Basic -2 Classes and Objects

1. Basic -2Classes and Objects

2. Classes and Objects • A class is a complex data TYPE • An object is an instance of a class. • Example: • Class: Person • Objects: several Person instances are all around you

3. Trivial Example: Class Person • A Person has some properties • Attributes or fields • In addition it may have some behavior • methods • A java class defines properties and behavior common to all people (all Person objects) • Each person gets their own field copies

4. Class Person - example class Person { String name; int height; // in inches int weight; // in pounds // we’ll worry about methods later void setWeight(int newWeight) { } void setHeight(int newHeight){ } }

5. Objects • An object is an instance of a class type • An object variable is a reference to the object’s data structure • Object variable can be seen as a pointer • Declaring an object variabile != creating an object • Declaration provides a null pointer • Must create an object with the newkeyword • Calls a constructor method of the class (optionally with arguments) • Dynamic allocation Variable declaration Person p1; p1 = new Person(); p1.name = “ your name”; Object instantiation

6. Object Variables • Multiple variables can point to same object • A variable can point to NO object • its value is null • NOTE: passing an object as parameter in a method actually passes a copy of the reference • Method manipulates the same referenced object Person p1, p2; p1=new Person(); p1.name = “John”; p2=p1; System.out.println(p1.name); System.out.println(p2.name); { Person p3; p3=new Person(); p3.name = “Dan”; p2=p3; System.out.println(p2.name); }

7. Illustration name: “John” height: 0 weight: 0 p1 p2 name: “Dan” height: 0 weight: 0 p3 Java graciously initializes primitive fields for you

8. Object creation caveats • What if we don’t use new? Person p; System.out.println(p.name); H:>javac Person.java Person.java:15: Variable p may not have been initialized. System.out.println(p.name); ^ 1 error

9. Second Try Person p = new Person(); p = null; System.out.println(p.name); • Compilation works fine but … H:>java Person Exception in thread "main" java.lang.NullPointerException: at Person.main(Person.java:16)

10. Object equality: == • RECALL: variables are references Person p1 = new Person(); Person p2 = new Person(); p1.name = “Jane”; p2.name = “Jane”; If(p1 == p2) System.out.println(“Same!"); else System.out.println(“Different!"); : Person “Jane ” p1 : Person “Jane” p2

11. More on equality: equals() • All classes are equipped with a method equals() • For comparing objects of same class • Can be (re-)defined wrt class logic boolean equals(Person otherPerson) { if name.equals(otherPerson.name) { System.out.println(“Same!"); return true; } System.out.println(“Different!"); return false; }

12. Object Destruction • Programmer cannot explicitly destroy an instantiated object • JVM takes care of “garbage collection” • Implicitly frees up memory from “useless” objects • Object is useless when program keeps no more references to it • e.g when program exits the context where object was created (block, method variables) • Or when variables point to other objects or are assigned the null value

13. Let’s play around with Person … p1.weight = 200; // bad but dealable p1.weight = 700; // unfortunate, but possible p1.weight = -20; // unreal Solution: Have class modify attribute. Allows sanity checks. Provide that behavior through a method p1.setWeight(700); // OK. Weight is now 700. p1.setWeight(-20); *** Error, weight must be positive number // weight still 700

14. Solution? … class Person { … void setWeight(int newWeight) { if (newWeight < 0) System.err.println( “*** Error, weight must be positive number”); else weight = newWeight; }

15. … New problem p1.setWeight(-20); *** Error, weight must be positive number p1.weight = -20; // Yo, I’m the boss Assigning weight directly bypasses sanity check by the setWeight() method. We still have a problem. Solution: change attribute visibility to make the bare weight attribute inaccessible from outside Person

16. New solution Class Person { private String name; private int height; // in inches private int weight; // in pounds public void setWeight(int newWeight) { if (newWeight < 0) System.err.println( “*** Error, weight must be positive number”); else weight = newWeight; } } Within same class no dot notation and no visibility issues

17. New solution in action class Test { public static void main(String args[]) { Person p1 = new Person(); p1.weight = 20; } } >javac Test.java >Test.java:4: Variable weight in class Person not accessible from class Test. p1.weight = 20; ^ 1 error Denied!

18. Finish up Need to add a getWeight(), since we can no longer access weight directly public int getWeight(void) { return weight; } Also, need get() and set() functions for name and height.

19. Accessor Functions for Encapsulation • Generally make fields private and provide publicgetField() and setField() accessor functions. • For this course – ALWAYS • unless there’s a specific reason not to • Usually there isn’t • Need to justify explicitly otherwise! • Encapsulation keeps programs tidy • Enables clear definition of interactions between classes • Public parts of a class represents its Application Programming Interfaces (API) • Private parts hide details of the class implementation • If interface remains stable, even if implementation changes, the rest of application is unaffected • Example: height in cm. and weight in Kg.

20. Point - float x,y + Point(float, float) + move(float dx, float dy) + String desc() More on Visibility /** Represents a 2d geometric point */ package 2d; class Point { private float x,y; // coordinates public Point(float coordX, float coordY){ x=coordX; y=coordY; } public void move(float dX, float dY){ x+=dX; y+=dY; } public String desc(){ return “Point (" + x + ", " + y + ")"; } } • Methods / Attributes • public • private • package (default) • protected Constructor with parameters At-a-glance look at the class API

21. Constructors public class Point { private float x,y; public Point() { x=y=0; // same as default } public Point(float coordX, float coordY) { x=coordX; y=coordY; } public Point(float xy) { this(xy,xy); } } • Multiple constructors with different sets of parameters • No need to specify result type • new operator invoked with parameters matching those of some constructor • If programmer provides no constructor, Java provides default constructor (no parameters) Constructor chaining

22. Default Constructor • Takes no parameters, initializes everything to defaults • 0, null etc. • Graciously provided by Java for absent-minded programmers ;-) • Only exists if there are no explicitly defined constructors • Any constructor, even one taking parameters, means no default

23. Overloading • Multiple constructors is a prominent example of overloading • Multiple methods can have same name but different signatures public class Point { private float x,y; public void move(float dx, float dy){ x+=dx; y+=dy; } public void move(float dxy){ move(dxy, dxy); } }

24. Strings • Strings in Java are objects of class java.lang.String • Java.lang is a fundamental library • String has a lot of nice features: check it out in the javadocs! char characters[] = {‘H', ‘e', ‘l', 'l', 'o'}; String salutation=new String(characters); System.out.println(salutation); System.out.println(salutation.length()); String s = “ World”; salutation += s; System.out.println (salutation);

25. Comparing Strings • if (s1 == s2) {…. • NO! • if (s1.equals(s2)) {… • Prominent case of equals() vs. ==

26. Turning objects into Strings • All classes have a method toString() to provide a textual description of objects • Can be (re-)defined wrt class logic public class Person { ... public String toString() { String ret; ret = name + “ is tall ”; ret += height; ret += “ inches and weighs ”; ret += weight; ret += “ pounds.” return ret; } ... }

27. Turning objects into Strings • toString()can be invoked implicitlly • When object is used within String manipulation operations Person assistant=new Person(“Mike”); System.out.println(“the assistant is: "+ assistant);

28. Introduction to the Java language What is Java, what is a Java program Style, Javadocs Language basics Types, variables Classes, objects, arrays Visibility and encapsulation Some utilities String, toString() System.out Summary