Re: Problem reconstructing decomposed int

On Sun, 28 Jun 2009 15:37:49 -0700, "Mike Schilling"
<mscottschilling@hotmail.com> wrote, quoted or indirectly quoted
someone who said :


I wrote a little program to prove my point:

import static java.lang.System.out;

public class StudyByte
   {
   static void display ( byte b )
      {
      out.println( b );
      }

   /**
    * test harness
    *
    * @param args not used
    */
   public static void main ( String[] args )
      {
      byte b = (byte) 254;
      display ( b );
      }
   }

Here is how Javap disassembles it.

Compiled from "StudyByte.java"
public class StudyByte extends java.lang.Object
  SourceFile: "StudyByte.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method #6.#17; // java/lang/Object."<init>":()V
const #2 = Field #18.#19; //
java/lang/System.out:Ljava/io/PrintStream;
const #3 = Method #20.#21; //
java/io/PrintStream.println:(I)V
const #4 = Method #5.#22; // StudyByte.display:(B)V
const #5 = class #23; // StudyByte
const #6 = class #24; // java/lang/Object
const #7 = Asciz <init>;
const #8 = Asciz ()V;
const #9 = Asciz Code;
const #10 = Asciz LineNumberTable;
const #11 = Asciz display;
const #12 = Asciz (B)V;
const #13 = Asciz main;
const #14 = Asciz ([Ljava/lang/String;)V;
const #15 = Asciz SourceFile;
const #16 = Asciz StudyByte.java;
const #17 = NameAndType #7:#8;// "<init>":()V
const #18 = class #25; // java/lang/System
const #19 = NameAndType #26:#27;// out:Ljava/io/PrintStream;
const #20 = class #28; // java/io/PrintStream
const #21 = NameAndType #29:#30;// println:(I)V
const #22 = NameAndType #11:#12;// display:(B)V
const #23 = Asciz StudyByte;
const #24 = Asciz java/lang/Object;
const #25 = Asciz java/lang/System;
const #26 = Asciz out;
const #27 = Asciz Ljava/io/PrintStream;;
const #28 = Asciz java/io/PrintStream;
const #29 = Asciz println;
const #30 = Asciz (I)V;

{
public StudyByte();
  Signature: ()V
  LineNumberTable:
   line 3: 0

  Code:
   Stack=1, Locals=1, Args_size=1
   0: aload_0
   1: invokespecial #1; //Method java/lang/Object."<init>":()V
   4: return
  LineNumberTable:
   line 3: 0

static void display(byte);
  Signature: (B)V
  LineNumberTable:
   line 7: 0
   line 8: 7

  Code:
   Stack=2, Locals=1, Args_size=1
   0: getstatic #2; //Field
java/lang/System.out:Ljava/io/PrintStream;
   3: iload_0
   4: invokevirtual #3; //Method java/io/PrintStream.println:(I)V
   7: return
  LineNumberTable:
   line 7: 0
   line 8: 7

public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
  LineNumberTable:
   line 17: 0
   line 18: 3
   line 19: 7

  Code:
   Stack=1, Locals=2, Args_size=1
   0: bipush -2 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
   2: istore_1
   3: iload_1
   4: invokestatic #4; //Method display:(B)V
   7: return
  LineNumberTable:
   line 17: 0
   line 18: 3
   line 19: 7

}

The key is bipush -2. It is pushing a 32-bit sign extended -2 onto the
the stack, not 0xfe as you thought. bytes are treated like signed
32-bit ints inside the JVM.
  
--
Roedy Green Canadian Mind Products
http://mindprod.com

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