9/23/15. Objec-ves. Assignment 2 Review. Review STATIC METHODS AND FIELDS. Sta-c Methods/Fields. Sta-c Methods

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1 Objec-ves Sta-c methods, variables Garbage collec-on Parameter passing in Java Inheritance Assignment 2 Review private int onevar; public Assign2(int par) { onevar = par; Is the above code correct? Sept 23, 2015 Sprenkle - CSCI209 1 Sept 23, 2015 Sprenkle - CSCI209 2 Review What method should we implement to allow premy prin-ng of objects we define? What method should we implement for determining if two objects are equivalent? STATIC METHODS AND FIELDS Sept 23, 2015 Sprenkle - CSCI209 3 Sept 23, 2015 Sprenkle - CSCI209 4 Sta-c Methods/Fields For related func-onality/data that isn t specific to any par-cular object java.lang.math Ø No constructor (what does that mean?) Ø Sta-c fields: PI, E Ø Sta-c methods: static double sin(double a) Sta-c Methods Do not operate on objects Cannot access instance fields of their class Can access sta$c fields of their class Similar to Python func$ons that are associated with the class Sept 23, 2015 Sprenkle - CSCI209 5 Sept 23, 2015 Sprenkle - CSCI

2 Review: Sta-c Fields A sta-c field is used when only one such field per class (not object!) All objects of a class share one copy of the sta-c field Review: Constant Sta-c Fields We used a sta-c field to designate a class constant: public class Converter { public static final double CM2IN = 2.54; The Math class has a sta-c constant, PI Ø Value can be accessed using the Math class: area = Math.PI * r * r; Ø Do not need an object of the Math class to use this constant Sept 23, 2015 Sprenkle - CSCI209 7 Sept 23, 2015 Sprenkle - CSCI209 8 Sta-c Fields Example public class Student { private int id; private static int nextid = 1; Each Student object has an id field There is only one nextid field, shared among all instances of the class Ø nextid field exists even when no Student objects have been constructed Sta-c Field Example public class Student { private static int nextid = 1; private int id = assignid(); private static int assignid() { int id = nextid; nextid++; return id; How could we use the nextid field to create unique IDs? Sept 23, 2015 Sprenkle - CSCI209 9 Sept 23, 2015 Sprenkle - CSCI Why is main sta-c? main() Most common sta-c method main() does not operate on any objects Ø Runs when a program starts there are no objects yet main() executes and constructs the objects the program needs and will use Ø Like the driver func$on for the program Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI

3 Analyzing java.lang.string String touppercase() Ø Converts all of the characters in this String to upper case static String valueof(boolean b) Ø Returns the string representa-on of the boolean argument Why can the second method be static? Sta-c Summary Sta-c fields and methods are part of a class and not an object Ø Do not require an object of their class to be created in order to use them When would we make a method static? Ø When a method does not have to access an object s state (fields) because all needed data are passed into the method Ø When a method only needs to access sta-c fields in the class Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI Explain the Error Representa-ve Error: error: non-static method mymethod() cannot be referenced from a static context Explain the Error Representa-ve Error: error: non-static method mymethod() cannot be referenced from a static context mymethod was called from a sta-c context Ø E.g., from the class (not an object) Ø From a sta-c method Can t then call a method for an object Ø Where did that object come from? Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI Summary: Class Design/Organiza-on Fields Ø Chosen first Ø Placed at the beginning or end of class defini-on Ø Have an access modifier, data type, variable name, and some op-onal other modifiers Ø Use this keyword to access the object Constructors Methods Ø Need to declare the return type Ø Have an access modifier Ø Decide if for the class or for object GARBAGE COLLECTION Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI

