However, when i try to obtain the 8 bits that forms the number, sometimes I get 7 bits or even more bits than 8. Can anyone help me with this problem? I want to get 8 bits so after that i can look at first 3 bits for example. I need that for returning the charset in this method.

I'll be very gratefull if you help me. I am looking forward to hearing from you soon.

Best regards, WaterKnight

I'm afraid it's pretty unclear what you are trying to achieve. You can help us out and thereby make it much more likely that you will receive a useful answer by editing your question to include a [mcve]. — John Bollinger, Oct 06 '18 at 15:25
What do you mean by "obtain the 8 bits"? Obtain where? You have the 8 bits right there in the `int` (along with 24 more bits). If you want to "look at first 3 bits", go on and look: `readByte & 0x07`. Or if you want to "look": [`Integer.toBinaryString(readByte)`](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#toBinaryString-int-) — Andreas, Oct 06 '18 at 15:26
@JohnBollinger I updated my code , just Would like to look at 3first bits of the int and know if it follows some kind of pattern. — WaterKnight, Oct 06 '18 at 15:31
`110` is a *decimal* number, equals to the *binary* number `0b1101110`, so `(byteLeido[0] >>> 5)==110` will never ever be true. Perhaps you meant `(byteLeido[0] >>> 5) == 0b110` or `(byteLeido[0] >>> 5) == 0x06` or `(byteLeido[0] >>> 5) == 6`? — Andreas, Oct 06 '18 at 15:31
@Andreas thats why I am asking to try to look at pattern at 3 or 2 first bits of the int that goes between 0 and 255. — WaterKnight, Oct 06 '18 at 15:33
Duplicate of [How to get 0-padded binary representation of an integer in java?](https://stackoverflow.com/q/4421400/5221149) — Andreas, Oct 06 '18 at 15:51

score 1 · Accepted Answer · answered Oct 06 '18 at 15:46

1

A byte is an 8-bit two's-complement signed number.

If you think a byte has values 0..255, then you are wrong. The 8 bits are -128..127, because bit 7 is the sign bit.

When you do bit-manipulation using operators like >>>, the byte is first widened to an int, which means that sign-extension is applied.

So, unsigned value 199 (example), is bits 11000111, which as a byte is value -57 aka 0xC7, and when sign-extended to an int becomes -57 aka 0xFFFFFFC7.

When you right-shift that by 5 (>>> 5), you get 0x07FFFFFE aka 134217726.

Since you're only interested in the lowest 3 bits of that value, you need to mask it: (val >>> 5) & 0x07, which will give you what you're looking for: 0x06 aka 0b110

answered Oct 06 '18 at 15:46

Andreas

154,647
11
152
247

I read that .read() of an input stream returned and int with values 0..255, so as this decimals values are a byte i would like to look into the 3 highest bits from the byte. In adittion, I tried your solution, System.out.println(Integer.toBinaryString((readByte >>> 5) & 0x07)); but in console I get lines with 3 bits, other ones with 2 bits and others with 1bit. – WaterKnight Oct 06 '18 at 15:54
@WaterKnight Did you not read my comment to you in the other answer? Or the link I provided in comment to your question? For your convenience, here both are: "You are getting 3 bits from all of the bytes read. Those are all 3-bit values, but, like any other formatted number, **leading zeroes have been removed**, so e.g. `11` means `011`, and `0` means `000`" and [How to get 0-padded binary representation of an integer in java?](https://stackoverflow.com/q/4421400/5221149) – Andreas Oct 06 '18 at 15:58
@Andres I just read your comment on the other answer and got it working as follows Integer.toBinaryString(byteLeido>>>5).equals("110"). Thank you very much. – WaterKnight Oct 06 '18 at 16:03

Karol Dowbecki · Answer 2 · 2018-10-06T15:34:45.467

0

[0, 255] range implies that you use all the 8 bits for value, without leaving the most significant bit for the sign.

FileInputStream.read() reads a byte of data. The int return type allows to spot the difference between -1 data and -1 used to signal end of the stream. In practice read() return value won't be greater than 255.

If you want to look at the last 3 bits use a bit mask. You can use binary literal to define your mask:

int mask = 0b111;
int lastBits = readBytes & mask;

edited Oct 06 '18 at 15:34

answered Oct 06 '18 at 15:21

Karol Dowbecki

43,645
9
78
111

when I make this i get the next input throught console: 1001100. I am using System.out.println(Integer.toBinaryString(b)); for getting a look into the bits. – WaterKnight Oct 06 '18 at 15:26
@TheScientificMethod I missed the point of the question, but you are right! – Karol Dowbecki Oct 06 '18 at 15:35
when I make the things from you updated answer all I get is: 100 11 1 0 1 10 0 1 10 10 100 ANSI. So i am not getting 3 bits from all of the bytes readed. – WaterKnight Oct 06 '18 at 15:40
@WaterKnight Sure you are getting 3 bits from all of the bytes read. Those are all 3-bit values, but, like any other formatted number, leading zeroes have been removed, so e.g. `11` means `011`, and `0` means `000`. – Andreas Oct 06 '18 at 15:49

How can I look for patterns in the first 3 bits of an int which is assigned from a FileInputStream.read() which returns an int between 0:255 or -1?

2 Answers2