convert byte to integer in java using & and |

Question

I need to do a conversion from 1 byte to integer using & and |. The output after conversion must be an integer with these bytes: (0xFF)(byte)(byte)(byte).

For example:
Input:

00000111  

Output:

11111111000001110000011100000111

My code works if the byte is in range [0, 127]

int integer=7;
byte a=(byte)integer;
int b=(0xFF<<24)|(a<<16)|(a<<8)|(a);
System.out.println(Integer.toBinaryString(b));

If the byte is in range [-128, -1], it doesn't work

input:

10000000

Wrong output:

11111111111111111111111110000000

Expected output:

11111111100000001000000010000000

Answer 1

It is happening because when you are shifting a, you are left with 1s on the front.

For example:

a<<16 gives you 11111111100000000000000000000000 but you want 100000000000000000000000

a<<8 gives you 11111111111111111000000000000000 but you want 1000000000000000

So we need to mask those additional 1s that are added on the front by doing & 0xFF

So replace this

int b=(0xFF<<24)|(a<<16)|(a<<8)|(a);

with this

int b = (0xFF<<24)|((a & 0xFF)<<16)|((a & 0xFF)<<8)|(a & 0xFF);

To know why we see 1s on the front, here is a great explanation: Odd behavior when Java converts int to byte?