Re: Eclipse generated hashCode() and Compiler Efficiency

Hi,

Eclipse generates hashCode() methods like this for a class with a
single int field "i":

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + i;
    return result;
  }

I always found it weird to define the constant "prime" at method
level. I wanted to know what happens and disassembled with

$ javap -c -verbose -private -classpath classes bytecode.T1

This is what I got for the method:

public int hashCode();
  Code:
   Stack=2, Locals=3, Args_size=1
   0: bipush 31
   2: istore_1
   3: iconst_1
   4: istore_2
   5: bipush 31
   7: iload_2
   8: imul
   9: aload_0
   10: getfield #18; //Field i:I
   13: iadd
   14: istore_2
   15: iload_2
   16: ireturn
  LineNumberTable:
   line 9: 0
   line 10: 3
   line 11: 5
   line 12: 15

  LocalVariableTable:
   Start Length Slot Name Signature
   0 17 0 this Lbytecode/T1;
   3 14 1 prime I
   5 12 2 result I

(This was from Eclipse compiler but Sun's javac yields the same albeit
with a different ordering of constants.)

Apart from other inefficiencies the weird thing is that in lines 0 and
2 the constant 31 is stored in local variable 1 ("prime") although it
is never read again. Is this an optimization they didn't bother to do
(or left for the JVM) or is there any reason why the local variable
must be there? So far I could not find any hints in JLS or JVM spec.

You can find the full examples here - including a variant where
"prime" is a constant on class level:https://gist.github.com/1063153

Instruction reference: http://java.sun.com/docs/books/jvms/second_edition=

Kind regards