7: Inheritance

What We Will Cover Reminders: Please silence all cell phones and pagers When class is over, please shut down your computer if it is on. Continuations 7.1: Introduction to Inheritance 7.1.1: Introducing Inheritance 7.1.2: Inheritance Example 7.1.3: Class Hierarchies 7.1.4: Adding Methods to Subclasses 7.1.5: Constructors and Inheritance 7.1.6: Using Access Modifiers 7.1.7: Final Classes, Methods and Parameters 7.1.8: Summary Exercise 7.1 7.2: Working With the Object Class 7.2.1: Methods of the Object Class 7.2.2: Determining an Object's Type 7.2.3: Casting Objects 7.2.4: Overriding the equals() Method 7.2.5: Overriding the hashCode() Method 7.2.6: Summary Exercise 7.2 7.3: Midterm Review 7.3.1: About the Midterm Exam 7.3.2: Recommended Preparation 7.3.3: Exam Taking Tips 7.3.4: Questions and Answers Wrap Up Assignments Due Next: A6-Card Games (3/24/10) Midterm Exam (4/7/10) Illuminations Questions from last class? Homework Questions? Note: I added the missing lab exercises! A6-Card Games (3/24/10) Homework Discussion Questions How difficult was it to work with enums? What type of sorting methods did people implement? ^ top 7.1: Introduction to Inheritance Objectives At the end of the lesson the student will be able to: Describe how inheritance is used in Java Use inheritance in your programs Inherit from Java's classes ^ top 7.1.1: Introducing Inheritance Inheritance: the ability to define new classes from existing ones. Inheritance is one of the important ideas of object-oriented programming It lets you code a class that is based on another class When used correctly, inheritance can simplify the overall design As you will discover, Java uses inheritance extensively in its API How Inheritance Works A class that is inherited from is called the superclass (a.k.a. base, parent) A class that inherits is called the subclass (a.k.a. derived, child) The subclass automatically contains all the variable and method definitions of the superclass In a subclass, you can define new member variables and methods not found in the superclass The two classes in the following diagram show how this works Here the superclass is a JFrame in the javax.swing package You use a JFrame to create a GUI window, which is called a frame The class has several public constants and methods used to set-up the JFrame For instance, setSize() lets you set the width and height of a JFrame In the HelloFrame subclass you can use all the constants and methods inherited from JFrame In addition, you can add new fields, constants and methods The example has added constants and a constructor New fields, constants and methods are specific to the subclass About the Diagram The first rectangular section of a class diagram contains the class name The second section describes the attributes (fields or variables) The third (bottom) section describes the operations (methods) A minus sign (-) marks attributes and operations that cannot be accessed by other classes A plus sign (+) marks attributes and operations that can be accessed by other classes Parameters are listed in parenthesis separated by commas The upward pointing arrow indicates inheritance ^ top 7.1.2: Class Hierarchies You can derive classes from derived classes Subclasses become superclasses for their descendants Child classes become parent classes in turn When you have multiple levels of subclasses, you create an inheritance hierarchy Classes higher in the hierarchy are called ancestor classes Classes lower in the hierarchy are called descendent classes A class can use the variables and methods of any of its superclasses For example, a Frame uses the variables and methods provided by both Window and Container Note that inheritance describes an is-a relationship For instance: a Dialog is a Window This is in contrast to the has-a relationship, where a class contains a reference to another class Diagram Showing an Inheritance Hierarchy The following diagram shows some classes from the Java API From the diagram, you can see that a superclass may have more than one subclass For instance, Window has two subclasses: Frame and Dialog Another Way to Draw Inheritance There are many ways to draw inheritance diagrams The following is another way to show the inheritance hierarchy for Container Note how the superclass is always drawn higher than the subclass Container | +--Panel | | | +--Applet | +--Window | +--Frame | +--Dialog Designing with Inheritance Note in the inheritance diagram above that a Frame is a type of Window When using inheritance in an application, it is important to keep "is a" in mind Make sure a subclass has an is-a relationship with the superclass A subclass must be a type of a superclass Which of the following are valid is-a relationships? Car ==> Vehicle Car ==> Motorcycle Student ==> Person Student ==> College Substitute the words "is a type of" for the arrow (==>) ^ top 7.1.3: Inheritance Example To deriving a new class from an existing one, you use the extends clause Syntax: [accessModifier] class SubclassName extends SuperclassName A subclass can only extend one superclass Java does not support multiple inheritance Interfaces (discussed later) achieve much of the same effect Example Superclass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 public class Person { private String name; public Person() { name = "No name yet"; } public Person(String initialName) { name = initialName; } public String getName() { return name; } public void setName(final String newName) { name = newName; } public void printAttributes() { System.out.println("Name: " + name); } public boolean sameName(Person other) { return (name.equalsIgnoreCase(other.name)); } private final void