Inheritance & Dynamic Binding

Class USBFlashDrive We better introduce a new class USMBFlashDrive and save() is defined as a method in that class Computer - String: mfr -Int: year -Double: price + set_mfr(…){..} + get_mfr(){..} + editAfile() {…} USBFlashDrive - String: loc + save() {…} class Computer { private string mfr; private int year; … public void set_mfr(…) {…} public string get_mfr() {…} public void editAfile(){… ; ;} } private USBFlashDrive usb; usb.save() public void set_usb(USBFlashDrive u){…} public USBFlashDrive get_usb(){…}

Now You can use the save() service You need to buy a computer and a usb flash drive Computer mylaptop = new Computer(“Dell”, 2010,900); USBFlashDrive myusb = new USBFlashDrive(…); mylaptop.editAfile(); Don’t forget to connect myusb to mylaptop!!! mylaptop.setusb(myusb);

How Can iPhone Be Used? Now, iPhone can be easily used as a USB Flash Drive

How Can Software Be Implemented Like this? We have two Classes: Computer - String: mfr -Int: year -Double: price + set_mfr(…){..} + get_mfr(){..} + editAfile() {…} USBFlashDrive - String: loc + save() {…} How Can iPhone be used WITHOUT CHANGE Computer’s Class??? IPhone + save() {…}

Inheritance Inheritance: is-a relationship. USBFlashDrive - String: loc + save() {…} IPhone + save() {…} Parent class Child class/subclaass Is-a relationship means: All subclass inherits all attributes and methods of parent class Subclass can override method of parent class. Subclass can have additional method that parent class doesn’t have + call(…){…}

How Can Software Be Implemented Like this? We have two Classes: class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } class USBFlashDrive{ … public void save() { // usb’s save here ……… } class IPhone extends USBFlashDrive { public void save() {// iphone’s save here ….. }

How Can Software Be Implemented Like this? In Computer’s Eye, iPhone is the same as usb flash drive when saving a file externally. Inheritance Relationship is also called “is-a” relationship!!! Iphone is a USBFlashDrive !!!

How Can Software Be Implemented Like this? Class Computer doesn’t distinguish between Iphone and USBFlashDrive. class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } Placehold that can reference to anything that is a USBFlashDrive, like iphone. But in runtime, which save() method is Called depends on the object referenced By usb.

So, Static Types vs Dynamic Types Every reference type variables have two types: static type and dynamic type. class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } Variable usb has two types: static type: USBFlashDrive dynamic type: USBFlashDrive or its subtypes (known at runtime)

Static Types Static types are used by compiler: class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; } } usb.save(); usb.call(); Compiler Complains: USBFlashDrive doesn’t have the call method!!! X

Dynamic Types Once compiler doesn’t complain, you can run the program. Dynamic Type decides which method is called class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } Which save() method is called depends On the dynamic type of usb

How it Works To run a program/get a service of save method, you need to buy a computer and iphone objects!!! Computer mylaptop = new Computer(“Dell”, 2010,900); IPhone iphone = new IPhone(…); mylaptop.set_usb(iphone); mylaptop.editAfile(…); class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } What does compiler do? How the runtime does?

More … To run a program/get a service of save method, you need to buy a computer and iphone objects!!! Computer mylaptop = new Computer(“Dell”, 2010,900); USBFlashDrive iphone = new IPhone(…); mylaptop.set_usb(iphone); mylaptop.editAfile(…); class Computer { private USBFlashDrive usb; … public void set_usb(USBFlashDrive u) { usb = u;} public USBFlashDrive get_usb() { return usb;} …. public void editAfile(){… ; usb.save();} } What does compiler do? How the runtime does?

How about this? Consider the following situation Computer mylaptop = new Computer(“Dell”, 2010,900); mylaptop.set_usb(iphone); mylaptop.editAfile(…); IPhone iphone = new USBFlashDrive(…); What does compiler say? NO !!! USBFlashDrive - String: loc + save() {…} IPhone + save() {…}