Using the Classes let class Vehicle extends Object { . } class Car extend

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Using the Classes let class Vehicle extends Object { . } class Car extend

Aquinas College, US has reference to this Academic Journal, Objects in addition to Classes David Walker CS 320 Advanced Languages advanced programming features ML data types, exceptions, modules, objects, concurrency, . fun so that use, but require special techniques so that compile in addition to optimize today will be looking at how so that compile objects in addition to classes similar so that those found in Java Appel chapter 14.1-14.4 Object Fun Add class declarations so that Fun: classes are a collection of field (variable) declarations method declarations every class declaration gives a new name so that a class a class may inherit methods in addition to fields from the class it extends the class ?object? sits at the top of the class hierarchy; it has no fields in addition to no methods

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An Object Fun Class let class Vehicle extends Object { var position := start method move (x : int) = (position := position + x) } in . end class name superclass that Vehicle inherits from field declaration method declaration Another Object Fun Class let class Vehicle extends Object { var position := start method move (x:int) = (position := position + x) } class Car extends Vehicle { var passengers := 0 method await(v:Vehicle) = if (v.position < position) then v.move(position ? v.position) else self.move(10) in . end v?s ?position? field current object?s ?position? field call so that inherited method new field declaration new method declaration Yet Another Object Fun Class let class Vehicle extends Object { var position := start method move (x:int) = position := position + x } class Car extends Vehicle { . } class Truck extends Vehicle { method move (x:int) = if x <= 55 then position := position * x } in . end method override Using the Classes let class Vehicle extends Object { . } class Car extends Vehicle { . } class Truck extends Vehicle {.} val t = new Truck val c = new Car val v : Vehicle = c in c.passengers := 2; c.move(60); v.move(70); c.await(t); end new object created subtyping allows a car so that be viewed in addition to used as a generic vehicle subtyping allows a truck so that be viewed in addition to used as a generic vehicle a car calls an inherited method Implementing Object Tiger Some key problems: how do we access object fields? both inherited fields in addition to fields in consideration of the current object? how do we access method code? if the current class does not define a particular method, where do we go so that get the inherited method code? how do we handle method override? Class Hierarchy The class hierarchy is the graph of inheritence relationships in a program: In a single-inheritence (SI) language, the graph is a tree In a multiple-inheritence (MI) language, the graph is a dag Multiple-inheritence languages are much trickier so that implement than single-inheritence languages Object Vehicle Car Truck

The Graduate School Application Process Introduction When do I start looking in consideration of schools? When do I start looking in consideration of schools? How do I choose schools? Why should I start looking so soon? Why should I start looking so soon? Why should I start looking so soon? Where do I look? Where do I look? Getting the Applications Completing Applications GRE Scores Essays Essays Essays Letters of Recommendation Letters of Recommendation Sending Applications Interview Selection Criteria Interview Selection Criteria Do I Need so that Attend the Interview? Do I Need so that Attend the Interview? Do I need so that Attend the Interview? Do I Need so that Attend the Interview? Do I Need so that Attend the Interview? What are the interviews like? What are the interviews like? What types of questions will be asked? Other Interview Information Other Interview Information Other Interview Information The Expenses The Expenses Good Luck!

Object Layout (SI) Objects are laid out somewhat like records each variable has a slot in the record in order so that implement field lookup we need so that have a systematic way so that find a given field eg: v.position the hard part is that v may be a generic Vehicle or it may be a Car or a Truck the same lookup code must work when we know the static type (say Vehicle) but we do not know the dynamic type of the object (say Vehicle, Car or Truck) we need so that put ?position? in the same place in the record that implements vehicles, cars in addition to trucks Using the Classes let class Vehicle extends Object { . } class Car extends Vehicle { var passengers := 0 method await(v:Vehicle) = if (v.position < position) then . } class Truck extends Vehicle {.} val t = new Truck val c = new Car val v : Vehicle = c in c.await(t); c.await(c); c.await(v) end assume v is a pointer in rv v.position (for SI) gets compiled into: load rtemp const(rv) what const do we use? must be the same constant regardless of whether v is a car, truck or vehicle the layout of fields in records that implement cars, trucks in addition to vehicles must be similar. Object Layout (SI) Solution: extension on the right lay out the inherited fields first in the same order as in the parent (SI => only 1 parent) lay out the newly declared so that the right

Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } A a class identifier; used so that implement downcasts; first field in all classes must be the class identifier first field named a; all objects extending A must have first field a Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } B a b c A a

Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } C a d B a b c A a Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } D a b c e C a d B a b c A a Static & Dynamic Methods The result of compiling a method is some machine code located at a particular address at a method invocation point, we need so that figure out what code location so that jump so that Java has static & dynamic methods so that resolve static method calls, we look at the static type of the calling object so that resolve dynamic method calls, we need the dynamic type of the calling object

Static Methods during semantic analysis, the compiler knows: static type (class) of the object calling the method the list of methods in each class in addition to determines the closest method (up the class hierarchy) alongside the given name in addition to inserts instructions so that pass object as self parameter a direct call so that the known method let class A extends Object { static method foo (x:int) = . static method bar (x:int) = . } class B extends A { static method foo (x:int) = . } var a : A = new A var b : A = new B var c : B = new B in a.foo(3); (* calls foo in class A *) b.foo(3); (* calls foo in class A *) c.bar(3); (* calls bar in class A *) c.foo(3); (* calls foo in class B *) Dynamic Methods Method called depends on object?s dynamic type During semantic analysis, may be unknown At run-time, we determine which code so that jump so that object stores a pointer so that its method table (v-table) as well as its object vars At compile-time, we generate code so that look up v-table in object extract method from table jump so that method body let class A extends Object { method foo (x:int) = . method bar (x:int) = . } class B extends A { method foo (x:int) = . } var a : A = new A var b : A = new B var c : A = if long-and-tricky-computation then a else b in c.foo(3) Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () }

Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A a A_f Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } B a A a A_f A_f B_g Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } C a B a A a A_f A_f B_g A_f C_g

Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } D a b C a B a A a A_f A_f B_g A_f C_g D_f C_g Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } D a b C a B a A a A_f A_f B_g A_f C_g D_f C_g D a b Multiple Inheritence Multiple inheritence is trickier so that implement than single inheritence because creating objects of a subclass from their subclass by ?extension on the right? doesn?t work if C inherits from both A in addition to B, we can?t put A?s variables at the front in addition to put B?s variables at the front of the object in the same place! we need so that do a global analysis so that determine object layout

Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } D a b c e C a d B b c A a d Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } D a b c e C a d B b c A a d Determine object layout by: global graph coloring! a node in consideration of each field name an interference edge between names that coexist in the same class (via inheritence or otherwise) Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } D a b c e C a d B b c A a d wasted space in every object

Other OO Features Down-casts in addition to type tests Java has casting mechanism ?(C) x? so that cast variable x so that class C at run time we look up x?s dynamic type in addition to determine whether it is a subtype of C these type casts are currently pervasive in addition to a source of both inefficiency in addition to errors soon, Java in addition to C# will be adding parametric polymorphism, a la ML, so that make many of these unnecessary casts go away Other OO Features Protection mechanisms so that encapsulate local state within an object, Java has ?private? ?protected? in addition to ?public? qualifiers private methods/fields can?t be called/used outside of the class in which they are defined during semantic analysis (type checking), the compiler maintains this information in the symbol table in consideration of each class Summary Object-oriented languages provide new challenges in consideration of compiler writers how so that find fields in addition to methods how so that make field in addition to method access just as efficient as ordinary function call in addition to variable lookup lots of ongoing research in OO language implementation tackles these in addition to other interesting questions

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Bianco, Sean is from United States and they belong to FreeVerse Editor and work for Shades Magazine in the AZ state United States got related to this Particular Article.

Journal Ratings by Aquinas College

This Particular Journal got reviewed and rated by Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } D a b C a B a A a A_f A_f B_g A_f C_g D_f C_g Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } D a b C a B a A a A_f A_f B_g A_f C_g D_f C_g D a b Multiple Inheritence Multiple inheritence is trickier so that implement than single inheritence because creating objects of a subclass from their subclass by ?extension on the right? doesn?t work if C inherits from both A in addition to B, we can?t put A?s variables at the front in addition to put B?s variables at the front of the object in the same place! we need so that do a global analysis so that determine object layout and short form of this particular Institution is US and gave this Journal an Excellent Rating.