Lesson 35..Inheritance

Transcription

1 Lesson 35..Inheritance 35-1 Within a new project we will create three classes BankAccount, SavingsAccount, and Tester. First, the BankAccount class: public class BankAccount public BankAccount(double amt) //Constructor balance = amt; public double getbalance( ) // You supply code here that returns the state variable, balance. public void deposit(double d) //You supply code here that adds d to balance. public void withdraw(double d) //You supply code here that subtracts d from balance. private double balance; Subclass and Superclass: This BankAccount class will be known as our Superclass. We will now create a SavingsAccount class that will be known as a Subclass. This SavingsAccount class is a perfect candidate to use as a subclass of BankAccount since it needs all the methods and state variable of the superclass, BankAccount. To make the SavingsAccount class inherit those methods and the state variable, use the key word extends as follows: public class SavingsAccount extends BankAccount public SavingsAccount(double amount, double rate) //Constructor super(amount); //Calls the constructor in interestrate = rate; //BankAccount and sets balance public void addinterest( ) double interest = getbalance( ) * interestrate / 100; deposit(interest); private double interestrate; There are some significant features of the constructor in SavingsAccount. In the absence of super(amount), it would have tried to automatically call the BankAccount constructor and would have failed, since that constructor requires a parameter and we would not have supplied one. By making super(amount) the first line of code, we are able to supply the needed parameter. When used, it must be the first line of code in the constructor.

2 There is also something interesting in the addinterest method above. Notice that we are calling the getbalance and deposit methods. This is completely legal even though they are not static methods and we are not accessing them with a BankAccount object. Why is it legal? It is because we have inherited these methods from BankAccount by virtue of the extends BankAccount portion of our class signature. Testing the subclass and superclass: And finally, we will create a class called Tester that we will use to test the interaction of our superclass and subclass: public class Tester public static void main(string[] args) //This begins a new account in which the initial balance is 200 // and the interest rate is 5%. SavingsAccount myaccount = new SavingsAccount(200, 5); //Make a deposit notice we use an inherited method, deposit myaccount.deposit(132.14); myaccount.addinterest( ); 35-2 //Here, we use another inherited method, getbalance System.out.println( Final balance is: + myaccount.getbalance( ) ); Important terms and ideas: Superclass the original class (sometimes called the base class) Subclass the one that says extends (sometimes called the derived class) abstract a. As applied to a class Example, public abstract class MyClass prevents objects from being instantiated from the class. Why would we want to do this? Perhaps the only way we would want our class used is for it to be inherited. b. As applied to a method Example, public abstract void chamfer( ); means that no code is being implemented for this method in this class. This forces the subclass that inherits this class to implement the code there. Note that the signature of an abstract method is immediately followed by a semicolon and that there can be no body (curly braces) for the method. If any method in a class is abstract, then that forces its class to be abstract too. final a. As applied to a class public final class MyClass means no other class can inherit this one.

3 35-3 b. As applied to a method public final void bisect( ) means it can t be overridden in a subclass. See the discussion below for the meaning of overriding. Overriding if a method is defined in a superclass and is also defined in a subclass then when objects are made from the subclass, the redundant method in the subclass will take precedence over the one in the superclass. This is overriding. There is a way to access a method in a superclass that has been overridden in the subclass. Let s suppose the method s signature in both classes is: public void trans(double x) From within some method of the subclass you can access the method trans in the superclass via a command like this: super.trans(15.07); private methods not inherited: Methods and state variables in the superclass that are designated as private are not inherited by the subclass. Shadowing is when a state variable in a subclass has a name identical to that of a state variable in the superclass. We do not say that the subclass variable overrides the other, rather that it shadows it. In such cases, uses within the subclass of the redundant variable give precedence to the subclass variable. It is, however, possible to access the shadowed variable in the superclass by creating a method in the superclass to access it. Suppose that the shadowed public variable in question is double y. Then, in the superclass create this method. public double gety( ) return y; Since this method is inherited by your subclass, use it to obtain the y value in the superclass. Assuming that an object created with your subclass is called myobj, consider the following code within the subclass: double d = myobj.gety( ); // returns y from the superclass double p = myobj.y; // returns y from the subclass There is also another type of shadowing. Let s look at a method that brings in the variable z as a parameter. Complicating things is a state variable also named z. public class MyClass... public void amethod(int z) z++; //This increments the local z which has precedence here within //this method. this.z = 19; //only way to access the state variable z from within //this method.... public int z;

