Concepts of Object-Oriented Programming 7 January 2019 OSU CSE 1 - PowerPoint PPT Presentation

Concepts of Object-Oriented Programming 7 January 2019 OSU CSE 1

Recall... Standard extends NaturalNumber- Kernel extends NaturalNumber implements implements NaturalNumber1L NaturalNumber2 7 January 2019 OSU CSE 2

The “Implements” Relation • The implements relation may hold between a class and an interface • If C implements I then class C contains code for the behavior specified in interface I – This means C has method bodies for instance methods whose contracts are specified in I – The code for C looks like this: class C implements I { // bodies for methods specified in I } 7 January 2019 OSU CSE 3

The “Implements” Relation • The implements relation may hold between The implements relation allows you to separate contracts from a class and an interface their implementations — a best • If C implements I then class C contains practice for component design. code for the behavior specified in interface I – This means C has method bodies for instance methods whose contracts are specified in I – The code for C looks like this: class C implements I { // bodies for methods specified in I } 7 January 2019 OSU CSE 4

The “Implements” Relation The Java compiler checks that C • The implements relation may hold between contains bodies for the methods in a class and an interface I , but does not check that those • If C implements I then class C contains bodies correctly implement the method contracts! code for the behavior specified in interface I – This means C has method bodies for instance methods whose contracts are specified in I – The code for C looks like this: class C implements I { // bodies for methods specified in I } 7 January 2019 OSU CSE 5

The “Extends” Relation • The extends relation may hold between: – Two interfaces (as on the earlier slide), or – Two classes • In either case, if B extends A then B inherits all the methods of A – This means B implicitly starts out with all the method contracts (for an interface) or all the method bodies (for a class) that A has – B can then add more method contracts (for an interface) or method bodies (for a class) 7 January 2019 OSU CSE 6

Caveats About Java Interfaces • “If B extends A then B inherits all the methods of A ” – Interfaces cannot have constructors • So there is no good place to write separate contracts for the constructors of classes that implement an interface – Interfaces cannot have static method contracts without also providing corresponding method bodies • So there is no good place to write separate contracts for public static methods of classes that implement an interface 7 January 2019 OSU CSE 7

Caveats About Java Classes • “If B extends A then B inherits all the methods of A ” – Constructors are not inherited • So in the situation above, the class B must include bodies for any constructors that are expected, even if they would be identical to those of A • The bodies of the constructors in B generally would simply invoke the constructors of A , which is done using the special notation super (…) – Static methods are inherited 7 January 2019 OSU CSE 8

“Implements” May Be Inferred I1 extends I2 implements C3 extends C4 7 January 2019 OSU CSE 9

“Implements” May Be Inferred I1 extends implements I2 implements C3 We may infer in extends this case that C3 implements I1 . C4 7 January 2019 OSU CSE 10

“Implements” May Be Inferred I1 We may also infer in this case that extends C4 implements I2 . I2 implements C3 implements extends C4 7 January 2019 OSU CSE 11

Interface Extension • If I1 and I2 are interfaces and I2 extends I1 , then the code for I2 looks like this: interface I2 extends I1 { // contracts for methods added in I2 } I1 extends I2 7 January 2019 OSU CSE 12

Interface Extension Remember, for interfaces all such methods are instance methods! • If I1 and I2 are interfaces and I2 extends I1 , then the code for I2 looks like this: interface I2 extends I1 { // contracts for methods added in I2 } I1 extends I2 7 January 2019 OSU CSE 13

Interface Extension • If I1 and I2 are interfaces and I2 extends I1 , then the code for I2 looks like this: interface I2 extends I1 { // contracts for methods added in I2 } Other terminology for this I1 situation: I2 is a subinterface of I1 extends I2 is a derived interface of I1 I2 I2 is a child interface of I1 7 January 2019 OSU CSE 14

Interface Extension • If I1 and I2 are interfaces and I2 extends I1 , then the code for I2 looks like this: interface I2 extends I1 { // contracts for methods added in I2 } Other terminology for this I1 situation: I1 is a superinterface of I2 extends I1 is a base interface of I2 I2 I1 is a parent interface of I2 7 January 2019 OSU CSE 15

