CS 541 Meeting -*- Outline -*-

* Java Overview
	refs: Gosling, Joy, Steele, The Java Language Specification,
                        Addison Wesley, 1996
              Arnold, Gosling, The Java Programming Language, Addison Wesley,
                        1996

** Goals
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           GOALS OF JAVA

"... a Java program should compute the
same result on all machines and in all
implementations"
       ("The Java Language Specification")

 - simplicity
 - avoid new, untested features
 - few implementation dependencies
        so very portable
 - avoid safety problems
        garbage collection
        checked array access
 - programmer productivity
        safety features
        exception handling
        multithreading
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        Q: Why does that quote seem important?
                because it's so unlike C or C++!
        Q: What's not here vs. C++?
                efficiency

        The general character of the language is "C++ simplified",
                although it's not clear that in the end it is simpler...

** Basics
------------------------------------------
          PARTS OF THE LANGUAGE

primitive expressions:
 data:
  values:
    boolean, int, short, long,
    char (unicode characters),
    float, double (IEEE 754 standard),
    references, null
  objects: arrays, class instances,
 variables
 assignments
 method calls

means of combination:
 statements (if, while, ...),
 methods,
 arrays, classes,
 inheritance (extends)

means of abstraction:
 classes, packages, interfaces,
 nested and anonymous classes,
 monitors (synchronzied methods and stmts)
------------------------------------------

        The IEEE 754 standard seemed like a good idea,
            but I don't like the fact that == and != don't satisfy the
            usual mathematical properties for NaN

        what's more significant is what they left out, in search of simplicity

------------------------------------------
          MISSING VS. C++

primitive expressions:
  data:
    values: 
      pointers, pointer arithmetic,
      references to stack locations
  objects:
    unions, structs

means of combination:
  goto (use labeled break and continue),
  data layout control,
  multiple inheritance (both kinds),
  private and protected inheritance

means of abstraction:
  macros,
  templates,
  namespaces (but has packages),
  friends
------------------------------------------

        and lots of syntax (C++ is HUGE!),
        although Java started out somewhat smaller, now Java is also big

** semantics of objects

   Java is basically like Smalltalk (and Lisp), unlike C++

------------------------------------------
        REFERENCES vs. POINTERS

A *pointer* is


A *reference* in Java is


C++ code:
   int i = 3;
   int * ip = &i;
   int * p2 = new int;
   ip = ip + 1;
   p2 = p2 + 1;
   int & ir = i;
   int & r2 = *(new int);
   

Java code:
    int i = 3;
    Integer j = new Integer(7);
    j = new Integer(8);
    j = null;
------------------------------------------
        ... an address (potentially 0 or even bogus)

        ... either null or the address of some object

        draw pictures

        In Java, can't have a reference to a variable,
                 can't have a reference to a non-object,
                 variables contain pure values or references


        Summary: Java has pointers but not pointer arithmetic.

------------------------------------------
          DOMAINS (KINDS OF VALUES)

ExpressibleValue =
  PureValue + Reference

PureValue =
  boolean + int + short + long + char
  + float + double

Reference =
  Object + null

DenotableValue = ExpressibleValue
------------------------------------------
        Explain the terms

        Q: What's a denotable value in C++ that isn't expressible?
                arrays

        Objects include arrays, but not variables

        Q: What benefit from not having references to variables?
        Q: What benefit from not having pointer arithmetic?

------------------------------------------
     PARAMETER PASSING AND ASSIGNMENT

Java is a language with:
        


direct model of arrays (Pascal, Ada):




indirect model of arrays (C, C++, Scheme):




------------------------------------------
        ... call by value
            and the indirect model of
                both arrays and objects

        ... array variables are bound directly
           to a sequence of cells,
           assignment and pass by value make copies
           no aliasing of array names

        ... array variables contain references to
            a sequence of cells (indirect binding)
            assignment and pass by value make aliases

        The main difference in Java is that almost everything
                uses the indirect model (references),
                as this is used for all objects

        copies are only made for pure values

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          EXAMPLE

import java.util.*;

class Demo {
  public static void main(String[] args){
    Vector x, y;
    x = new Vector();
    y = new Vector();
    x.addElement(new Integer(3));
    x.addElement(new Integer(4));
    y = x;
    y.setElementAt(new Integer(7), 0);
    System.out.println(x.elementAt(0));
  }
}
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        draw a picture, discuss

        Summary, the basic semantics in Java is like Smalltalk (or LISP)