Object-oriented Programming in Java

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

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();

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 new HourlyEmployee() instanceof Employee is true  Often used before downcasting To avoid an exception To avoid an exception

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 Calls the function corresponding to the type the reference indicates at the moment Invisibly to the program Invisibly to the program  Employee e = new HourlyEmployee(); e.computePay(); e = new SalariedEmployee(); e.computePay(); // A different function!  Example: Figure 2

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

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

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 Both for classes and methods

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

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

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

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)); } int x, yColor color Instance variables in parent class, Point: Instance variable in child class, ColorPoint:

Floating-point Fields  Don’t compare for equality with ==! Has to do with special values (NaN, etc.) 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 ==

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) Boolean: (f ? 0 : 1) byte, char, or short: (int) f byte, char, or short: (int) f Long: (int)(f ^ (f>>>32)) Long: (int)(f ^ (f>>>32)) Float: Float.floatToIntBits(f) Float: Float.floatToIntBits(f) Double: Double.doubleToLongBits(f) Double: Double.doubleToLongBits(f) Combine with prime numbers: h = 37*f1; h = 37*h;… Combine with prime numbers: h = 37*f1; h = 37*h;… Objects: combine their hashcodes (see pp ) Objects: combine their hashcodes (see pp )