Exercises Software Development I. 08 Objects II. Generating and Releasing Objects (Constructors/Destructors, this, Object cloning) December 3rd, 2014

Transcription

1 Exercises Software Development I 08 Objects II Generating and Releasing Objects (Constructors/Destructors, this, Object cloning) December 3rd, 2014 Software Development I Winter term 2014/2015 Priv.-Doz. Dipl.-Ing. Dr. Andreas Riener Institute for Pervasive Computing Johannes Kepler University Linz riener@pervasive.jku.at

2 Classes and Objects: State of Objects Variable/Object Types Primitive variables (static or class variables) Object variables - A container for objects; to such variables objects can be assigned to, i.e., a variable of type from any class Example: Person p; - The opposite of object variables are primitive variables, e.g. int age; - Default initial state defined at time of object creation with new Operator e.g., new ObjectName(); specified by default initial state of all variables contained in that object ( s description) Member variables, instance variables - Synonym use of these two terms - A variable declared within a class and which is created every time the class is instantiated - Can be either modified by method invocation (=call) or direct (should be avoided data encapsulation ) Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 2

3 Classes and Objects: Generating Objects Basics: Consider the following statement ObjectName myobject = new ObjectName(); What exactly is 'new ObjectName()';??? a function/method call? Each and every class defines at least one constructor method name of the constructor equals the name of the class Standard constructor: no parameters, no specific (variable) initialization automatically generated by Java (but can me overwritten) What happens during object generation? Processing of 'new' creates a dynamic instance of the class, e.g. instantiation provides an object of that class with all the member attributes (One of) the constructor(s) of the class is called, the just created object is implicitely pased to this constructor (using the this reference; see later) Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 3

4 Classes and Objects: Generating Objects Example: Different options to instantiate a new 'Person' object Option 1 (call it the bad ) Person pbad = new Person(); pbad.name = "Michael"; pbad.age = 21; Option 2 ( the better ) Person pbetter = new Person(); pbetter.setname("michael"); pbetter.setage(21); Option 3 ( the best ) Person pbest = new Person("Michael", 21); self defined constructor with 2 parameters (String, int) proper constructor selected during object generation; if no constructor is specified and no parameters are passed, a standard constructor will be created and used) Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 4

5 Classes and Objects: Generating Objects What is a constructor; what is its function? Constructors are sort of special methods that are used to define (assign) the initial state of objects Constructors are always named after the class, i.e., constructors of the class Person are named Person(...) Constructors never have a return type, i.e., they do not allow to return results to the caller no return type means: nothing, not even void implicitly, the constructor has a return type; it returns an instance (an object) of the class It is possible to define more than one constructor per class differentiation by number and/or data types of the parameters (also the order of parameters is used to differentiate between similar constructors) this concept is called constructor overloading Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 5

6 I Classes and Objects: Generating Objects Types of constructors Default constructor ObjectName(); this constructor is automatically created by the Java compiler (javac) might be overwritten by the programmer (in this case, the compiler does not create another one implicitly) creates an object in its initial (default) state; every object contained in the just created object will also be generated and initialized in its default state, e.g., public class Flat { int norooms; int sqmeters; Person owner; public Flat() { norooms = 2; owner = new Person(); public class Person { String name; float rate; // Eur/sqm public Person() { name = ""; rate = 7.5f; Flat myflat = new Flat(); System.out.println(myFlat.owner.rate); // value? 7.5 Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 6

7 I Classes and Objects: Generating Objects Types of constructors Copy constructor ObjectName(ObjectName o); creates a new object as copy of the object 'o' field-by-field assignment has to be implemented by the programmer (no standard concept) Don t confuse with the clone()-concept defined in class Object (see later) Special constructor ObjectName(DataType1 para1,...); any number of constructors similar (same) name, but have to be unique: (1) number of parameters (2) data types of parameters (3) order of parameter list! public Flat(int sqmeters, Person owner); public Flat(Person owner, int sqmeters); Two different constructors: Differentiated by the order of parameters (data types) Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 7

8 Classes and Objects: Generating Objects Usage example (correct) public class Person { private String name; private int age; // overwritten default constructor // assigns standard values to all member variables public Person () { name = ""; age = 0; //... // general constructor with two parameters 's', 'i public Person (String s, int i) { name = s; // initialize name with value given in 's' age = i; // initialize age with value given in 'i' Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 8

