Java Object Model. Or, way down the rabbit hole

Transcription

1 Java Object Model Or, way down the rabbit hole 1

2 Type Definition: a set of values and a set of operations that can be applied to those values Java is a strongly-typed language: compiler & runtime system ensure that programs don t violate type system rules Compiler catches most illegal operation attempts Java virtual machine catches a few (e.g. invalid casts) By contrast, C/C++ not strongly typed 2

3 Types in Java Primitive: int, double, char, boolean, etc. Class Interface Array Array in itself is distinct type Individual array elements are of some component type Null 3

4 Values in Java Every value is one of the following: Value of a primitive type (e.g. 1.4, -9, c ) Reference to a class object Reference to an array null Notes: No such thing as a value of an interface type Void is not a data type just a method tag to indicate no return value 4

5 Subtype relationship Subtype contains subset of values of a given type Can use subtype where supertype is specified 5

6 S is a subtype of T if: 1. S and T are the same type 2. S and T are both class types, and T is a direct or indirect superclass of S 3. S is a class type, T is an interface type, and S or one of its superclasses implements T 4. S and T are both interface types, and T is a direct or indirect superinterface of S 5. S and T are both array types, and the component type of S is a subtype of the component type of T 6. S is not a primitive type and T is the type Object 7. S is an array type and T is Cloneable or Serializable 8. S is the null type and T is not a primitive type 6

7 Examples ListIterator is a subtype of Iterator Container is a subtype of Component FlowLayout is a subtype of Layout Manager JButton is a subtype of Component 7

8 Examples Rectangle[] is a subtype of Shape[] int[] is a subtype of Object int is not a subtype of long long is not a subtype of int int[] is not a subtype of Object[] 8

9 Array Types Recall rule 5: S[] is a subtype of T[] if S is a subtype of T Example: Rectangle [] r = new Rectangle[10]; Can store object r in an array of Shape, since Rectangle is a subset of Shape: Shape [] s = r; Variables r and s both refer to the same array 9

10 Example continued What happens if a non-rectangle Shape is stored in s[]? Throws ArrayStoreException at runtime Every array object remembers its component type Could store a subtype of Rectangle (but not just of Shape) without throwing exception 10

11 Primitive type wrappers Many services in Java API deal with Objects; for example, ArrayLists are collections of Objects Primitive types are not subtypes of Object Useful to have classes to wrap primitive types so they can avail themselves of Object services 11

12 Wrapper classes in Java Wrapper classes exist for all primitive types: Integer, Short, Long Byte, Character Float, Double Boolean Wrapper classes are immutable 12

13 Wrapper class methods Constructor take primitive type argument: double num = ; Double d = new Double(num); Can unwrap to get primitive value: num = d.doublevalue(); Some wrappers have parse methods that convert from String to primitive: String s = ; int n = Integer.parseInt(s); 13

14 Type inquiry Can use instanceof operator to test whether an expression is a reference to an object of a given type or one of its subtypes Operator returns true if expression is a subtype of the given type; otherwise (including in the case of a null expression) returns false 14

15 Example using instanceof JPanel picpanel = new JPanel(); ArrayList images = new ArrayList(); for (int x=0; x<images.size(); x++) { Object o = images.get(x); if (o instanceof Icon) { Icon pic = (Icon)o; picpanel.add(pic); } } 15

16 Notes on instanceof Can test whether value is subtype of given type, but doesn t distinguish between subtypes In previous example, o could be any kind of Icon (since Icon is an interface, couldn t be an Icon per se) 16

17 Getting exact class of object Can get exact class of object reference by invoking getclass() method Method getclass() returns an object of type Class; can invoke getname() on Class object to retrieve String containing class name: System.out.println(o.getClass().getName()); 17

18 Class object Type descriptor Contains information about a given type, e.g. type name and superclass 18

19 Ways to obtain Class object Call getclass() on an object reference Call static Class method forname(): Class c = Class.forName( java.awt.rectangle ); Add suffix.class to a type name to obtain a literal Class object: Class c = Rectangle.class; 19

20 Notes on Class type There is exactly one Class object for every type loaded in the virtual machine Class objects describe any type, including primitive types Can use == to test whether two Class objects describe same type: if (o.getclass() == Rectangle.class) True if o is exactly a Rectangle (not a subclass) 20

21 Arrays and getclass() When applied to an array, getclass() returns a Class object that describes the array type Use Class method getcomponenttype() to get a Class object describing the array s components Use Class method isarray() to determine whether or not an object is an array 21

22 Arrays and Class.getName() Array name is built from these rules: [type Where type is one of the following: B: byte C: char D: double F: float I: int J: long S: short Z: boolean For non-primitive types, replace type with Lname; (where name is the name of the class or interface note semicolon) 22

23 Reflection Mechanism by which a program can analyze its objects & their capabilities at runtime Java API includes several reflection classes, described on next slide 23

24 Reflection Classes Class: describes a type Package: describes a package Field: describes field; allows inspection, modification of all fields Method: describes method, allows its invocation on objects Constructor: describes constructor, allows its invocation Array: has static methods to analyze arrays 24

