O-O-Overloading & D-Dispatch. Scott Kilpatrick 02 March 2011

Transcription

1 O-O-Overloading & D-Dispatch Scott Kilpatrick 02 March

2 Overloading and dispatch: in natural languages Overloading: one word has many meanings court court court 2

3 Overloading and dispatch: in natural languages Overloading: one word has many meanings Dispatch: map word occurrence to meaning court court court dispatch A man and woman court each other over wine. 2

4 Overloading and dispatch: in programming languages Many langs: overload for convenience println(); println(5); { print string, newline } { print integer, newline } { print newline } OOP langs: overload for specialization someobject.equals(another); hello!!!.equals(another); { Object equality } { String equality } 3

5 Overloading and dispatch: in programming languages Many langs: overload for convenience println(); println(5); { print string, newline } { print integer, newline } { print newline } OOP langs: overload for specialization someobject.equals(another); hello!!!.equals(another); { Object equality } { String equality } What if? Object someobject = hello!!! ; 3

6 Methods define overloaded functions Receiver.methodName(Parameters) {... code... } Acts like function with extra this/self Determines unique impl. of methodname { Object.toString() } { } class Object class String { String.toString() { } } extends Object String.indexOf(String) { } { } 4

7 Overloading: a terminological distinction overloading vs. overriding convenience specialization Overload methods with different parameters Override methods with same parameters, specialized receivers String.indexOf(char) String.indexOf(char, int) Object.toString() String.toString() 5

8 Dispatch maps calls to methods at compile-time Find methods that fit the call site e.g. foo.equals( bar ); { String.equals(Object) { fit foo.equals( bar ) 6

9 Dispatch maps calls to methods at compile-time Find methods that fit the call site 2. Pick the best-fitting method e.g. foo.equals( bar ); { String.equals(Object) { fit foo.equals( bar ) 2. best fit: String.equals(Object) 6

10 Dynamic dispatch chooses a better method at run-time Use the receiver s run-time value Reuse other args types C++: virtual methods Java: all methods Server s = getmyserver(); s.process(req); Server.process(Request) MyServer.process(Request) 7

11 Dynamic dispatch chooses a better method at run-time Use the receiver s run-time value Reuse other args types C++: virtual methods Java: all methods Server s = getmyserver(); s.process(req); Server.process(Request) MyServer.process(Request) compiles to call receiver s method process(request) 7

12 Dynamic dispatch chooses a better method at run-time Use the receiver s run-time value Reuse other args types C++: virtual methods Java: all methods Server s = getmyserver(); s.process(req); Server.process(Request) MyServer.process(Request) compiles to call receiver s method process(request) is run as Server.process(Request) MyServer.process(Request) 2. new MyServer().process(req); 7

13 Dynamic dispatch chooses a better method at run-time Use the receiver s run-time value Reuse other args types C++: virtual methods Java: all methods Server s = getmyserver(); s.process(req); Only overriding methods Server.process(Request) MyServer.process(Request) compiles to will be chosen dynamically! call receiver s method process(request) is run as Server.process(Request) MyServer.process(Request) 2. new MyServer().process(req); 7

14 Common mistake with dynamic dispatch Never define MyClass.equals(MyClass)! Object me = getmyclass(); Object it = getmyclass(); me.equals(it); MyClass.equals(Object) MyClass.equals(MyClass) Remember overloading/overriding distinction overrides MyClass.equals(Object) MyClass.equals(MyClass) overloads 8

15 Common mistake with dynamic dispatch Never define MyClass.equals(MyClass)! Object me = getmyclass(); Object it = getmyclass(); me.equals(it); MyClass.equals(Object) MyClass.equals(MyClass) Remember overloading/overriding distinction overrides MyClass.equals(Object) MyClass.equals(MyClass) overloads 8

16 Common mistake with dynamic dispatch Never define MyClass.equals(MyClass)! Object me = getmyclass(); Object it = getmyclass(); me.equals(it); 2. MyClass.equals(Object) MyClass.equals(MyClass) Remember overloading/overriding distinction overrides MyClass.equals(Object) MyClass.equals(MyClass) overloads 8

17 Common mistake with dynamic dispatch Never define MyClass.equals(MyClass)! Object me = getmyclass(); Object it = getmyclass(); me.equals(it); 2. MyClass.equals(Object) MyClass.equals(MyClass) Remember overloading/overriding distinction overrides MyClass.equals(Object) MyClass.equals(MyClass) overloads 8

18 Thanks 9