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Java object model

Part 3: Serialization & Reflection

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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

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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

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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();

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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

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Standard Java classes & serialization

• Many standard classes implement Serializable

• Examples:– String– ArrayList: means entire list of objects could be

stored in a single operation

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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

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Reflection

• Mechanism by which a program can analyze its objects & their capabilities at runtime

• Java API includes several reflection reflection classesclasses, described on next slide

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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

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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

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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)

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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

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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

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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());

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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

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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(“)”);

}

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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)

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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

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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[] {});

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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

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Example - saying hello the hard wayimport java.lang.reflect.*;import java.io.*;public class SayHello{

public static void main(String[]args) throwsNoSuchMethodException, IllegalAccessException,

InvocationTargetException{

Method m = PrintStream.class.getDeclaredMethod(“println”, new Class[] {String.class});m.invoke(System.out, new Object[] {“Hello!});

}}

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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();

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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);

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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

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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);

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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;