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Introduction to Inheritance in Object-Oriented Programming

Learn about inheritance, how to inherit and override superclass methods, invoke superclass constructors, and understand the common superclass Object and its toString method.

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Introduction to Inheritance in Object-Oriented Programming

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  1. COMP 121 Week 3: Inheritance

  2. Objectives • To learn about inheritance • To understand how to inherit and override superclass methods • To be able to invoke superclass constructors • To understand the common superclass Object and to override its toString method

  3. Introduction to Inheritance • Inheritance • Used with object types that have a certain amount of commonality • Mountain bikes, racing bikes, road bikes, and recumbent bikes all share characteristics common to bicycles • Squares, triangles, rectangles, and octagons all share characteristics common to polygons • Deans, professors, administrators, and technical support personnel all share characteristics common to university employees • Surgeons, family doctors, pediatricians, heart specialists all share characteristics common to doctors • Savings accounts, checking accounts, money market accounts all share characteristics common to bank accounts

  4. Introduction to Inheritance • Inheritance relationship • Mountain bike inherits from bicycle • Triangle inherits from polygon • Professor inherits from university employee • Pediatrician inherits from doctor • Savings account inherits from bank account • Classes are extended by adding methods and fields • Savings account is a bank account with interest • Instance field would be added for the interest rate • Method would be added to add interest to the account

  5. Inheritance vs. Implementing an Interface • Inheriting from class is not the same as implementing an interface • Subclass inherits behavior and state • Like interfaces, one advantage of inheritance is code reuse Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  6. Inheritance Diagram • Every class extends the Object class either directly or indirectly Inheritance Inheritance Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  7. Inheritance Diagram (BlueJ) Inheritance

  8. UML Diagram (BlueJ) Uses Is-A Inheritance

  9. Inheritance Syntax class SubclassName extends SuperclassName{ methods    instance fields } • The extended class is called the superclass • The extending class is called the subclass Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  10. Question: • If the class Car extends Vehicle, which is the subclass and which is the superclass? Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  11. Subclass versus Superclass Superclass Subclass

