DCO10103 Object-Oriented Programming and Design

Inheritance and Polymorphism

Java allows an object to be referred to by a reference variable of an ancestor type
recall that everything is an Object
we can add(Object, Employee, Boss, CommissionWorker, PieceWorker, or HourlyWorker) to the Vector PayrollDB
we can also calculate earning of Boss, CommissionWorker, PieceWorker, or HourlyWorker using ((Employee) payrollDB.get(i)).earnings()
instance methods from the ancestor that are overridden by a descendant's method are not accessed when the ancestral reference is used, the descendant's copy is accessed
e.g. payrollDB.toString() will use the toString() method of PayrollDB, not Vector.toSting(), or Object.toString()
however, the casting is required in ((Employee) payrollDB.get(i)).earnings() because the Object class does not have a method called earnings()
access to hidden instance variables and hidden static methods is different, the ancestor's versions are accessed when a ancestor reference is used
Example: RealPointTester and ChildTester
overloading Vs overriding

comparing the two Student classes

    public boolean equals(Student student)             // overloading equals()
    {
        return getNumber() == student.getNumber();
    }

Student implements Comparable

    public boolean equals(Object obj)                 // overriding Object.equals()
    {
        if(obj instanceof Student)
        {
            Student student = (Student)obj;
            return getNumber() == student.getNumber();
        } else
        {
            return false;
        }
    }

    public int compareTo(Object obj)                  // overriding Comparable.compareTo()
        throws ClassCastException
    {
        int i = getGrade();
        int j = ((Student)obj).getGrade();
        return i != j ? i >= j ? 1 : -1 : 0;
    }

it is your job to ensure correct reference is used e.g. in "((Student)obj).getGrade()", obj is indeed a Student object which has the getGrade() method or else ClassCastException will be resulted

handling ClassCastException

Range  ran = new  Range();
Account  acc = new  Account();
try  {
	if (ran.equals((Range) acc))  {
		stdOut.println("Execution would not get here...");
	}
} catch(ClassCastException  cce)  {
	stdOut.println(cce.toString());
}

making use of Inheritance and Polymorphism Store anything (Collection) and Sort everything (Comparable)

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