4 35-4 Cosmic Superclass Every class that does not extend another class automatically extends the class Object (the cosmic superclass). Following are the signatures and descriptions of four of the most important methods of the Object class. Signature String tostring( ) boolean equals(object obj) Object clone( ) int hashcode( ) Description Returns a string representation of the object. For example, for a BankAccount object we get something like BankAccount@1a28362 Tests for the equality of objects. This tests to see if two variables are references to the same object. It does not test the contents of the two objects. Produces a copy of an object. This method is not simple to use and there are several pitfalls and is therefore, rarely used. Returns an integer from the entire integer range. In many classes it is commonplace to override the inherited methods above with corresponding methods that better suit the particular class. For example, the String class overrides equals so as to actually test the contents. Table 35-1 Creation of objects: Suppose we have a superclass called SprC and a subclass called SbC. Let s look at various ways to create objects: a. SbC theobj = new SbC( ); SprC anotherobj = theobj; Since anotherobj is of type SprC it can only access methods and state variables that belong to SprC; however, overridden methods will be accessed in SbC. b. SprC hallmark = new SbC( ); Since hallmark is of type SprC it can only access methods and state variables that belong to SprC; however, overridden methods will be accessed in SbC. c. SbC obj = new SprC( ); //illegal Expecting a particular object type: Any time when a parameter is expecting to receive an object of a particular type, it is acceptable to send it an object of a subclass, but never of a superclass. This is because the passed subclass object inherits all the methods of the object. Otherwise, the expected object may have methods not in a superclass object. Consider the following hierarchy of classes where each class is a subclass of the class immediately above it. Person Male Boy

5 35-5 Suppose there is a method with the following signature: public void themethod(male ml) The method themethod is clearly expecting a Male object; therefore, the following calls to this method would be legal since we are either sending a Male object or an object of a subclass: Male m = new Male( ); themethod(m); //ok to send m since it s expecting a Male object Boy b = new Boy( ); themethod(b); //ok to send b since b is created from a subclass of Male Since themethod is expecting a Male object, we can t send an object of a superclass. Person p = new Person( ); themethod(p); //Illegal themethod( (Male)p ); //Legal if we cast p as a Male object Using the same classes from above, the following examples illustrate legal and illegal object creation. Notice when we use a class on the left, the class on the right must be either the same class or a subclass. Person p = new Male( ); //legal Person p = new Boy( ); //legal Male m = new Boy( ); //legal Boy b = new Male( ); //illegal Boy b = new Person( ); //illegal Male m = new Person( ); //illegal instanceof This method tells us if an object was created from a particular class. Suppose Parent is a superclass, Child is one of its subclasses, objp is a Parent object, and objc is a Child object. Also, assume that Circle is some unrelated class. d. (objc instanceof Child) returns a true e. (objc instanceof Parent) returns a true f. (objc instanceof Circle) returns a false g. (objp instanceof Child) returns false h. (objp instanceof Parent) returns true i. (objp instanceof Circle) returns false Notice the syntax of instanceof is that an object precedes instanceof and a class, subclass or interface follows. The big picture: The following shows the function of each part in the declaration and creation of an object in which a superclass, subclass, or interface may be involved.

6 <class, superclass, or interface name> objectname = new <class or subclass name( )>; 35-6 This specifies the object type and what methods the object can use. This tells us where the methods are implemented that we are to use ( including the constructor(s) ). Fig Determining object type, legal methods, and where the methods are implemented. If a method in the subclass overrides that of the superclass, then code in the subclass runs. Inheritance is considered one of the most important, but, unfortunately, one of the most difficult aspects of Java. See Appendix U for an enrichment activity in which you would be able to participate in electronic communities in the form of message boards (forums). Investigate the questions and answers that other programmers post concerning this topic. Exercise (A) on Lesson 35 public class Red extends Green public int blue(double x)... public String s; private int i; public class Green public double peabody(double y) return mm; private boolean crunch( )... private double mm; public long xx; 1. Which of the above two classes is the base class? 2. Which of the above two classes is the subclass? 3. Which of the above two classes is the superclass?

7 Which of the above two classes is the derived class? 5. Is this legal? If not, why not? (Assume this code is in some class other than the two above) Red myobj = new Red( ); boolean bb = myobj.crunch( ); 6. Is this legal? If not, why not? (Assume this code is in some class other than the two above) Red myobj = new Red( ); int bb = myobj.blue(105.2); 7. Write code for the blue method that will printout the mm state variable. 8. Write code for the blue method that will printout the xx state variable. Use the following two classes for problems 9-12: public class Red extends Green public int blue(double x)... public double peabody(double vv) public String s; private int i; public class Green public Green(long j) xx = j; public double peabody(double y) return mm; private Boolean crunch( )... private double mm; public long xx;