special() { System.out.println("I am special"); } } Example Subclass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 public class Student extends Person { private int studentNumber; public Student() { super(); studentNumber = 0; } public Student(String name, int number) { super(name); studentNumber = number; } public Student(String initialName) { this(initialName, 0); } public int getStudentNumber() { return studentNumber; } public void printAttributes() { super.printAttributes(); System.out.println("Student Number : " + studentNumber); //special(); } } Example Test Application 1 2 3 4 5 6 7 8 9 10 11 public class StudentApp { public static void main(String[] args) { Student noName = new Student(); Person ed = new Student("Ed", 10); Student buford = new Student("Buford"); noName.printAttributes(); ed.printAttributes(); buford.printAttributes(); } } ^ top 7.1.4: Adding Methods to Subclasses We added a member variable to our Student subclass public class Student extends Person { private int studentNumber; A Student has this attribute in addition to a name name is inherited from Person Also, we added a new instance method to the subclass: public int getStudentNumber() { return studentNumber; } A Student has this method in addition to all the methods of the superclass Overriding Methods In addition to adding new members, we can change the way a works in a subclass Changing the way a method works in a subclass is known as overriding a method We overrode this method in our Student subclass public void printAttributes() { System.out.println("Name: " + getName()); System.out.println("Student Number : " + studentNumber); } Both superclass and subclass has a printAttributes method Both methods have the same parameters (i.e. none) Thus, they have the same signature The method from the subclass overrides the superclass method Note that a subclass method will NOT override a superclass method if the parameters are different Would have different signatures Sometimes people confuse overriding with overloading See the table below for the similarities and differences Note that you can call an overridden method of the superclass from a subclass To do so, you use the keyword super as shown here: public void printAttributes() { super.printAttributes(); System.out.println("Student Number : " + studentNumber); } Overriding Verses Overloading Overriding Overloading Same method name Same method name Same signature One method in superclass, one in subclass Different signature Both methods can be in same class ^ top 7.1.5: Constructors and Inheritance Constructors are not inherited by a subclass Instead, superclass constructors are called by the subclass constructor Superclass constructors are called either implicitly or explicitly using super() Examples of Subclass Constructors Here is a no-parameter constructor for Student: public class Student extends Person { private int studentNumber; public Student() { super(); studentNumber = 0; } ... The first statement of the constructor is super() which calls the constructor of the superclass After the superclass constructor finishes, this constructor then initializes the attribute studentNumber to 0 We could have left out the call to super() because Java automatically includes it if not explicitly stated If you want to call a different constructor, then you use super() with arguments of the correct type: ... public Student(String name, int number) { super(name); studentNumber = number; } This constructor passes the parameter name to the constructor of the superclass After which the Student() constructor initializes the instance variable studentNumber Note that super() must be first action in a constructor definition More About Subclass Constructors Constructors cannot be overridden because they are not inherited However, as we have seen before, constructors can be overloaded within the same class Another capability of constructors is that they can call other constructors -- known as chaining To call another constructor, you use the construct this() Like a call to super(), Java will call the constructor (within the same class) that matches the types of arguments with the constructor parameters Here is an example using this() and super() together Example of this() and super() Constructor Call Class Student has a constructor with two parameters: String for the name attribute int for the studentNumber attribute public Student(String name, int number) { super(name); studentNumber = number; } Another constructor within Student takes just a String argument Initializes the studentNumber attribute to a value of 0: public Student(String initialName) { this(initialName, 0); } Calls the constructor having two arguments within the same class Note that you can call either this() or super() in a constructor, but not both Whichever is used, it must be the first action taken by the constructor Only one of them can be first, so if you want to invoke both: Use a call with this() to call a constructor using super() ^ top 7.1.6: Using Access Modifiers You use the keywords public, private and protected to control access to class members Controls access to class instance variables and methods Only a particular definition (unlike C++) public Access: Interface Access Least restrictive of all access modifiers public fields and methods can be accessed from anywhere the class is accessible: public class MyClass { public double x; public void foo() {} } private Access: Don't Touch That! Most restrictive of all access modifiers private fields and methods cannot be accessed from outside of the class Good design practice to make all variables private and to provide accessor methods where needed Auxiliary ("helper") methods are