Java object model

1. Java object model Part 3: Serialization & Reflection

2. Serialization • Denotes the process of storing an object to a stream without converting to an external representataion • Serialization allows an object to be persistent: that is, to exist separate from the program that created it • When an object is serialized, it is transformed into a sequence of bytes; this can later be restored to the original object

3. Java object serialization • In order for an object to be serialized, it must implement the Serializable interface • Like Cloneable, Serializable is devoid of methods • To serialize an object, invoke the writeObject() method of the ObjectOutputStream class; to deserialize, invoke the readObject() method of ObjectInputStream

4. Example // write data to output file: MyClass[] students = new MyClass[12]; ... ObjectOutputStream outs = new ObjectOutputStream (new FileOutputStream(“myfile.dat”)); outs.writeObject(students); outs.close(); // read data from input file: ObjectInputStream ins = new ObjectInputStream (new FileInputStream(“myfile.dat”)); MyClass students = (MyClass[]) ins.readObject(); ins.close();

5. Serialization & dependent objects • Serialization automatically takes into account any additional referenced objects - follows all references contained in the object being serialized & serializes them • Thus, any objects contained within a serializable object must also implement Serializable

6. Standard Java classes & serialization • Many standard classes implement Serializable • Examples: • String • ArrayList: means entire list of objects could be stored in a single operation

7. The transient modifier • We may wish to exclude some information when serializing an object: • passwords or other confidential data • extraneous data (whether or not a particular graphical object was in a selected state, e.g.) • Reserved word transient can be used to modify variable declaration; such a variable will not be represented when the object is serialized

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

9. 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

10. 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

11. Class class methods • getSuperClass() returns Class object that describes superclass of a given type; returns null is 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)

12. 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 & package-visible fields • includes both static & instance fields • does not include superclass fields

13. 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

14. 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());

15. 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

16. Example: printing Rectangle constructor information Constructor cons = Rectangle.class.getDeclaredConstructors(); for (int=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(“)”); }

17. 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)

18. 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

19. 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[] {});

20. Method methods • invoke(): can be used to call a method described by a Method object - need to: • supply implicit parameter (null for static methods) • 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

21. 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!}); } }

22. 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();

23. 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);

24. 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

25. 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);

26. 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;