25 Class class object Includes class name and superclass of an object, as we have already seen; also includes: interface types class implements package of class names & types of all fields names, parameter types, return types of all methods parameter types of all constructors 25

26 Class class methods getsuperclass() returns Class object that describes superclass of a given type; returns null if type is Object or is not a class getinterfaces() returns array of Class objects describing interface types implemented or extended; if type doesn t implement or extend any interfaces, returns array of length 0 (only returns direct superinterfaces) 26

27 Class class methods getpackage() returns Package object; Package has getname() method which returns String containing package name getdeclaredfields() returns array of Field objects declared by this class or interface includes public, private, protected & packagevisible fields includes both static & instance fields does not include superclass fields 27

28 Field class methods getname(): returns String containing field name gettype(): returns Class describing field type getmodifiers(): returns an int whose various bits are set to indicate whether field is public, private, protected, static, or final: use static Modifier methods ispublic, isprivate, isprotected, isstatic, isfinal to test this value 28

29 Example - prints all static fields of java.lang.math Field[] fields = Math.class.getDeclaredFields(); for (int x = 0; x < fields.length; x++) if(modifier.isstatic(fields[x].getmodifiers())) System.out.println(fields[x].getName()); 29

30 More Class class methods getdeclaredconstructors() returns array of Constructor objects describing class constructors Constructor object has method getparametertypes that returns an array of Class objects describing the constructor s parameters 30

31 Example: printing Rectangle constructor information Constructor cons[] = Rectangle.class.getDeclaredConstructors(); for (int x=0; x < cons.length; x++) { Class [] params = cons[x].getparametertypes(); System.out.println( Rectangle( ); for (int y=0; y < params.length; y++) { if(y > 0) System.out.print(, ); System.out.print(params[y].getName()); } System.out.println( ) ); } 31

32 Output from example Rectangle() Rectangle(java.awt.Rectangle) Rectangle(int, int, int, int) Rectangle(int, int) Rectangle(java.awt.Point, java.awt.dimension) Rectangle(java.awt.Point) Rectangle(java.awt.Dimension) 32

33 Class class s getdeclaredmethods() method Returns array of Method objects Method object methods include: getparametertypes(): returns array of parameter types getname(): returns method name getreturntype(): returns Class object describing return value type 33

34 Obtaining single Method or Constructor objects Class s getdeclaredmethod() (note the singular) returns a Method object if supplied with a method name and array of parameter objects: Method m = Rectangle.class.getDeclaredMethod( contains, new Class[]{int.class, int.class}); For Constructor object: Constructor c = Rectangle.class.getDeclaredConstructor(new Class[] {}); 34

35 Method methods invoke(): can be used to call a method described by a Method object - need to: supply implicit parameter (null for static methods) first argument, representing calling object supply array of explicit parameter values (need to wrap primitive types) if method returns a value, invoke returns an Object; need to cast or unwrap return value, as appropriate 35

36 Example - saying hello the hard way import java.lang.reflect.*; import java.io.*; public class SayHello { public static void main(string[]args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException { Method m = PrintStream.class.getDeclaredMethod ( println, new Class[] {String.class}); m.invoke(system.out, new Object[] { Hello! }); } } 36

37 Example with return value Method m = Math.class.getDeclaredMethod( sqrt, new Class[] {double.class}); Object r = m.invoke(null, new Object[] {new Double(10.24)}); double x = ((Double) r).doublevalue(); 37

38 Inspecting objects Can use reflection mechanism to dynamically look up object fields as program runs To allow access to a field, call its setaccessible() method; example: Class c = object.getclass(); Field f = c.getdeclaredfield(name); f.setaccessible(true); 38

39 Inspecting objects Once granted access, you can read and write any field of the object: Object value = f.get(object); f.set(object, value); Notes: f must be a Field that describes a field of object; otherwise, get and set throw exception if field type is primitive, get returns & set expects a wrapper if field is static, supply null for object 39

40 Inspecting array elements Field allows read/write access to arbitrary field of object; Array works similarly on array objects for array a with index x: get() method: Object value = Array.get(a,x) set() method: Array.set(a,x,value); 40

41 Inspecting array elements Finding length of array a: int n = Array.getLength(a); Creating new array (twice as large) with static method newinstance(): Object newarray = Array.newInstance( a.getclass().getcomponenttype(), 2 * Array.getLength(a) + 1); System.arraycopy(a, 0, newarray, 0, Array.getLength(a)); a = newarray; 41

42 Object class Superclass for all Java classes Any class without explicit extends clause is a direct subclass of Object Methods of Object include: String tostring() boolean equals (Object other) int hashcode() Object clone() 42

43 Method tostring() Returns String representation of object; describes state of object Automatically called when: Object is concatenated with a String Object is printed using print() or println() Object reference is passed to assert statement of the form: assert condition : object 43

44 Example Rectangle r = new Rectangle (0,0,20,40); System.out.println(r); Prints out: java.awt.rectangle[x=0,y=0,width=20,height=40] 44