declared private often as well Generally do not declare a class private -- how would you use it? public class MyClass { private double x; private void foo() {} } protected Access: Inheritance Access Accessible in the package of this class and any subclass More restrictive than public accessibility but less restrictive than default public class MyClass { protected double x; protected void foo() {} } Package Access: the Default No access modifier: known as default or package accessibility Only accessible to other classes in the same package (directory) Less restrictive than private accessibility, but more restrictive than public or protected access class MyClass { double x; void foo() {} } ^ top 7.1.7: Final Classes, Methods and Parameters The keyword final is used in many ways Final Classes Use final to prevent a class from being extended This is an important part of creating immutable classes For example: public final class String {} String is an example of an immutable class from the Java API Final Methods Specifies that a method definition cannot be overridden with a new definition in a subclass For example: private final void special() { System.out.println("I am special"); } Since you would never want the method to do anything else, it makes sense to make it final Allows the compiler to generate more efficient code Final Parameters Use final in a parameter list to prevent a method from assigning a new value For example: public void doSomething(final int i, final int j) { //... } This can prevent a bug where you might set new values to the parameter variables in error The keyword final is not part of the method signature and does not affect how a method is overridden ^ top 7.1.8: Summary You can use inheritance to create a subclass (derived, child) Inherits fields and methods from a superclass (base, parent) Inheritance allows you to develop hierarchies of classes Subclasses can have access to fields and methods of the hierarchy Deriving a new class from an existing one requires the extends clause public class SubclassName extends SuperclassName You can define new fields and methods in a subclass You can call constructors of the superclass using the super() method super(name); Also, you can chain constructors together using this() Accessibility is controlled using keywords: public, private and protected If a method in a subclass has the same signature as a method in the superclass, the method from the subclass overrides (replaces) the superclass method Will not override the superclass if the parameters are different Would have a different signature You can prevent classes form being extended with the final modifier You can prevent methods from being overridden using the final modifier as well ^ top Exercise 7.1 Take one minute to prepare an answer the following question: Given the following code, which one of the following constructors could be added to MySub without causing a compile time error? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 class MySuper { int number; MySuper(int i) { number = i; } } public class MySub extends MySuper { int count; MySub(int cnt, int num) { super(num); count = cnt; } // Add new constructor here } MySub() {} MySub(int cnt) { super(); count = cnt; } MySub(int cnt) { count = cnt; super(cnt); } MySub(int cnt) { this(cnt, cnt); } MySub(int cnt) { super(cnt); this(cnt, 0); } ^ top 7.2: Working With the Object Class Objectives At the end of the lesson the student will be able to: Use methods of the Object class Override methods of the Object superclass Write code to cast objects up and down inheritance chains ^ top 7.2.1: Methods of the Object Class Every class implicitly extends java.lang.Object This makes the Object class the ancestor of all Java classes Including all programmer-defined classes Thus, every class you write has access to the fields and methods of the Object class That means you need to know how to work with the Object class The following is a summary of some of the methods of the Object class Method clone() is complicated and I refer you to the textbook, chapters 8 and 13, for instructions Overriding finalize() is usually unnecessary and should be avoided We already looked at overriding toString(), which you should do for almost every class you write In the following sections we will look at how to make use of the other methods Object Class Methods Method Description clone() Returns a copy of this object as an Object object. You must implement the Cloneable interface to use this method. (Cover later) equals(Object obj) Returns true if this object refers to the same space in memory as another object. Otherwise, returns false even if the other object contains the same data. finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. getClass() Returns a Class object that represents the class of this object. hashCode() Returns a hash code value for the object, which is supported for the benefit of hashtables. toString() Returns a String object containing the class name followed by an @ symbol and the memory location (in hexadecimal) for the object. ^ top 7.2.2: Determining an Object's Type To work with methods of the Object class, we need to know how to check the type of an object Many times you have a parameter of type Object For instance, when you use the equals() method to compare two objects public boolean equals(Object obj) Also, some methods, like clone(), return an Object type protected Object clone() To work with these methods, you need to know the actual type of the object Java has two main ways of determining this information: instanceof operator Run-time type identification (Reflection API) Using the instanceof Operator The instanceof operator checks