Java程序辅导

Java: An Operational 
Semantics
Gaurav S. Kc
B. Eng. Project
Department of Computing
Semantics of Java -- why?
n Semantics:
– Assignment of meanings to programs
n Java: 
– A simple, object-oriented, distributed, 
interpreted, robust, secure, architecture 
neutral, portable, high-performance, 
multithreaded, and dynamic language.
Why? … contd.
n Better “mental model” of language
n Acceptance
– reliability
– expected behaviour
n Java : Security v.s. functionality
n Widespread use
n Reasoning: towards a compromise
Project Goals
n Semantics extension
n Better understanding of Java
n Research based -- no implementation
Break-down into parts
n Existing features: 
– inheritance
– instance variables
– overloading and overriding
n Additions:
– access modifiers
– final, static, abstract
– constructors
An example in the new syntax
abstract class Animal extends Object {
int age; 
Animal() {
super();} 
int getAge() {...} }
public final class Dog extends Animal {
final int legs = 4; 
final static boolean hasTail = yes; 
Dog() {
this(“Laika”); } 
Dog(String s) {
super(); … } 
int getAge(String name) {…} }
Access Modifiers
n Public
n Protected
n Private
n [default]
n No packages
l Accessibility checks
Semantics extension ...
Final and Abstract modifier
n Classes
– sub-classing not permitted
– instantiating not permitted
n Fields
– Constant behaviour?
l Well-formedness
l Constructor invocation
l Assignment
Semantics extension ...
Static modifier
n Fields that don’t belong to objects
n Class and interface fields
n State extension: 
– Class/interface entries
– References
l Runtime checks
l Class or interface v.s. object
Semantics extension ...
Constructor
A constructor is used in the creation of an 
object that is an instance of a class.
It is the basis with which the run-time 
system allocates space from memory to 
objects during execution.
n Instance fields & [inherited] instance fields
n OutOfMemory exception
n Static initialisation
Syntax of a Constructor
n Explicit constructor 
invocation
– this();
n same class
– super();
n parent class
n static initialisation
n Statements
public class C extends B  {
int x = 5;
public C (int n) {
this(true);
print(n); 
}
private C (boolean b)  {
super();
if (b) …
}
}
[boolean] C(true), s
[] B(); x=5; if (b) …, s
Constructor
execution
new C(true);
Other research work on Java
n Within the Department of Computing:
– Exceptions SLURP@DoC
– Concurrency SLURP@DoC
– Binary Compatibility SLURP@DoC
n In other research institutions:
– Generics PLT@Rice
– Security Issues SIP@Princeton
Other research, … contd.
n A comparison perhaps?
– Different aspects of Java
– Post-grad & post-doc work
Conclusions
n Boring? Certainly not!
n Acquired skills
n Taste of pure research
n Lots of non-trivial work
n State of the art technology
n Continued research in Java Semantics
n Improved know-how of the Java system
Acknowledgements
n Krysia Broda
n Sophia Drossopoulou
n Susan Eisenbach
n Tanya Valkevych