Reference Types

2 Objectives Introduce reference types –class –array Discuss details of use –declaration –allocation –assignment –null –parameter –aggregation Introduce garbage collection

3 Reference Variable declaration of class type gives reference –handle only –no object created declaration yields references Stock ibm; Stock sun; ibm sun class Stock {... } class type references, not objects

4 Instance Instance of class types is reference/object pair –object created using new –accessed through reference name price shares name price shares ibm sun declare references Stock ibm; Stock sun; ibm = new Stock(); sun = new Stock(); allocate objects

5 Assignment Assignment of reference types copies only the reference –does not create new object name price shares ibm s Stock ibm = new Stock(); Stock s; s = ibm; now refer to same object

6 Reassignment Reference may be reassigned –can refer to different objects at different times –not required to stay bound to same object Stock ibm = new Stock(); Stock sun = new Stock(); Stock s; s = ibm;... s = sun;... s refers to ibm s refers to sun

7 Parameters Parameters of reference type pass only reference –do not copy object Typical to pass reference by value –but can pass ref or out if needed void Liquidate(Stock s) { s.Sell(s.shares); } Stock ibm = new Stock();... Liquidate(ibm); parameter is reference to passed object pass reference type as parameter name price shares 0 s ibm

8 compare references? Comparison Two possible meanings for reference comparison –identity: do the two references refer to the same object? –equality: do the objects contain equal data? name IBM price 56.0 shares 100 name Sun price 7.50 shares 200 ibm sun compare data?

9 Identity test Use library method to guarantee identity test with references –static method Object.ReferenceEquals Stock ibm = new Stock(); Stock sun = new Stock(); if (Object.ReferenceEquals(ibm, sun))... false, refer to different objects

10 operator == Operator == may perform either identity or equality test –default behavior is identity test –type can customize behavior by overloading operator Stock ibm = new Stock(); Stock sun = new Stock(); if (ibm == sun)... refer to documentation of Stock class to determine behavior

11 Keyword null Assign null to indicate reference does not refer to object –invalidates reference –does not destroy object Stock ibm = new Stock();... ibm = null;... invalidate

12 Using null reference Error to use null reference –can catch and handle NullReferenceException at runtime –can test reference before use and avoid exception Stock s = null; s.Buy(50);... error: s is null, runtime exception generated Stock s;... if (s != null) s.Buy(50);... test reference before use

13 Reference field Common for class to contain reference as field –reference only –not embedded object class Point { int x; int y;... } reference class Circle { Point center; int radius;... } Circle c = new Circle(); center radius c

14 Reference field initial value Reference fields default to null Circle c = new Circle(); center null radius 0 c center will be null

15 Contained objects Objects for reference fields typically allocated at initialization allocate center Point class Circle { public Circle(int x, int y, int radius) { this.center = new Point(x, y); this.radius = radius; }... } Circle c = new Circle(1, 2, 3); center radius 3 c x 1 y 2

16 Array Array declaration gives reference –does not create array object references int [] a; bool[] b; a b

17 Array creation Arrays created using new operator references int [] a; bool[] b; a = new int [5]; b = new bool[3]; create arrays a b

18 Array assignment Array assignment copies only the reference int[] a = new int[5]; int[] b; b = a;... refer to same array a b

19 Array parameters Array parameter passes reference –does not copy array Typical to pass array reference by value –but can pass ref or out if needed int Total(int[] a) { int total = 0; foreach (int i in a) total += i; return total; } int[] data = new int[5];... int t = Total(data); reference to passed array pass array a data

20 Array of class reference Array of class type is array of references –elements default to null –must allocate objects with new and store reference in array Stock[] stocks = new Stock[3]; stocks[0] = new Stock(); stocks[1] = new Stock(); stocks[2] = new Stock(); array of references objects stocks name price shares name price shares name price shares

21 Managed heap Memory for reference types obtained from managed heap –area of memory dedicated for use by the application –allocation optimized to incur only small runtime overhead heap name price shares name price shares ibm sun Stock ibm = new Stock(); Stock sun = new Stock(); int [] a = new int [10]; bool[] b = new bool[5]; a b

22 Garbage collection Memory of unused objects automatically reclaimed –not explicitly released by programmer –called garbage collection void Method() { int[] a = new int[5]; Stock s = new Stock(); } objects no longer in use, now eligible to be reclaimed

23 Garbage collection algorithm Garbage collector finds unreferenced objects –begins at root references such as live local variables –all objects found from some root are still in use –objects not reachable from a root can be collected heap a b references X X X

24 Timing of garbage collection Garbage collector execution managed by system –runs as needed void Allocate() { int[] a = new int[10000];... } collection of unused objects may occur if needed to satisfy this request

25 Control of garbage collection GC class provided to give programmer control if needed –Collect method runs garbage collector Should rarely control garbage collection explicitly –already highly optimized –collection is expensive, waste of time if done unnecessarily public class GC { public static void Collect() {... }... } force collection

26 Allocation failure Heap may eventually run out of available memory –attempted allocation will fail –OutOfMemoryException will be generated –application can catch and handle exception if desired int[] a = new int[10000]; allocation may fail if out of memory

27 Summary Reference/object pair used for class instance and array Reference semantics apply in –assignment –comparison –parameter passing Garbage collection automatically reclaims unused memory –relieves programmer of responsibility