if a test object is in the inheritance hierarchy of a class type Syntax: testObject instanceof ClassName Where: testObject: the object to test ClassName: the class to compare again For example: Person ed = new Student("Ed", 10); System.out.println(ed instanceof Student); instanceof returns true if the testObject is a descendant of any ClassName Using Reflection You can use reflection to determine the exact type of an object When Java runs an application, it keeps track of all the objects it loads using a class named Class For each object it loads, Java creates a Class object that has information about the object The getClass() method of Object returns the run-time class of an object For example: Person ed = new Student("Ed", 10); Class info = ed.getClass(); System.out.println(info); Thus, you can get the Class information about any object Among other methods, the Class object has a method named getName() that returns the name of the type You can use these methods to determine the exact type of an object: System.out.println(info.getName()); More Information Trail: The Reflection API ^ top 7.2.3: Casting Objects Once you know what type of object you have, you may need to cast objects to their actual type This allows you to use the methods of the more specialized object Following is the inheritance chain for Student Java will implicitly cast an object up the inheritance chain (upcasting) However, you must explicitly cast an object down the inheritance chain (downcasting) Student ed = new Student("Ed", 1); Object obj = ed; // cast Student to Object Student ed2 = (Student) obj; // cast Object to Student Following shows how casting affects the methods you can call Object obj = new Student("Ed", 1); String studStr = obj.toString(); // OK // String name = obj.getName(); // does not compile Student ed = (Student) obj; // cast Object to Student String name = ed.getName(); // OK Once we cast obj to Student, we can use the getName() method Note that if you try to cast to an incompatible type you will get a compiler error String str = (String) ed; // compiler error ^ top 7.2.4: Overriding the equals() Method To test if two objects refer to the same location, you use the equals() method of the Object class For example: 1 2 3 4 5 6 7 8 9 10 public class EqualsTestApp { public static void main(String[] args) { Person p1 = new Person("Ed"); Person p2 = new Person("Ed"); if (p1.equals(p2)) System.out.println("true"); else System.out.println("false"); } } This is often not the behavior you want and then you will need to override the equals() method When overriding the equals() method, you need to make sure that: It returns false if the argument is null: p1.equals(null) == false; It returns true or false consistently if the objects are used multiple times without changing It returns true if an object is compared to itself: p1.equals(p1) == true; It returns true only if an object and the argument of the equals method can be interchanged and still return true: p1.equals(p2) == p2.equals(p1); For any three reference variables p1, p2 and p3, if p1.equals(p2) == true and p2.equals(p3) == true, then p1.equals(p3) == true. This may look daunting but is quite straightforward As an example, we build a correct equals() method for Person: public boolean equals(Object obj) We want our Person to work if the passed reference is null: if (obj == null) { return false; } We also want our equals() to work for any kind of object reference Thus, if the passed object is not the same kind, we return false To test the exact type, we use the getClass() method: if (getClass() != obj.getClass()) { return false; } If our test object passes these cases, then we compare all the instance variables of our class for equality To do this, we must first cast our object to the actual type and then make the comparisons: Person p = (Person) obj; if (name.equals(p.getName())) { return true; } return false; Putting all this together we have: public boolean equals(Object obj) { if (obj == null) { return false; } else if (getClass() != obj.getClass()) { return false; } else { // now safe to cast Person p = (Person) obj; if (name.equals(p.getName())) { return true; } } return false; } ^ top 7.2.5: Overriding the hashCode() Method If you override equals(), then you should override the hashCode() method as well. Otherwise, your code will not work correctly with the hash-based classes of the Java Collection like HashTable A hashtable is like an array but allows non-integer indexes You will learn about hashtables later For now, this is information we will refer back to when we discuss hashtables When an object is inserted in a hashtable, the hashCode() method is called The value returned by hashCode() determines where to insert the object in the table Producing an optimal hash code can be tricky, but we can produce a good approximation using the following recipe The recipe is from the book, Effective Java by Joshua Bloch pp. 36-41 (see more information below) Note that computing a hash code for a class may be more complex than shown below if the equals() method is complex If you have a complex equals() method you should read the chapter linked to in the More Information section below Simple Hash Code Recipe Store a nonzero constant value like 42 in an int variable named result int result = 17; // arbitrary nonzero value For every instance variable x used in the comparison do the following: Compute an int hash code named c for the instance variable using the following: If the type is a boolean, compute: c = x ? 