How to find most significant bit (MSB)

I want to know which value the first bit of a byte has.

For example:

I have byte m = (byte) 0x8C;

How could I know if the first bit is an 1 or a 0 ?

Can anyone 开发者_如何学JAVAhelp me out ?

It depends what you mean by "first bit". If you mean "most significant bit" you can use:

// 0 or 1
int msb = (m & 0xff) >> 7;

Or if you don't mind the values being 0x80 or 0, just use:

// 0 or 0x80
int msb = m & 0x80;

Or in fact, as a boolean:

// Uses the fact that byte is signed using 2s complement
// True or false
boolean msb = m < 0;

If you mean the least significant bit, you can just use:

// 0 or 1
int lsb = m & 1;

If the first bit is the lowest bit (ie bit 0), then

if((m & 1) >0) ...

should do it.

In general,

if ((m & (1<<N)) > 0) ...

will give you whether or not bit N is set. If, however, you meant the highest bit (bit 7), then use N=7.

Assuming you mean leftmost bit, bitwise and it with 0x80 and check if it is zero nor not:

public boolean isFirstBitSet(byte b) {
    System.out.println((b & (byte)0x80));
    return (b & (byte)0x80) < 0;
}

If you mean lowest order bit you will need to and with 0x01 and check a different condition:

public boolean isFirstBitSet(byte b) {
    System.out.println((b & (byte)0x01));
    return (b & (byte)0x80) > 0;
}

Use the bitwise and operator.

public class BitExample {
    public static void main(String args[]) throws Exception {
        byte m = (byte)0x8C;
        System.out.println("The first bit is " + (m & (byte)0x01));
        m = (byte)0xFF;
        System.out.println("The first bit is " + (m & (byte)0x01));
    }
}

// output is...
The first bit is 0
The first bit is 1

Its a bit of a hack but you can use

if(x >> -1 != 0) // top bit set.

This works for byte, short, int, long data types.

However for most types the simplest approach is to compare with 0

if (x < 0) // top bit set.

This works for byte, short, int, long, float, or double

(Ignoring negative zero and negative NaN, most people do ;)

For char type you need to know the number of bits. ;)

if (ch >>> 15 != 0) // top bit set.

This code gives you msb(most significant bit)/biggest number which is a power of 2 and less than any given number.

class msb{
void main(){
        int n=155;//let 110001010
        int l=(Integer.toBinaryString(n)).length();//9
        String w="0"+Integer.toBinaryString(i).substring(1);//010001010
        i=Integer.parseInt(w,2);
        System.out.println(n^i);//110001010^010001010=100000000
}
}