Object-oriented Programming in Java

1. Object-oriented Programming in Java

2. What is OOP? • The goal is (subtype) polymorphism • Achieved by • Classes (user-defined types) • Inheritance (is-a, substitutability) • Dynamic Function Binding • Function to run is bound according to the dynamic type of the object

3. Upcasting • Storing a reference to a subtype in a supertype variable • Makes sense because of the is-a relationship • Allows any subtype object to be used in a context expecting a supertype object • Employee e = new SalariedEmployee();

4. The instanceof operator • Returns true if the left operand is the same type or a subtype of the right operand • new HourlyEmployee() instanceof Employee is true • Often used before downcasting • To avoid an exception

5. Polymorphism • Dynamic variation of behavior • According to an object’s dynamic type • Calls the function corresponding to the type the reference indicates at the moment • Invisibly to the program • Employee e = new HourlyEmployee();e.computePay();e = new SalariedEmployee();e.computePay(); // A different function! • Example: Figure 2

6. Benefits of OOP • High degree of separation among components • Low coupling • Insulates components from changes in other components • Example: payroll program • Uses the Employee interface • The dynamic types are invisible

7. Downcasting • When an object must be interpreted as a subtype • When extracting from a collection, for example • Must use with care • The object may not be the type you’re expecting • Checked at runtime • Examples: Figures 4, 5

8. Abstract Classes • Not meant to be instantiated • Define an interface • Also define part of the implementation • Subclasses complete the implementation • abstract keyword • Both for classes and methods

9. java.lang.Object • The Mother of all Classes • All objects are subtypes of Object • Object methods: • int hashCode( ); • boolean equals(Object); • String toString( ); • Among others…

10. Object.toString • Provides a String representation of an object • Prints the class name and the hashCode • Should override

11. Object.equals(Object) • Used for a value-based equality test • Override carefully: • Test == this first • if true, return true • Test instanceof TheClass • if false, return false • Then check all the fields • Use equals() recursively for object fields • Can super.equals() for inherited fields • Examples: ColorPoint (next slide) and SuperEquals.java • Always override hashCode() if you override equals()

12. ColorPoint public boolean equals(Object other) { if (this == other) return true; else if (!(other instanceof ColorPoint)) return false; ColorPoint p = (ColorPoint) other; return (super.equals(p) && p.color.equals(this.color)); } Instance variables in parent class, Point: Instance variable in child class, ColorPoint: int x, y Color color

13. Floating-point Fields • Don’t compare for equality with ==! • Has to do with special values (NaN, etc.) • Convert floats to int with Float.floatToIntBits • Convert doubles to long with Double.doubleToLongBits • Compare the resulting integers with ==

14. hashCode • Used to associate an object with an int • Used in hash tables and other containers • Should be “as unique as possible” • Rules: • Boolean: (f ? 0 : 1) • byte, char, or short: (int) f • Long: (int)(f ^ (f>>>32)) • Float: Float.floatToIntBits(f) • Double: Double.doubleToLongBits(f) • Combine with prime numbers: h = 37*f1; h = 37*h;… • Objects: combine their hashcodes (see pp. 197-198)