Encapsulation, Inheritance & InterfacesThree Pillars of Object-Oriented ProgrammingEncapsulationInheritance (from before exam)InheritanceOne superclassMethod OverridingInterfaceInterfacesDefining an InterfaceJava Code: InterfacesImplementing an InterfaceJava Code: Implementing an InterfaceInterface FactsInheritance RecapClassA extends ClassBSlide 17Slide 18Slide 19Slide 20What’s the difference?Encapsulation, Inheritance & InterfacesCSE 115 Spring 2006February 27, March 1 & 3, 2006Three Pillars of Object-Oriented ProgrammingEncapsulationInheritancePolymorphismEncapsulationKeeping data and operations that work on that data in one computational unit“public face” versus “private implementation”Inheritance (from before exam)Ability to extend another class and inherit all its public capabilities.InheritanceEvery class in Java uses inheritance.Either specify a specific superclass by using the keyword extends when defining the class.Use default superclass, which is java.lang.ObjectOne superclassIn Java, a class can extend one class (ie – only one class name can appear after the keyword extends in a class definition)However, if the class that is extended has a superclass, the new subclass inherits from both its direct superclass as well as that class’ superclass.Method OverridingDefining a method in a subclass that has the same signature as one in the superclass.Can:Call the superclass’ methodAdd on to the superclass’ methodCompletely redefine the method.InterfaceContract (of methods)If a class chooses to enter into the agreement with the interface, the class promises to have methods with the same signatures as those in the interface.InterfacesAre NOT classesBut they are types (just like classes are). So, they can be the type ofVariables (local or instance)Return typesFormal ParametersDefining an InterfaceNo instance variables allowed.Everything is public.No implementation of methods, just method signatures followed by ;These types of methods are called abstract methods because they have no method bodies. The keyword abstract might be seen in the method signatures of methods in an interfaceJava Code: Interfacespublic interface InterfaceName {public abstract void method1();abstract public Object method2();void method3(Object obj);//Note that all three method signatures are valid for an //interface and which you use is a matter of style.}Implementing an InterfaceA class realizes an interface using the keyword implements.The formal name for this relationship is realization and the informal name is “implements” or “implements an interface”Inside the code, the class must create methods with the same signatures as those in the interface and give each method a method body.Java Code: Implementing an Interfacepublic class name implements InterfaceName {public void method1() { }public Object method2() { return new Object(); }public void method3(Object obj) { }//Note that each method must have a method body, but //that method body could be empty. }Interface FactsWe can implement more than one interface.We can have a superclass and implement one or more interfaces.An interface can extend another interface or multiple other interfaces.You can NEVER create an instance of an interface.Inheritance RecapThe next set of slides presents a recap of all the topics we have discussed with inheritance.ClassA extends ClassBClassA now inherits (can access and use) all public and protected elements of ClassBWe can expect the behavior from the subclass that the superclass had.ClassA extends ClassBWhen the constructor from ClassA is called, it automatically calls the no-argument constructor for ClassB. If ClassB does not have a no-argument constructor, ClassA must explicitly call one of the other constructors from ClassB using the keyword super.ClassA extends ClassBA subclass is the specialization of the superclass, so we can add instance variables, add methods, change parameters on methods, or change the way methods behave.ClassA extends ClassBWhen we have a method that has the same signature in a subclass as one in the superclass, we have method overriding. Method overriding is how we change the behaviors of methods in a subclass.InheritanceLooking at inheritance in reverse – if we have a lot of commonality amongst some classes, we can create one superclass to generalize amongst the subclasses. Ex) Got a lot of classes that do the same thing? Create a common superclass!What’s the difference?Method overloadingMethod inheritanceMethod