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Tip of the Day
Language: Java Language
Expertise: Beginner
Feb 26, 1998

byte array to int conversion

Question:
I have just read a packet of data into a byte array from a socket. At offsets 5-8, 9-12, and 13-16 are binary integers. How can I get them into Java int variables?

Answer:
The recommended way of doing this is to use java.io.ObjectInputStream and its readInt() method. However, there are cases where you will want to deal with bytes directly, such as when you are converting little-endian integers to big-endian integers. The easiest way to address how to do this is simply to give an example. The following program takes four bytes in big-endian order and turns them into a Java integer:

public final class bytesToInt {

  public static final void main(String[] args) {
    byte[] bytes;
    int result, bitmask;

    bytes = new byte[4];
    bytes[0] = 1; // 00000001
    bytes[1] = 2; // 00000010
    bytes[2] = 3; // 00000011
    bytes[3] = 4; // 00000100

    result = 0;
    result |= ((0xff & bytes[0]) << 24);
    result |= ((0xff & bytes[1]) << 16);
    result |= ((0xff & bytes[2]) << 8);
    result |= (0xff & bytes[3]);

    System.out.println("int : " + result);
    System.out.print("bits: ");

    // Print the individual bits to show they were set correctly
    bitmask = 0x80000000;
    for(int i=0; i < 32; i++) {
      if((result & bitmask) == 0)
	System.out.print('0');
      else
	System.out.print('1');
      bitmask >>>= 1;
    }
    System.out.println();
  }

}
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