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AP JAVA NOTES: Chapter 10 INHERITANCE AND POLYMORPHISM Static Variables and Methods: When are static variables and methods used? Example of a static variable: Example of a static (class) method: Inheritance – a relationship defined between objects that share characteristics. A subclass can itself be a superclass for another subclass, leading to an inheritance hierarchy of classes. Example: The arrow designates the is-a relationship. Person Employee Student GradStudent UnderGrad The inheritance relationship between a subclass and a superclass is specified in the declaration of the subclass, using the keyword extends. Example: public class Person { public String myName; public String myID; … } public class Student extends Person { public final static int NUM_TESTS = 3; private int [] myTests; private String myGrade; //constructors public Student () { super(); myTests = new int [NUM_TESTS]; myGrade = “”; } public Student (String name, int[]tests, String grade) { super (name); myTests = tests; myGrade = grade; } public void setGrade (String newGrade ) { myGrade = newGrade;} public computeGrade () { if myName.equals(“”)) myGrade = “No grade”; else if (getTestAverage() >=65) myGrade = “Pass”; else myGrade = “fail”;} public double getTestAverage() { //Calculates and returns the average of all tests } } public class UnderGrad extends Student { public Undergrad() {super();} public class GradStudent extends Student { private int myGradID; //default constructor public GradStudent() { super(); myGradID=0; } //constructor public GradStudent (String name, int[]tests, String Grade, int gradID) { super (name, tests, grade); myGradID = gradID; } public int getID() {return myGradID; } public void computeGrade () {super.computeGrade(); if (getTestAverage() >90) setGrade (“Pass with Honors”); } public Undergrad (String name, int []tests, String grade) { super (name, tests, grade);} public void computeGrade() { if (getTestAverage () >=70) setGrade (“Pass”); else setGrade (“Fail”); } } } Rules for Subclasses: • A subclass can add new private instance variables. • A subclass can add new public, private or static methods. • A subclass can override inherited methods. • A subclass may not redefine a public method as private. • A subclass may not override static methods of the superclass. • A subclass must define its own constructors. • A subclass cannot access the private members of its superclass. Declaring Subclass Objects: When a variable of a superclass is declared in a client program, that reference can refer not only to an object of the superclass, but also to objects of any of its subclasses. Thus each of the following is legal: Student s = new Student(); Student g = new GradStudent(); Student u= new UnderGrad(); Since Student is not necessarily a GradStudent nor an UnderGrad, the following declarations are not valid: GradStudent g = new Student (); Undergrad u = new Student(); Valid Declarations: Student s = new Student (“Brian Sea”, new int[] {90, 94, 99}, “none”); Student g = new GradStudent (“Kevin Carter”, new int [] {87, 89, 90}, “none”); What happens when the following method calls are implemented? s.computeGrade(); g.computeGrade(); u.computeGrade(); Polymorphic: A method that has been overridden in at least one subclass. Polymorphism – the mechanism of selecting the appropriate method for a particular object in a class hierarchy. ** With polymorphism, the programmer can deal in generalities and let the execution-time environment concern itself with the specifics. The correct method is chosen because, in Java, method calls are always determined by the type of the actual object, not the type of the object reference. EXAMPLE: public class StudentTest { public static void computeAllGrades (Student [] studentList) { for (int j = 0; j