45 More on tostring() Default tostring() method just prints (full) class name & hash code of object Not all API classes override tostring() Good idea to implement for debugging purposes: Should return String containing values of important fields along with their names Should also return result of getclass().getname() rather than hard-coded class name 45

46 Overriding tostring(): example public class Employee { public String tostring() { return getclass().getname() + "[name=" + name + ",salary=" + salary + "]"; }... } Typical String produced: Employee[name=John Doe,salary=40000] 46

47 Overriding tostring in a subclass Format superclass first Add unique subclass details Example: public class Manager extends Employee { public String tostring() { return super.tostring() + "[department=" + department + "]"; }... } 47

48 Example continued Typical String produced: Manager[name=Mary Lamb,salary=75000][department=Admin] Note that superclass reports actual class name 48

49 Equality testing Method equals() tests whether two objects have same contents By contrast, == operator test 2 references to see if they point to the same object (or test primitive values for equality) Need to define for each class what equality means: Compare all fields Compare 1 or 2 key fields 49

50 Equality testing Object.equals tests for identity: public class Object { public boolean equals(object obj) { return this == obj; }... } Override equals if you don't want to inherit that behavior 50

51 Overriding equals() Good practice to override, since many API methods assume objects have well-defined notion of equality When overriding equals() in a subclass, can call superclass version by using super.equals() 51

52 Requirements for equals() method Must be reflexive: for any reference x, x.equals(x) is true Must be symmetric: for any references x and y, x.equals(y) is true if and only if y.equals(x) is true Must be transitive: if x.equals(y) and y.equals(z), then x.equals(z) If x is not null, then x.equals(null) must be false 52

53 The perfect equals() method public boolean equals(object otherobject) { if (this == otherobject) return true; if (otherobject == null) return false; if (getclass()!= otherobject.getclass()) return false;... } 53

54 Hashing Technique used to find elements in a data structure quickly, without doing full linear search Important concepts: Hash code: integer value used to find array index for data storage/retrieval Hash table: array of elements arranged according to hash code Hash function: computes index for element; uses algorithm likely to produce different indexes for different objects to minimize collisions 54

55 Hashing in Java Java library contains HashSet and HashMap classes Use hash tables for data storage Since Object has a hashcode method, any type of object can be stored in a hash table 55

56 Default hashcode() Gives hash value based on memory address of object; consistent with default equals() method If you override equals(), you should also redefine hashcode() For class you are defining, use product of hash of each field and a prime number, then add these together result is hash code 56

57 Example public class Employee { public int hashcode() { return 11 * name.hashcode() + 13 * new Double(salary).hashCode(); }... } 57

58 Object copying Shallow copy Copy of an object reference is another reference to the same object Happens with assignment operator Deep copy Actual new object created, identical to original A.K.A cloning 58

59 Method clone() must fulfill 3 conditions X.clone()!= X X.clone().equals(X) X.clone().getClass() == X.getClass() 59

60 Object.clone() Default method is protected If a class wants clients to be able to clone its instances, must: Redefine clone() as public method Implement Cloneable interface 60

61 Cloneable Interface that specifies no methods: public interface Cloneable {} Strictly tagging interface; can be used to test if object can be cloned: if (x instanceof Cloneable) If class doesn t implement this interface, Object.clone() throws a CloneNotSupportedException (checked) 61

62 Example clone() method (v 1.0) public class Employee implements Cloneable { public Object clone() { try { return super.clone(); } catch(clonenotsupportedexception e) { return null; // won't happen } }... } 62

63 Shallow cloning Object.clone() uses assignment makes shallow copy of all fields If fields are object references, original and clone share common subobjects Not a problem for immutable fields (e.g. Strings) but programmer-written clone() methods must clone mutable fields Rule of thumb: if you extend a class that defines clone(), redefine clone() 63

64 Example clone() method (v 2.0) public class Employee implements Cloneable { public Object clone() { try { Employee cloned = (Employee) super.clone(); cloned.hiredate = (Date) hiredate.clone(); return cloned; } catch(clonenotsupportedexception e) { return null; } }... } 64

65 Components Component: a software construct that encapsulates more functionality than a single class Can use a set of components to construct an application without much additional programming Visual Basic/ActiveX controls exemplify the component model of programming

66 Components & Java Beans JavaBeans: Java s (partial) implementation of the component model Capabilities of a bean: Can execute methods Can reveal properties Can emit events

67 Bean Properties Not necessarily instance variables may be stored externally Have getter and setter methods (which may do more work than just/getting setting an instance variable, even if the property is an instance variable)

68 Bean Properties Java doesn t really support components directly: beans rely on naming conventions for standardization Setter method names start with set, getter with get (exception Boolean getters use is)

69 Beans & Façade pattern Java Beans work according to the Façade pattern: Each bean may consist of multiple classes, but a single class is chosen as the façade Clients call on façade method, which can call methods from other classes Façade class reveals subsystem class capabilities; subsystem classes don t need to be aware of the facade

70 Beans & Reflection Beans are intended to be edited in a builder environment (such an environment is built into NetBeans) When bean is loaded into builder environment, façade class is searched for methods that match the conventional naming scheme this is an example of the reflection mechanism at work