Example: Interface Extension clear newInstance Standard transferFrom + extends multiplyBy10 NaturalNumber- divideBy10 Kernel isZero = clear multiplyBy10 newInstance divideBy10 transferFrom isZero 7 January 2019 OSU CSE 16

Example: Interface Extension clear NaturalNumberKernel actually newInstance Standard has all these methods, even though transferFrom their contracts are in two separate + extends interfaces. multiplyBy10 NaturalNumber- divideBy10 Kernel isZero = clear multiplyBy10 newInstance divideBy10 transferFrom isZero 7 January 2019 OSU CSE 17

Example: Interface Extension clear The extends relation for interfaces newInstance Standard allows you to separate contracts into transferFrom smaller chunks — arguably a best + extends practice for component design. multiplyBy10 NaturalNumber- divideBy10 Kernel isZero = clear multiplyBy10 newInstance divideBy10 transferFrom isZero 7 January 2019 OSU CSE 18

Class Extension • For classes, extension can serve two different purposes: – To add method bodies that are not already in the class being extended (similar to the use of extension for interfaces) – To override methods that are already implemented in the class being extended, by providing new method bodies for them 7 January 2019 OSU CSE 19

Class Extension When pronounced, this may sound • For classes, extension can serve two like “overwrite”, but that is not a different purposes: correct interpretation! – To add method bodies that are not already in the class being extended (similar to the use of extension for interfaces) – To override methods that are already implemented in the class being extended, by providing new method bodies for them 7 January 2019 OSU CSE 20

Class Extension • For classes, extension can serve two For now, we are concerned only with this use of class extension. different purposes: – To add method bodies that are not already in the class being extended (similar to the use of extension for interfaces) – To override methods that are already implemented in the class being extended, by providing new method bodies for them 7 January 2019 OSU CSE 21

Class Extension Important note: Overriding a method is different from overloading a method! A method (name) is overloaded when two or • For classes, extension can serve two more methods have the same name, in which case the methods must differ in the number different purposes: and/or types of their formal parameters (which – To add method bodies that are not already in the compiler uses to disambiguate them). the class being extended (similar to the use of extension for interfaces) – To override methods that are already implemented in the class being extended, by providing new method bodies for them 7 January 2019 OSU CSE 22

Class Extension • If C1 and C2 are classes and C2 extends C1 , then the code for C2 looks like this: class C2 extends C1 { // code for methods added or // overridden in C2 } C1 extends C2 7 January 2019 OSU CSE 23

Class Extension Remember, for classes these may be either static methods or instance methods. • If C1 and C2 are classes and C2 extends C1 , then the code for C2 looks like this: class C2 extends C1 { // code for methods added or // overridden in C2 } C1 extends C2 7 January 2019 OSU CSE 24

Class Extension • If C1 and C2 are classes and C2 extends C1 , then the code for C2 looks like this: class C2 extends C1 { // code for methods added or // overridden in C2 } Other terminology for this situation: C1 C2 is a subclass of C1 C2 is a derived class of C1 extends C2 is a child class of C1 C2 7 January 2019 OSU CSE 25

Class Extension • If C1 and C2 are classes and C2 extends C1 , then the code for C2 looks like this: class C2 extends C1 { // code for methods added or // overridden in C2 } Other terminology for this situation: C1 C1 is a superclass of C2 C1 is a base class of C2 extends C1 is a parent class of C2 C2 7 January 2019 OSU CSE 26

Example: Overriding a Method ... power NaturalNumber2 ... extends NaturalNumber2- power Override 7 January 2019 OSU CSE 27

Example: Overriding a Method ... power NaturalNumber2 ... extends NaturalNumber2- power Override There is a method body for power in NaturalNumber2 ... 7 January 2019 OSU CSE 28

Example: Overriding a Method ... power NaturalNumber2 ... extends NaturalNumber2- power Override ... and there is another method body for power in NaturalNumber2Override . 7 January 2019 OSU CSE 29