4 Memory Management In C++ and some other OOP languages, classes have explicit destructor methods that run when an object is no longer used Java does not support destructors because it provides automa+c garbage collec+on Ø Waits un-l there are no references to an object Ø Reclaims memory allocated for the object that is no longer referenced Do you know what Python does? Garbage Collector Garbage collector is low- priority thread Ø Or runs when available memory gets -ght Before GC can clean up an object, the object may have opened resources Ø Ex: generated temp files or open network connec-ons that should be deleted/closed first GC calls object s finalize() method Ø Object s chance to clean up resources Discussion: Benefits and limitations of garbage collection? Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI Garbage Collec-on Benefits Fewer memory leaks Ø Less buggy code Ø But, memory leaks are s-ll possible Code is easier to write Limita@ons Garbage collec-on may not be as efficient as explicit freeing memory finalize() Inherited from java.lang.object Called before garbage collector sweeps away an object and reclaims its memory Should not be used for reclaiming resources Ø i.e., close resources as soon as possible Ø Why? When method is called is not determinis-c or consistent Only know it will run some-me before garbage collec-on Clean up anything that cannot be atomically cleaned up by the garbage collector Ø Close file handles, network connec-ons, database connec-ons, etc. Note: no finalizer chaining Ø Must explicitly call parent object s finalize method Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI Alterna-ves to finalize Recall: unknown when finalize will execute or if it will execute Ø Also heavy performance cost Solu-on: create your own termina-ng method Ø User of class terminates when done using object Examples: File s or Window s close method May s-ll want a finalize method as a safety net if user didn t call the terminate method Ø Log a warning message so user knows error in code PARAMETER PASSING Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI

5 Method Parameters in Java Java always passes parameters into methods by value Ø Methods cannot change the variables used as input parameters Ø A subtle point, so we need to go through several examples Python is something that s not quite pass- by- value it depends on if the object is mutable or immutable Ø Pass- by- alias is one term used Sept 23, 2015 Sprenkle - CSCI Method Parameters in Java public static void main(string[] args) { int x = 10; int squared = square(x); System.out.println("The square of " + x + " is " + squared); public static int square(int num) { return num*=num; Draw the stack as it changes (similar to Python): main x 10 Sept 23, 2015 Sprenkle - CSCI public static void main(string[] args) { int x = 27; doublevalue(x); static void doublevalue(int p) { p = p * 2; public static void main(string[] args) { int x = 27; doublevalue(x); static void doublevalue(int p) { p = p * 2; Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI Pass by Value: Objects Primi-ve types are a limle more obvious Ø Can t change original variable For objects, passing a copy of the parameter looks like public void methodname(chicken c) Pass Chicken object to methodname when calling method methodname(chicken); chicken = height height = = 45 c = Sept 23, 2015 Sprenkle - CSCI Pass by Value: Objects What happens in this case? methodname(chicken); chicken = c = public void methodname(chicken c) { if( c.getweight() < MIN ) { c.feed(); height height = = 45 Sept 23, 2015 Sprenkle - CSCI Does chicken change in calling method? 5

6 Pass by Value: Objects What happens in this case? methodname(chicken); chicken = c = public void methodname(chicken c) { if( c.getweight() < MIN ) { c.feed(); height height = = 45 Sept 23, 2015 Sprenkle - CSCI Does chicken change in calling method? YES! Both chicken and c are pointing to the same object Farm farm = new Farm("OldMac"); Chicken sal = new Chicken("Sallie Mae", 5, 23); farm.feedchicken(sal); // From Farm class Sept 23, 2015 Sprenkle - CSCI Farm farm = new Farm("OldMac"); Chicken sal = new Chicken("Sallie Mae", 5, 23); farm.feedchicken(sal); // From Farm class c = new Chicken(c.getName(), c.getweight(), c.getheight() ); Sept 23, 2015 Sprenkle - CSCI Tracing through Execu-on Farm farm = new Farm("OldMac"); Chicken sal = new Chicken("Sallie Mae", 5, 23); farm.feedchicken(sal); // From Farm class c = new Chicken(c.getName(), c.getweight(), c.getheight() ); sal = height height = = 23 c = Sept 23, 2015 Sprenkle - CSCI Tracing through Execu-on c = new Chicken(c.getName(), c.getweight(), c.getheight() ); sal = height height = = 23 name name = "Sallie = Mae" Fred 5 Summary of Method Parameters Everything is passed by value in Java An object variable (not an object) is passed into a method Ø Changing the state of an object in a method changes the state of object outside the method Ø Method does not see a copy of the original object c = x0affbf 5 height height = = 23 name name = "Sallie = Mae" Fred Sept 23, 2015 Sprenkle - CSCI Sept 23, 2015 Sprenkle - CSCI

7 To Do Assignment 4a Ø Wri-ng sta-c methods Sept 23, 2015 Sprenkle - CSCI