Question of the Day

Question of the Day • Write valid mathematical equation using:2 3 4 5oneaddition operator (‘+’)oneequality operator (‘=’) • Should have equal values on both sides of equals sign • Not limited to what is possible in Java

Question of the Day • Write valid mathematical equation using:2 3 4 5 one addition operator (‘+’)one equality operator (‘=’) • Should have equal values on both sides of equals sign • Not limited to what is possible in Java 5 + 4 = 32

Lecture 11:Abstract Classes & Interfaces

Announcement

Inheritance Issues • Multiple inheritance causes many problems • What if there are multiple superclasses declare field? • Which constructor will super()chain to? • Since classes extend only 1 class, Java avoids problem • Often mix multiple ideas within class, however

Inheritance Issues • Multiple inheritance causes many problems • What if there are multiple superclasses declare field? • Which constructor will super() chain to? • Since classes extend only 1 class, Java avoids problem • Often mix multiple ideas within class, however public class Mammal { … } public class Bird { … } public class Platypus extends

Inheritance Issues • Multiple inheritance causes many problems • What if there are multiple superclasses declare field? • Which constructor will super()chain to? • Since classes extend only 1 class, Java avoids problem • Often mix multiple ideas within class, however public class Mammal { … } public class Bird { … } public class Platypus extends

abstract • Can declare methods as abstract • Methods cannot be defined; promise functionality • Methods can have parameters, return type, etc. • Bodies are illegal; declaration only lists signature public abstract void foo(inti);public abstract Beer bar(String o);

abstract class • If we have a class containing abstract methods • Class considered abstractno matter how declared • abstractclasses similar to regular classes… • static & non-static fields allowed in class • Class will always extend exactly 1 other class • Mix of abstract& normal methods can be included • May use default one or maydefine a constructor • Can be extended by other classes

abstract class • If we have a class containing abstract methods • Class considered abstractno matter how declared • abstractclasses similar to regular classes… • …but cannot instantiate abstract class • And abstractmethods inherited by subclasses

abstract class • If we have a class containing abstract methods • Class considered abstractno matter how declared • abstractclasses similar to regular classes… • …but cannot instantiate abstract class • And abstractmethods inherited by subclasses • But subclass can override method & make it concrete

Interfaces in Real World

Interfaces • Can only declare important constant fields • publicstaticfinalmust be used for fields • Interface declares publicabstractmethods • Methods callable anywhere by any class • But method’s body cannot be defined in interface

Adding to an Interface • Interfaces have sub-interfaces • Sub-interfaceextendssuper-interface • Super-interface’s methods inherited by sub-interface • Methods from super-super-interface also inherited • Sub-interface can extendmultiple interfaces • Methods not defined so does not cause problems

Declaring Interfaces publicinterfaceDrawable{public void setColor(Color c); public ColorgetColor(); public void draw(Graphics g);} public interfaceTranDrawextendsDrawable{ public void setTransparency(inttLevel);} public interfaceMoveDrawextendsDrawable{ public void setPosition(int x, int y); }

Interfaces • Classesimplementinterfaces • Implementing classes define interface’s methods • Any number of interfaces may be implemented • Multiple inheritance issues ignored -- methods empty • Unrelated to superclasschosen or used by a class • Classes must implement all interface’s methods • Includes methods inherited from super-interface

Implementing Interfaces public class SquareimplementsDrawable {private Color c;private int x, y, side;public Square(Colorcol, intlen) { c = col; side = len;}public void setColor(Colorcol){ c=col; }public ColorgetColor() { return c; }public void draw(Graphics g) { g.drawRect(x, y, side, side, c);} }

Implementing Interfaces public class Oval implementsDrawable {private Color c;private int x, y, majRad, minRad;public Square(Color co, intmaj, int min){ c = co;majRad = maj;minRad = min;}public void setColor(Color col){ c=col; }public ColorgetColor() { return c; }public void draw(Graphics g) { g.drawOval(x, y, majRad, minRad, c);} }

Using Interfaces • Cannot instantiate an interface… • Not possible, since only create instances of class • Variables of interface type are perfectly legal • Variable can refer to implementing class instance • Methods must be implemented in actual class • Remember: which method called by instance type public voiddrawRed(Drawabled, Graphics g) {d.setColor(Color.red);d.draw(g); }

Interface vs. Abstract Class • Both concepts serve similar purposes • Cannot instantiate either of these types • But can be used for variable, field, & parameter types • Used in other classes to identify important features • But very important differences define when each used

Interface vs. Abstract Class • Canextendclasses & implementinterfaces • Both used to markinstances have methods defined • Fields, params, & locals can use either as their type • Use abstractclass when… • …classes should use common method or private field • …place in object hierarchy as subclass of existing class • Otherwise use interfacefor abstract methods • More flexible: class can implement many interfaces • Can mark classes; do not have to declare methods

Typecasting inti = 13;Square s = ((Square)i); • Only exist to “assist” compiler with code • Changes variable’s type so compilation continues • Not in *.class file – does not affect instance at all • Only when KNOW instance & variables types differ • Errors at runtime instead of during compilation • Illegal code will compile, but still illegal

Narrowing Conversions • Java cannot compile narrowing conversions • Assigns superclass/interface to lower variable • Compiler will not allow it, but could be legal • Typecasting required for these assignments Object obj = new String(“bye”);

Narrowing Conversions • Java cannot compile narrowing conversions • Assigns superclass/interface to lower variable • Compiler will not allow it, but could be legal • Typecasting required for these assignments Object obj = new String(“bye”); String sad = obj; //  Does not work

Narrowing Conversions • Java cannot compile narrowing conversions • Assigns superclass/interface to lower variable • Compiler will not allow it, but could be legal • Typecasting required for these assignments Object obj = new String(“bye”); String sad = obj; //  Does not work String glad = (String)obj; //  works!

Your Turn • Get into your groups and complete activity

For Next Lecture • Review your OO concepts & understand ideas • Some fun & different problems for Monday’s quiz • Covers all material since last quiz (must know Java, too) • Focus will be on: inheritance, interfaces, abstract classes, typecasting,how to tell if calls are legal, … • There is weekly assignment problem on Angel • Due by 5PM Tuesday (via e-mail) • Problem #2 graded using the provided JUnit tests

Question of the Day