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How to convert a byte[] to an integer

Posted on 2002-03-28
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Last Modified: 2012-05-04
I have a byte[] with four bytes!
for example:
byte [] k={23,39,23,40}
how to convert k[] to an integer?
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Question by:liluqun
  • 3
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6 Comments
 
LVL 16

Expert Comment

by:Peter Kwan
ID: 6904262
For LittleEndian:

    byte[] k={23,39,23,40};
     int y=0;

     y=k[0];
     for (int i=1; i<4; i++) {
        y = y << 8;
        y += k[i];
     }

For BigEndian:
    byte[] k={23,39,23,40};
     int y=0;

     y=k[3];
     for (int i=2; i>=0; i--) {
        y = y << 8;
        y += k[i];
     }

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Expert Comment

by:Ovi
ID: 6904583
Or, not worying about manual conversion, like

byte[] byteArray = {1, 2, 3, 4, 4, 5, 6};
int value = -1; // -1 considered as invalid value for this operation.
try {
  value = Integer.parseInt(new String(byteArray));
} catch(Exception e) {
  value = -1;
}
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Expert Comment

by:Peter Kwan
ID: 6904597
Ovi, your method does not work.
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LVL 9

Expert Comment

by:Ovi
ID: 6904607
Sorry about that, I've tested myself after I posted. Please ignore the comment.
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Author Comment

by:liluqun
ID: 6904635
Now try this code:
===========================================
import java.io.*;

public class Test {

  public Test() {
  }
  public static void main(String[] args) {
    Test test1 = new Test();
    try {
      RandomAccessFile rf=new RandomAccessFile("d:\\ok.txt","rw");
      rf.writeInt(777678);        
      rf.close();
    }
    catch (Exception ex) {
       System.out.print(ex);
    }

  }
}
===============================================
After run the above code , we can get a 4-bytes file!
Now we use bytes arrarys to restore the integer777678,
I know use rf.readInt() can restore the integer 777687 at easy,this time ,suppose I can get a byte arrary from the 4-bytes file ok.txt
Now retore it with the following code:
===============================================
import java.io.*;

public class Test {

  public Test() {
  }
  public static void main(String[] args) {
    Test test1 = new Test();
    try {
      RandomAccessFile rf=new RandomAccessFile("d:\\ok.txt","rw");
     // rf.writeByte();
     //rf.writeInt(777678);
      byte [] w=new byte[4];
      rf.read(w);
    int y=0;

    y=w[0];
    for (int i=1; i<4; i++) {
       y = y << 8;
       y += w[i];
    }

System.out.print(y);
      rf.close();
    }
    catch (Exception ex) {
       System.out.print(ex);
    }

  }
}
============================
we can only get: an integer:711886
       not 777678!!
Why
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LVL 16

Accepted Solution

by:
Peter Kwan earned 20 total points
ID: 6904659
This is because "byte" is 8-bit 2's complement.

Here is a working version:

 if (w[0] < 0)
   y += 256;       // 2^8 = 256
 y=w[0];
 for (int i=1; i<4; i++) {
    y = y << 8;
    if (w[i] < 0)
      y+=256;
    y += w[i];
 }
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