MyClass.java

public class MyClass
{
public static void main(String args[])
{
  System.out.println("Hello World".hashCode());
}
}

=> -862545276

String.java

/* taken from jdk-7 sourcecode */
public String {

    public int hashCode() {
        int h = hash;
        if (h == 0 && count > 0) {
            int off = offset;
            char val[] = value;
            int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }
}

string_hash_code.rb

class JavaHashCode
	attr_accessor :string
	def initialize(_string)
		@string = _string
	end


  def hashCode
    sum(parts)
  end

  def hashCode2
    h = 0
    self.string.chars.each do |char|
      h = (31*h) + char.ord
    end
    to_signed_int(h)
  end
private
  
  def parts
  	size = self.string.size
  	hash = 0
  	parts = []
  	self.string.chars.each_with_index do |ch, i|
  	  parts << to_signed_int(ch.ord * 31 ** (size - (i+1)))
  	end
	  parts
  end

  def sum(summands)
    summands.inject(0) {|h, i|
      to_signed_int(h + i)
    }
  end

  # http://stackoverflow.com/questions/7365868/how-do-i-emulate-integer-overflow-on-a-fixnum-variable - copied from the last anser
  def to_signed_int(i)
    if i < -2147483648
        i & 0xffffffff 
    elsif i > 2147483647 
        -(-(i) & 0xffffffff)
    else  
        i
    end    
end
end

puts JavaHashCode.new("Hello World").hashCode
=> -862545276
puts JavaHashCode.new("Hello World").hashCode2
=> -862545276