8 Consider the following constructor in the Red class: public Red( ) //What code would you put here to invoke the constructor in the //superclass and send it a parameter value of 32000? 10. Is there any method in Red that is overriding a method in Green? If so, what is it? 11. Look at the peabody method inside Red. Write the code inside that method that will allow you to access the same method inside its superclass, Green. Let the parameter have a value of Consider the following constructor in the Red class: public Red( ) String s = Hello ; super(49); Is this code legal? If not, why not? 13. Assume that the following fragments of code are all in a subclass. Match each to an item from the sentence bank to the right. this(x,y) this.z super(j) super.calc( ) a. refers to a constructor in the superclass b. refers to a constructor in the subclass c. refers to an overridden method in the super class d. refers to a data member in the subclass

9 35-9 Exercise (B) on Lesson 35 The following code applies to problems 1-3: public abstract class Hammer public abstract void duty( ); public abstract int rule(int d); public class Lurch extends Hammer public void duty( ) int x = Y; public int rule( int d) Y = d + 1; return Y; private int Y = 30; private int x; 1. What is the purpose of making the two methods above abstract? 2. Write out the full signature of the rule method. 3. Which class actually implements the duty method? 4. A class for which you cannot create objects is called a (an) class. 5. public abstract class Felix... Is the following attempt at instantiating an object from the Felix class legal? If not, why? Felix myfelix = new Felix( );

10 Is the following legal? If not, why? public abstract class Lupe public abstract void fierce( )... public final double PI = 3.14; 7. What is the main reason for using abstract classes? 8. Modify the following class so it is impossible to make subclasses from it. public class MyClass Why would the following code be pointless? public final abstract class MyClass... //there are no static methods 10. public class ChevyChase public void Chicago(int x)... Modify the above code so as to make it impossible for a subclass that extends ChevyChase to override the Chicago method. 11. Is it possible to override instance fields (also called state variables)? 12. What is shadowing (as the term applies to superclasses and subclasses)?

11 35-11 The following code applies to problems 13 14, 18-20: public class Parent public void rubydoo( ) public int x = 0; public class Child extends Parent public void busterstein( ) public int x = 39; 13. Consider the following code in a Tester class: Child mychild = new Child( ); System.out.println(myChild.x); //What gets printed? 14. Consider the following code in a Tester class: Child mychild = new Child( ); Is there any way using the mychild object to retrieve the x state field within the Parent class? Write the code that will do this. You may write a new method for either class if you need to. 15. What is the name of the Cosmic Superclass? 16. What is the name of the class that every class (that does not extend another class) automatically extends? 17. What are the four main methods of the Object class?

12 Is the following legal? If not, why not? Child theobj = new Child( ); Parent newobj = theobj; newobj.busterstein( ); 19. Is the following legal? If not, why not? Child theobj = new Child( ); Parent newobj = theobj; newobj.rubydoo( ); 20. Is the following legal? If not, why not? Parent meatloaf = new Child( ); For problems 21-25, consider the following. In each problem either state what is printed or indicate that it won t compile: public class A public A (int x) this.x = x; public int f( ) return x; public int g( ) return x; public int x;

13 35-13 public class B extends A public B (int x, int y) super(x); this.x = y; public int f( ) return x + g( ); public int zorro( ) return x + g( ); public int x; 21. A a = new B(5, 10); System.out.println(a.g( )); 22. A a = new B(5, 10); System.out.println( a.f( ) ); 23. A a = new B(5, 10); System.out.println( a.x ); 24. B a = new B(5, 10); System.out.println( a.x ); 25. A a = new B(5, 10); System.out.println( a.zorro( ) ); *********************************************************** 26. Consider the classes Food, Cheese, and Velveeta where Cheese is a subclass of Food and Velveeta is a subclass of Cheese. State which of the following lines of code are legal. Cheese c = new Food( ); Velveeta v = new Food( ); Cheese c = new Velveeta( ); Food f = new Velveeta( ); Food f = new Cheese( );

