r = obj2.m( 0, 1 ); s = obj2.f; r = obj2.m( ); r = obj2.anothermethod( 0, 1 ); s = obj2.anotherfield;

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Cora Parks
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2 ! Objects(access(fields( and(methods(of(other( objects((! A(safe(language( detects(situa7ons( where(the(receiver( object(does(not(have( the(accessed(field(or( method((! Type(systems(can(be( used(to(detect(such( errors(( r = obj2.m( 0, 1 ); s = obj2.f; r = obj2.m( ); r = obj2.anothermethod( 0, 1 ); s = obj2.anotherfield;

3 Sta7c(type(checking(! Each(expression(of(a( program(has(a(type%%! Types(of(variables(and( methods(are(declared% explicitly(or(inferred%%! Types(of(expressions(can( be(derived(from(the%types% of%their%cons2tuents%%! Type%rules(are(used(at( compile%2me%to(check( whether(a(program(is( correctly%typed%% Compile time Errors

4 Run(7me(type(checking(in(! Most(sta7c(type( systems(rely(on( dynamic(checks(for( certain(opera7ons((! Common(example:( type(conversions%by( casts((! Run82me%checks% throw(an(excep7on(in( case(of(a(type(error(( sta7c(type(systems(

5 Sta7c(vs(Dynamic(! Sta7c(checkers(need( to(approximate%run8 2me%behavior% (conserva7ve(checks)(! Dynamic(checkers( support(on8the8fly% code%genera2on%and( dynamic(class(loading(

7 Dynamic(Type(Checking(! instanceof%can(be( used(to(avoid(run7me( errors((! instanceof%makes( type(informa7on( available(to(program( execu7ons((

8 ! A(program(can( observe%and(modify% its(own(structure%and( behavior%%! Simplest%form% o Type(informa7on(is( available(at(run(7me((! Most%elaborate%% o All(compileI7me( informa7on(can(be( observed(and(modified( Reflec7on(

9 Why(we(need(reflec7on( As(a(programming(language(becomes(higher&and& higher&level,(its(implementa7on(in(terms(of( underlying(machine(involves(more&and&more& tradeoffs,(on(the(part(of(the(implementor,(about( what(cases(to(op7mize(at(the(expense(of(what( other(cases.( (the(ability&to&cleanly&integrate( something(outside(of(the(language s(scope( becomes&more&and&more&limited (( Kiczales(1993(

10 What(is(Reflec7on?(! Reflec2on%is(the(ability(of(a(program(to(manipulate&as&data( something(represen7ng(the(state&of&the&program(during(its( own(execu7on.(( o There(are(two(aspects(of(such(manipula7on:(introspec7on( and(intercession.(! Introspec2on%is(the(ability(for(a(program(to(observe(and( therefore(reason(about(its(own(state.((! Intercession%is(the(ability(for(a(program(to(modify(its(own( execu7on(state(or(alter&its&own&interpreta7on(or(meaning.( o Both(aspects(require(a(mechanism(for(encoding(execu7on( state(as(data:(providing(such(an(encoding(is(called( reifica7on. (( o Bobrow,(Gabriel(&(White,(1993(

11 Consequences( o A(reflec7ve(system(has(an(internal&representa7on&of& itself.( o A(reflec7ve(system(is(able(to(act&on&itself(with(the( ensurance(that(its(representa7on(will(be(causally( connected((up(to(date).(( o A(reflec7ve(system(has(some(sta7c(capacity(of(self: representa7on(and(dynamic(self:modifica7on(in( constant(synchroniza7on(

12 Metaobjects(vs(metaclasses(! Need(dis7nc7on(between(metaclass(and( metaobject!( o A(metaclass(is(a(class(whose(instances(are(classes( o A(metaobject(is(an(object(that(describes(or( manipulates(other(objects(! Different(metaobjects(can(control(different(aspects(of( objects(

13 Some(MetaObjects(! Structure:(( o Behavior,(ClassDescrip7on,(Class,(Metaclass,(ClassBuilder(! Seman+cs:-- o Compiler,(Decompiler,(IRBuilder(! Behavior:(( o CompiledMethod,(BlockContext,(Message,(Excep7on(! ControlState:(( o BlockContext,(Process,(ProcessorScheduler(! Resources:(( o WeakArray(! Naming:(( o SystemDic7onary(! Libraries:(( o MethodDic7onary,(ClassOrganizer(

14 Reflec7on(in(Java( For(every(loaded(class,(the(JVM(maintains(an( associated(class(object( The(Class(object( reflects (the(class(it( represents( Instances(of(the(class(Class(store(informa7on( about(classes( Class(name( Inheritance( Interfaces(implemented( Methods,(members,(etc.(

15 Class(object( Instances of the class Class represent classes and interfaces in a running Java application. Class has no public constructor. Class objects are constructed automatically by the JVM as classes are loaded

16 Class c = "foo".getclass(); Returns the Class for String void printclassname(object obj) { System.out.println("The class of " + obj + " is " + obj.getclass().getname()); Class object to print the class name of an object

17 Introspec7on(

20 public static void showtype(string classname) Class thisclass = Class.forName(className); String flavor = thisclass.isinterface()? "interface" : "class"; System.out.println(flavor + " " + classname); Class parent = thisclass.getsuperclass(); if (parent!= null) { throws ClassNotFoundException { System.out.println("extends " + parent.getname()); Class[] interfaces = thisclass.getinterfaces(); for (int i=0; i<interfaces.length; ++i) { System.out.println("implements "+ interfaces[i].getname());