  12. Inheritance Example public class SavingsAccount extends BankAccount { // Fields and methods unique to a SavingsAccount … } • BankAccount is the superclass • SavingsAccount is the subclass • SavingsAccount automatically inherits all methods and instance fields of BankAccount Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  13. Subclass Fields and Methods • In subclass: • Add new instance fields and methods • Change or override methods public class SavingsAccount extends BankAccount { public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } private double interestRate; } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  14. Subclass Object • SavingsAccount object inherits the balance instance field from BankAccount • Additional instance field is interestRate Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  15. Encapsulation and Inheritance public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } • Encapsulation • addInterest calls getBalance rather than updating the balance field of the superclass (field is private) • addInterest calls getBalance without specifying the implicit parameter • Call applies to the same object (this) Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  16. Inheritance Hierarchies • Sets of classes can form complex inheritance hierarchies Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  17. Inheritance Hierarchies: Swing Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  18. Hierarchy of Bank Accounts • Consider a bank that offers its customers the following account types: • Checking account: no interest; small number of free transactions per month, additional transactions are charged a small fee • Savings account: earns interest that compounds monthly Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  19. Hierarchy of Bank Accounts Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  20. Hierarchy of Bank Accounts Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  21. Subclass vs. Superclass Methods • All bank accounts support the getBalance method • All bank accounts support the deposit and withdraw methods, but the implementations differ (checking account needs to count transactions) • Checking account needs a method deductFees • Savings account needs a method addInterest Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  22. Overriding vs. Inheriting Methods • Overriding a method • Supply a different implementation of a method that exists in the superclass • Must have same signature (same name and same parameter types) • If method is called on an object of the subclass type, the overriding method is executed Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  23. Overriding vs. Inheriting Methods • Inheriting a method (behavior) • Don't supply a new implementation of a method that exists in superclass • Superclass method can be called on the subclass objects Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  24. Defining New Methods • Add a method • Supply a new method that doesn't exist in the superclass • New method can be called only on subclass objects Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  25. Inheriting Instance Fields • Can't override fields • All fields from the superclass are automatically inherited • New fields that don't exist in the superclass can be added to the subclass Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  26. Inheriting Instance Fields • What if you define a new field with the same name as a superclass field? • Each object would have two instance fields of the same name • Fields can hold different values • Legal but extremely undesirable • DON’T DO IT!!! Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  27. CheckingAccount Implementation • Overrides deposit and withdraw to increment the transaction count public class CheckingAccount extends BankAccount { public void deposit(double amount) {. . .} public void withdraw(double amount) {. . .} public void deductFees() {. . .} // new method private int transactionCount; // new instance field } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  28. CheckingAccount Implementation • Each CheckingAccount object has two instance fields: • balance (inherited from BankAccount) • transactionCount (new to CheckingAccount) Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  29. CheckingAccount Implementation • You can apply four methods to CheckingAccount objects: • getBalance() (inherited from BankAccount) • deposit(double amount) (overrides BankAccount method) • withdraw(double amount) (overrides BankAccount method) • deductFees() (new to CheckingAccount) Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  30. Accessing Private Fields in Superclass • Consider deposit method of CheckingAccount • Can't just add amount to balance • balance is a private field of the superclass public void deposit(double amount) { transactionCount++; // now add amount to balance . . . } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  31. Accessing Private Fields in Superclass • A subclass has no access to private fields of its superclass • Subclass must use public interface So how can we add the amount to the balance? Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  32. Invoking a Super Class Method • Can't just calldeposit(amount)in deposit method of CheckingAccount • That is the same as this.deposit(amount) • Calls the same method (infinite loop) • Instead, invoke superclass methodsuper.deposit(amount) Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  33. Invoking a Superclass Method public void deposit(double amount) { // Increment the transaction count transactionCount++; // Now add amount to balance super.deposit(amount); } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  34. Subclass Construction • The super keyword is also used to call a constructor • super followed by parameters in parentheses indicates a call to the superclass constructor • Must be the first statement in subclass constructor • If subclass constructor doesn't explicitly call superclass constructor, the default superclass constructor is used • Default constructor -- constructor with no parameters • If all superclass constructors require parameters, it’s a compile error error Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  35. Subclass Construction public class CheckingAccount extends BankAccount { public CheckingAccount(double initialBalance) { // Construct superclass super(initialBalance); // Initialize transaction count transactionCount = 0; } . . . } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  36. Converting Between Subclass and Superclass Types • Can convert a subclass reference to superclass reference SavingsAccount collegeFund = new SavingsAccount(10); BankAccount anAccount = collegeFund; Object anObject = collegeFund; • The three object references stored in collegeFund, anAccount, and anObject all refer to the same object of type SavingsAccount Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  37. Converting Between Subclass and Superclass Types Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  38. Converting Between Subclass and Superclass Types • Superclass references don't know about methods in the subclass • When you convert a subclass object to its superclass type • The value of the reference stays the same (actual memory location of the object) • Less information is known about the object anAccount.deposit(1000); // OK anAccount.addInterest(); // Not OK Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  39. Converting Between Subclass and Superclass Types • Why would anyone want to know less about an object? • Reuse code that knows about the superclass but not the subclass: • Can be used to transfer money from any type of BankAccount public void transfer(double amount, BankAccount other) { withdraw(amount); other.deposit(amount); } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  40. Converting Between Subclass and Superclass Types • Occasionally you need to convert from a superclass reference to a subclass reference • If object types are unrelated (not a superclass/ subclass relationship), an exception is thrown at runtime BankAccount anAccount = (BankAccount) anObject; Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  41. instanceof operator • Tests whether an object belongs to a particular type • Returns true if the object on the left hand side of the operator is an instance of the type on the right hand side of the operator • Otherwise, returns false if (anObject instanceof BankAccount) { BankAccount anAccount = (BankAccount) anObject; . . . } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  42. Object: The Cosmic Superclass • All classes defined without an explicit extends clause automatically extend Object Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  43. Useful Methods in the Object Class • String toString() • boolean equals(Object otherObject) • Object clone() • Good idea to override these methods in your classes • Will not be overriding clone() in this course Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  44. Overriding the toString Method • Returns a string representation of the object • Useful for debugging Rectangle box = new Rectangle(5, 10, 20, 30); String s = box.toString(); // Sets s to "java.awt.Rectangle[x=5,y=10,width=20,height=30]" Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  45. Overriding the toString Method • toString is called whenever you concatenate a string with an object: • Object.toString default behavior is to print the class name and the hash code of the object "box=" + box; // Result: "box=java.awt.Rectangle[x=5,y=10,width=20,height=30]" BankAccount momsSavings = new BankAccount(5000); String s = momsSavings.toString(); // Sets s to something like "BankAccount@d24606bf" Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  46. Overriding the toString Method public String toString() { return "BankAccount[balance=" + balance + "]"; } • In BankAccount: • Converts BankAccount object to a String • Used by System.out.println BankAccount momsSavings = new BankAccount(5000); String s = momsSavings.toString(); // Sets s to "BankAccount[balance=5000]" Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  47. Inheritance and toString Method • toString method often returns the class name and the names and values of the instance fields in a class • In a superclass, to be usable by subclasses, get the class name instead of hard-coding it • For example, in BankAccount: public String toString() { return getClass().getName()+"[balance =" + balance + "]";} Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  48. Inheritance and toString Method • In a subclass, first get the string that represents the superclass, then add the subclass information • For example, in SavingsAccount: public String toString() { return super.toString() + "[interestRate = " + interestRate + "]"; } Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  49. Summary • Inheritance is used to extend existing classes by adding methods and fields • The more general class is called the superclass • The more specialized class that inherits from the superclass is called the subclass • The subclass inherits both state and behavior from the superclass • Code reuse is one advantage of inheritance Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

  50. Summary (continued) • Methods in the superclass can be changed or overridden in the subclass • A subclass cannot access the private fields of the superclass • The super keyword is used to call a method of the superclass • The super keyword may also be used as the first statement of the subclass constructor to call the superclass constructor Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

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