14 Inheritance Contest Type Problems 1. What replaces <*1> and <*2> in the code to the right to indicate that objects cannot be instantiated and that the methods are not being defined? A. <*1>: abstract <*2>: abstract B. <*1>: abstract <*2>: final C. <*1>: final <*2>: abstract D. <*1>: final <*2>: final 2. The interest earned on a loan is the product of 1/12, the principle, the rate, and the months. What replaces <*3> in the code to the right to correctly compute the interestearned( ) method? A. months / 12 * rate * ad.getprinciple( ) B. months * rate * ad.getprinciple( ) / 12 C. Loan.months /12 * Loan.rate * ad.getprinciple D. months * rate * ad.getprinciple( ) * (1/12) E. More than one of these 3. Assume that the class Info is a subclass of AccountDetails and has a constructor which receives a double and a String. Which of the following builds a Loan p object with rate.07, months 4, and principle $450? A. Loan p (.07, 4, new Info(450, Bob )); B. Loan p = Loan(.07, 4, new Info(450, Bob )); C. Loan p = new Loan(.07, 4, Info(450, Bob )); D. Loan p = new Loan(.07, 4, new Info(450, Bob )); 4. What is output by the code below? Parent pr = new Parent(7); System.out.print(pr.work( )); A. 1 B. 0 C. 3 D What is output by the code below? Parent pr = new Child(4, 11); System.out.print(pr.work( )); A. 11 B. 4 C. 1 D public <*1> AccountDetails public <*2> double getprinciple( ); public <*2> String getname( ); public <*1> class Financial public <*2> double interestearned( ); public <*2> double paymentdue( ); public class Loan extends Financial public Loan(double rate, int months, AccountDetails ad) this.rate = rate; this.months = months; this.ad = ad; public double interestearned( ) return <*3>; public double paymentdue( ) //code not shown private double rate; private int months; AccountDetails ad; public class Parent public Parent(int q) this.q = q; public int work( ) return q; private int q; public class Child extends Parent public Child(int q, int y) super(q); this.y = y; public int work( ) return y + super.work( ); private int y;

15 6. What replaces <*1> in the code to the right that causes Z to inherit class A? A. implements A B. subclass of A C. subclass of class A D. extends A E. inherits A 7. Which of the following replaces <*1> in the code to the right so that the default constructor builds a Triangle object with base 2 and altitude 5? A. this(2, 5); B. Triangle (2, 5); C. super(2, 5); D. this(base) = 2; this(altitude) = 5; E. More than one of these 8. Assume that <*1> has been filled in correctly. Which of the following returns the area of EquilateralTri et? A. (EquilateralTri)et.area( ) B. et.super.area( ) C. et.(equilateraltri)area( ) D. et.area( ) 9. Given a Triangle tri that is initialized to hold a Triangle and an EquilateralTri et that is initialized to hold a Triangle, which of the following expressions evaluates to true? A. Triangle instanceof EquilateralTri B. tri instanceof et C. tri instanceof EquilateralTri D. Triangle instanceof Object 10. Suppose st is a Street object. Which of these is a valid call to method House.getInfo( ) using st as an argument? A. st.getinfo(town t) B. House.getInfo( (Town)st ) C. House.getInfo( Town(st) ) D. House.getInfo(Town.st) 11. Suppose st is a Street object. What is the value of this expression? st instanceof Town A. 0 B. true C. 1 D. false public class Z <*1> //methods and data not shown public class Triangle public Triangle( ) <*1> public Triangle(int bs, int alt) base = bs; altitude = alt; public double area( ) return.5 * base * altitude; private int base; private int altitude; public class EquilateralTri extends Triangle public EquilateralTri (int s) super(s, s * Math.sqrt(3)/2); this.s = s; private int s; public class Town //code not shown public class Street extends Town //code not shown public class House public static void getinfo(town t) //code not shown 35-15

16 12. Given the declarations below, which of the following expressions is true? Car cr = new Car( ); Chevy chv = new Chevy( ); Lumina lm = new Lumina( ); A. cr instanceof Chevy B. chv instanceof Lumina C. cr instanceof Lumina D. lm instanceof Car E. More than one of these 13. Suppose that the static method dostuff( ) of class Engine takes a parameter of type Lumina. Given the declarations below, which of these is a valid call to dostuff( )? Car cr = new Lumina( ); Chevy chv = new Lumina( ); Lumina lm = new Lumina( ); A. Engine.doStuff(chv) B. Engine.doStuff(cr) C. Engine.doStuff(lm) D. dostuff((car)lm) public class Car //methods and data not shown public class Chevy extends Car //methods and data not shown public class Lumina extends Chevy //methods and data not shown Suppose that Insect is an abstract class, that Bee is a class that extends Insect, and that Drone is a class that extends Bee. Given the following declaration, which of these is true? Drone d = new Drone( ); A. d instanceof Object B. d instanceof Insect C. d instanceof Bee D. d instanceof Drone E. All of these 15. If class Man is a subclass of class Person, what is the syntax for calling a private method of Person named meth( ) from within a private method of Man? A. this.meth( ) B. meth( ) C. super.meth( ) D. super( meth( ) ) 16. Which of the following replaces <*1> in the code to the right to call the constructor for the Pasta class with the parameter g? A. this(g); B. super(g); C. x.super( ); D. Pasta(g); public class Spaghetti extends Pasta public Spaghetti(int g, int h) <*1> remaining code not shown