21 static void showmethods(object o) { Class c = o.getclass(); Method[] themethods = c.getmethods(); for (int i = 0; i < themethods.length; i++) { String methodstring = themethods[i].getname(); System.out.println("Name: " + methodstring); System.out.println(" Return Type: " + themethods[i].getreturntype().getname()); Class[] parametertypes = themethods[i].getparametertypes(); System.out.print(" Parameter Types:"); for (int k = 0; k < parametertypes.length; k ++) { System.out.print(" " + parametertypes[k].getname()); System.out.println();

22 Employee number level print() Only((par7al( (view(of(the( classes ( MonthlyEmployee print() HourlyEmployee print() public final class MonthlyEmployee extends Employee { public void print() { System.out.println("I m a Monthly Employee"); public final class HourlyEmployee extends Employee { public void print() { System.out.println("I m a Hourly Employee");

23 Employee e; e = new MonthlyEmployee(); Class c = e.getclass(); System.out.println("class of e = " + c.getname()); e = new HourlyEmployee(); c = e.getclass(); System.out.println("class of e = " + c.getname()); class of e = MonthlyEmployee class of e = HourlyEmployee

24 We can ask a method object to invoke the method it represents we must provide it with the implicit and explicit arguments Employee e = new HourlyEmployee(); Class c = e.getclass(); Method m = c.getmethod("print", null); m.invoke(e, null); the output produced: I m a Hourly Employee

25 Unit(Tes7ng( class Cell { int value; Cell( int v ) { value = v; int get( ) { return value; void set( int v ) { value = v; void swap( Cell c ) { int tmp = value; value = c.value; c.value = tmp; class TestCell { void testset( ) {... void testswap( ) { Cell c1 = new Cell( 5 ); Cell c2 = new Cell( 7 ); c1.swap( c2 ); assert c1.get( ) == 7; assert c2.get( ) == 5;

26 public static void testdriver( String testclass ) { Class c = Class.forName( testclass ); Object tc = c.newinstance( ); Method[ ] methods = c.getdeclaredmethods( ); for( int i = 0; i < methods.length; i++ ) { if( methods[ i ].getname( ).startswith( "test ) && methods[ i ].getparametertypes( ).length == 0 ) methods[ i ].invoke( tc ); Basic mechanism behind JUnit

27 Safety checks have to be done at run-time Type checking: Does the Class-object represent a concrete class? Does the class have a parameter-less constructor? Accessibility: Are the class and the parameter-less constructor accessible?

29 Due to Java s erasure semantics, generic type information is not represented at run time

31 Code(genera7on(! If(code(is(represented(as(data,(we(can(as(well( allow(programs(to(create%code%from%data%%! Generate(code(dynamically(according(to(user( input(and(execu7on(environment((! Examples(( o Class(loading(in(Java( o Expression(trees(in(C#((

32 public class MainClass { public static void main(string[] args){ ClassLoader classloader = MainClass.class.getClassLoader(); try { Class aclass = classloader.loadclass("myclass"); System.out.println("aClass.getName() = " + aclass.getname()); catch (ClassNotFoundException e) { e.printstacktrace();

33 package java.lang; public abstract class ClassLoader { public Class loadclass(string name); protected Class defineclass(byte[] b); public URL getresource(string name); public Enumeration getresources(string name); public ClassLoader getparent()

34 public class A { public void dosomething() { B b = new B(); b.dosomethingelse(); B b = new B() is semantically equivalent to B b = A.class.getClassLoader().loadClass( B ).newinstance()

35 import java.util.logging.level; import java.util.logging.logger; public class ClassLoaderTest { public static void main(string args[]) { try { //printing ClassLoader of this class System.out.println("ClassLoaderTest.getClass().getClassLoader() : " + ClassLoaderTest.class.getClassLoader()); //trying to explicitly load this class again using Extension class loader Class.forName("test.ClassLoaderTest", true, ClassLoaderTest.class.getClassLoader().getParent()); catch (ClassNotFoundException ex) { Logger.getLogger(ClassLoaderTest.class.getName()).log(Level.SEVERE, null, ex);

36 Code(manipula7on(! If(code(is(represented(as(data,(we(can(as(well( allow(programs(to(modify%the%code%%! Adapt(program(dynamically(according(to(user( input(and(execu7on(environment((! Apply(systema7c(changes(to(programs(( o Code(instrumenta7on(

37 import java.io.*; public class HelloWorld { public static void main(string[] argv) { BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); String name = null; try { System.out.print("Please enter your name> "); name = in.readline(); catch(ioexception e) { return; System.out.println("Hello, " + name);

38 ClassGen cg = new ClassGen("HelloWorld", "java.lang.object", "<generated>", ACC_PUBLIC ACC_SUPER, null); ConstantPoolGen cp = cg.getconstantpool(); // cg creates constant pool InstructionList il = new InstructionList();

Concepts of Object-Oriented Programming Peter Müller

Concepts of Object-Oriented Programming Peter Müller Chair of Programming Methodology Autumn Semester 2017 1.2 Introduction Core Concepts 2 Meeting the Requirements Cooperating Program Parts with Well-Defined