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Announcements Lab 8 Was Due Today Lab 7 Regrade Due Thursday
(no regrades) Lab 7 Regrade Due Thursday No class Friday
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Today in COMP 110 Possible solution for Program 4
Inheritance recap from last time More Inheritance
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Program 4
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Inheritance Define a general class
Later, define specialized classes based on the general class These specialized classes inherit properties from the general class
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Inheritance Hierarchies
Put these classes into an inheritance hierarchy Crocodile Human Reptile Animal Mammal Whale Animal Reptile Mammal Crocodile Human Whale
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The is-a relationship This inheritance relationship is known as an is-a relationship A Doctoral student is a Grad student A Grad student is a Student A Student is a Person Is a Person a Student? Not necessarily!
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The Keyword extends public class Derived_Class_Name extends Base_Class_Name { Declaration_of_Added_Instance_Variables Definitions_of_Added_And_Overridden_Methods } public class Student extends Person { // stuff goes here A derived (child) class inherits the public instance variables and public methods of its base (parent) class
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Overriding Methods Java handles this situation as follows:
If a derived class defines a method with the same name, number and types of parameters, and return type as a method in the base class, the derived class’ method overrides the base class’ method The method definition in the derived class is the one that is used for objects of the derived class
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Overriding methods Why is it useful?
You often want derived classes to perform custom behavior public class Shape { public void draw(Graphics g) { } public class Circle extends Shape { public void draw(Graphics g) { g.drawOval(…arguments…); } public class Rectangle extends Shape { public void draw(Graphics g) { g.drawRect(…arguments…); }
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The Keyword super Last time we saw that the keyword super could be used to invoke the constructor of the base class public class Person { private String name; public Person() { name = "No name yet"; } public class Student extends Person { private int id; public Student() { super(); id = -1; } Student student = new Student(); //name is set to No name yet, and id is set to -1
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The Keyword super The use of super must always be the first action taken in the constructor Not required to be specified, but if it is, it must be first Java will automatically call the default constructor of the base class if super is not specified public Student() { super(); id = -1; } public Student() { id = -1; } public Student() { id = -1; super(); } OK OK ERROR
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The Keyword super You cannot use repeated supers
If specified, specify only once public Student() { super(); id = -1; } public Student() { super(); super("No name yet"); id = -1; } OK ERROR
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The Keyword super What if the base class is a derived class?
public class Person { private String name; public Person() { name = "No name yet"; } public class Student extends Person { private int id; public Student() { super(); id = -1; } public class Undergraduate extends Student { private int year; public Undergraduate() { super(); //calls constructor of Student, not Person year= 0; }
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Method Overriding public class Person { // ... public void printInfo() { System.out.println("Name: " + name); } public class Student extends Person { // ... public void printInfo() { System.out.println("Name: " + getName()); System.out.println("ID: " + getID()); } Overridden Is there a way to call the printInfo() method of the class Person from within the class Student? Yes
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Calling Overridden Methods
public class Person { // ... public void printInfo() { System.out.println("Name: " + name); } public class Student extends Person { // ... public void printInfo() { super.printInfo(); System.out.println("ID: " + getID()); } } The keyword super can also be used to call overridden method of a base class
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Inheritance and Overloading
public class Person { private String name; public void setData(String newName) { name = newName; } public class Student extends Person { private int id; public void setData(String newName, int newID) { setData(newName); id = newID; } Overloaded Methods with a different number or type of parameters are overloaded, not overridden
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Type Compatibilities Given this inheritance heirarchy… Person Athlete
HighJumper ExtremeAthlete Skydiver XGamesSkater
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Is This Code Legal? Person p = new Person(); Yes! Person Athlete
HighJumper ExtremeAthlete Skydiver XGamesSkater
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Is This Code Legal? HighJumper h = new HighJumper(); Yes! Person
Athlete HighJumper ExtremeAthlete Skydiver XGamesSkater
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Is This Code Legal? Person p = new Athlete();
Yes! An Athlete is a Person, so this is okay Person Athlete HighJumper ExtremeAthlete Skydiver XGamesSkater
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Is This Code Legal? Skydiver s = new Person();
No! A Person is not necessarily a Skydiver, so this is illegal Person Athlete HighJumper ExtremeAthlete Skydiver XGamesSkater
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Is This Code Legal? Athlete ath = new Athlete(); XGamesSkater xgs = ath; No! An Athlete is not necessarily an XGamesSkater, so this is illegal Person Athlete HighJumper ExtremeAthlete Skydiver XGamesSkater
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When to Use Inheritance
Use inheritance when an is-a relationship is present A Student is a Person, Student inherits from Person Do not confuse an is-a relationship with a has-a relationship A Student has a date of enrollment, so an object of class Date, should be an instance variable of the Student class
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The Class Object Every class in Java is derived from the class Object (implied) Every class in Java is an Object Object Animal Person Reptile Mammal Student Employee Crocodile Human Whale
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Dynamic Binding Person person = new Person("John Smith"); person.printlnfo(); //calls printInfo of Person class Student student = new Student("John Smith", ); student.printlnfo(); //calls printInfo of Student class Person p = new Student(“Tom Jones", ); p.printInfo(); //which printInfo method is called? The printInfo method of the Student class is called because p is really a student!
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The Operator instanceof
We can test whether an object is of a certain class type using the instanceof operator: public void objectType(Person p) { if(p instanceof Student) { System.out.println("p is an instance of the class Student"); } else if(p instanceof GradStudent) { System.out.println("p is an instance of the class GradStudent"); … Syntax: object instanceof Class_Name
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The Class Object The Java class Object provides methods that are inherited by every class For example equals, toString These methods should be overridden with methods appropriate for the classes you create
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Printing Objects to Screen
The method println is overloaded to accept objects as an argument System.out.println(int i) System.out.println(double d) System.out.println(Object o) When printing an object with System.out.println(Object o), the object’s toString() method is called and the result is printed public void println(Object o) { System.out.println(o.toString()); //dynamic binding }
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Overriding toString public class Person { private String name;
public Person() { name = "No name yet"; } public void setName(String newName) { name = newName; //converts a Person object to a String, overrides toString in the class Object public String toString() { return "Name: " + name + "\n"; public static void main(String[] args) { Person person = new Person(); person.setName("John Smith"); System.out.println(person); //prints "Name: John Smith" to screen (without quotes)
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Overriding toString public class Student extends Person {
private int id; public Student() { super(); id = -1; } public void setID(String newName) { name = newName; //converts a Person object to a String, overrides toString in the class Person public String toString() { return "Name: " + name + "\nID: " + id + "\n"; public static void main(String[] args) { Student student = new Student(); student.setName("John Smith"); student.setID( ); System.out.println(student); //prints "Name: John Smith" // "ID: " to screen (without quotes)
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Programming Demo Programming with inheritance
Sports Define the following classes Person Athlete HighJumper ExtremeAthlete Skydiver XGamesSkater
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Sports Each class has a method called jump Person Athlete HighJumper
Simply prints a message to screen Person System.out.println("Whee!"); Athlete System.out.println("I jump well!"); HighJumper System.out.println("I jump REALLY well!"); ExtremeAthlete System.out.println("EXTREMEEEEEEEEEE JUMP!"); SkyDiver System.out.println("EXTREMEEEEEEEEEE fall!"); XGamesSkater System.out.println("I'm doing some sort of 360 or such. Tally-ho.");
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Sports Write a class called Sports that demonstrates TypeCompatibility
Dynamic binding Instanceof operator
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Friday No recitation, enjoy the Holidays
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