1 : 0 If the type is byte, char, short or int, compute: c = (int) x If the type is long, compute: c = (int) (x ^ (x >>> 32)) If the type is float, compute: c = Float.floatToIntBits(x) If the type is double, compute: long t = Double.doubleToLongBits(d); and then compute the hash code for a type long If the type is a class, compute: c = (x == null) ? 0 : x.hashCode(); If the type is an array, compute the hash code for every element of the array and combine the elements as shown in step 2b Combine the hash code c computed in step 2a as follows: result = 31 * result + c; Example Class Computing Hash Codes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 public class HashCodeRecipe { private boolean b = true; private char ch = 'X'; private long bow = 424242424242L; private float f = 1.234F; private double d = 1.234; private String str = "Hi Mom!"; private int[] a = {90, 95, 87, 89, 98}; public int hashCode() { final int ARBITRARY_NONZERO_VALUE = 17; int result = ARBITRARY_NONZERO_VALUE; int c = 0; // temporary hash code c = b ? 1 : 0; result = 31 * result + c; c = (int) ch; result = 31 * result + c; c = (int) (bow ^ (bow >>> 32)); result = 31 * result + c; c = Float.floatToIntBits(f); result = 31 * result + c; long t = Double.doubleToLongBits(d); c = (int) (t ^ (t >>> 32)); result = 31 * result + c; c = str.hashCode(); result = 31 * result + c; for (int i : a) { c = (int) i; result = 31 * result + c; } return result; } public static void main(String[]args) { HashCodeRecipe recipe = new HashCodeRecipe(); System.out.println(recipe.hashCode()); } } For our Person class, we can use the hashCode() of the String object: public int hashCode() { int result = 17; // arbitrary nonzero value result = 37 * result + name.hashCode(); return result; } More Information Methods Common to All Objects from the book Effective Java ^ top 7.2.6: Summary Every class extends from Object Thus you can use methods from Object in your own classes You can cast an object up and down its inheritance chain Will not lose any data stored in the object You can determine the actual type of an Object reference using either the: instanceof operator getClass() method You can override methods of the Object class to improve the behavior of your class Object method toString() is almost always overridden in subclasses Return a String describing your object data Object method equals() is overridden when you do not like the default behavior When you override equals(), you must override hashcode() Hash codes are tricky to optimize but we covered a "simple" recipe ^ top Exercise 7.2 Take one minute to prepare an answer the following question: Given the following code, what will be printed? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 public class InstanceTester { public static void main(String[] args) { B b = new C(); A a = b; if (a instanceof A) System.out.println("A"); if (a instanceof B) System.out.println("B"); if (a instanceof C) System.out.println("C"); if (a instanceof D) System.out.println("D"); } } class A {} class B extends A {} class C extends B {} class D extends C {} A will be printed B will be printed C will be printed D will be printed The Person class we covered previously has the following instance variable: public class Person { private String name; // methods omitted } Using the hash code recipe we discussed, what is an acceptable hashCode() method for class Person? Check answer ^ top 7.3: Midterm Review Learner Outcomes At the end of the lesson the student will be able to: Describe how to prepare for the midterm exam Describe how to take the midterm exam ^ top 7.3.1: About the Midterm Exam Important Midterm Exam Information Date and Time: 04/07/10 @ 12:40 PM to 2:00 PM Location: Blackboard (Online) You must take the exam at the specified time or you will receive a score of zero (0) Except by prior arrangement with the instructor I am using Blackboard to administer the test Do not come to the classroom for the test Instead, find a computer with a fast Internet connection (such as the CTC) The exam is designed to take about one hour to complete However, you will have 75 minutes during the scheduled time to complete the exam You may have less time if you are late to the exam Since the exam is online, it is open book and open notes and you can use an editor and a compiler or even an IDE You are honor bound NOT to communicate with anyone but the instructor during the exam; thus: You may NOT call anyone but the instructor on a phone during the exam You may NOT use the Internet to email anyone but the instructor during the exam The instructor will be online and monitoring Blackboard during the exam Thus you may call or email the instructor during the exam about any questions or problems you have with the exam You will be able to view scores only after the instructor finishes grading ^ top 7.3.2: Recommended Preparation Try the Example Midterm in Blackboard These questions are intended to help you get a "feel" for taking an exam in Blackboard The questions are NOT intended to tell you everything that is on the exam Review your notes and prepare your 3" x 5" card Review your homework assignments and solutions You should be prepared to write short programs ^ top 7.3.3: Exam Taking Tips Save after every answer -- you can always choose another answer If you get stuck on a question, make your best guess and return later If you are equally uncertain between two choices, go with first impression You do not need to comment code for tests and exams Unless specifically instructed to in the exam question Use the full time available Check your work if you finish early ^ top 7.3.4: Questions and Answers Any questions? ^ top Wrap Up Due Next: A6-Card Games (3/24/10) Midterm Exam (4/7/10) ^ top Home | Blackboard | Schedule | Room Policies | Syllabus Help | FAQ's | HowTo's | Links Last Updated: April 11 2010 @21:40:11