9 Classes and Objects: Generating Objects Usage example (correct) public class Person { private String name, phone; private int age, weight, height; private int birthd, birthm, birthy; private int svn; Ok works, but hard to read here... Parameter semantics cannot be determined/derived from parameter names Ok, why not using descriptive parameter names: 'name', 'phone', 'age', etc.? // overwritten default constructor // assigns standard values to all member variables public Person () { name = ""; age = 0, weight = 0, height = 0, birthd= 0, birthm = 0, birthy = 0; // general constructor with many parameters public Person (String s, String p, int a, int b, int c, int d, int e, int f ) { name = s; // initialize name with value given in 's' phone = p; // initialize phone with value given in 'p' age = a; // initialize age with value given in 'a' birthd = b; // initialize birth day with value given in 'b' birthm = c; // initialize birth month with value given in 'c'... Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 9

10 Classes and Objects: Generating Objects Usage example (faulty) public class Person { private String name, phone; private int age; // overwritten default constructor // assigns standard values to all member variables public Person () { name = ""; phone = ""; age = 0; //...!??? // general constructor with two parameters 'name', 'age' public Person (String name, int age) { name = name; age = age;??? Which attributes are meant here? Instance variables or parameter? Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 10

11 Classes and Objects: Generating Objects - Constructors How to solve the problem of duplicate identifiers (in the constructor)? Use the reference operator 'this' 'this' is a special instance variable that is defined (can be used) within methods of an object [during method invocation] is a container for instance variables whose method or constructor is currently executed can be used like any other normal instance variable solves the problem of similar names (i.e., parameter or local variable overloading object variables) this concept allows to address/identify member attributes always correctly Usage example (correct, using this-reference) public class Person { private String name; private int age; // general constructor with two parameters 'name', 'age' public Person (String name, int age) { this.name = name; this.age = age; Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 11

12 Classes and Objects: Generating Objects this-reference: Demonstration example - class Person public class Person { private String name; private String phone; private int age; public Person(String name, String phone, int age){ this.name = name; this.phone = phone; this.age = age; public void printpersondetails() { System.out.println(this.name + " (" + this.age + " years): " + this.phone); public static void main (String[] args) { Person instructor = new Person ("Andreas Riener", " ", 55); Person student = new Person ("Bart Simpson", " ", 18); Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 13

13 Classes and Objects: Generating Objects this-reference: Demonstration example - class Person public class Person { private String name; private String phone; private int age; public Person(String name, String phone, int age){ this.name = name; this.phone = phone; this.age = age; public void printpersondetails() { System.out.println(this.name + " (" + this.age + " years): " + this.phone); public static void main (String[] args) { Person instructor = new Person ("Andreas Riener", " ", 55); Person student = new Person ("Bart Simpson", " ", 18); instructor.printpersondetails(); student.printpersondetails(); Andreas Riener (55 years): Bart Simpson (18 years): Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 14

14 I Classes and Objects: Generating Objects this-reference: Demonstration example - class Car public class Car { private int hp; private String color; // horsepower // paint void anymethod (Car c) { if (this == c) System.out.println("true"); else System.out.println("false"); public static void main (String[] args) { Car audi = new Car(); audi.hp=250; audi.color="red"; Car bmw = new Car(); bmw.hp=400; bmw.color="black ; audi.anymethod(audi); // true audi.anymethod(bmw); // false bmw.anymethod(audi); // false bmw=audi; bmw.anymethod(audi); // true Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 15

15 Classes and Objects: Generating Objects Copy constructor Create a new object based on an already existing object, i.e., with its characteristics/values Approach: (1) create a new (blank) object; (2) copy all instance variables Example public class Person { private String name; private int age; //... // copy constructor Why this() and not Person()? "The method Person() is undefined for the type Person" public Person (Person p) { this(); // reference to any matching constructor in this class // has to be the first statement in a constructor if (p!= null) { // copy all values from object 'p' to the new object this.name = p.name; // also allowed: name = p.name (unambiguous assignment) this.age = p.age; // age = p.age Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 16

16 Classes and Objects: Generating Objects Copy constructor II Dependent on the already existing other constructors, it might be possible to implement the copy constructor by calling an existing constructor Example public class Person { private String name; private int age; public Person (String name, int age) { this.name = name; this.age = age; // copy constructor public Person (Person p) { this(p.name, p.age); // first statement in the constructor Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 17

17 I Classes and Objects: Generating Objects Constructor overloading: Remember unique signature Example int addition (int a, int b){ return a + b; float addition (float a, float b){ return a + b; The call value = addition (4,6); executes the first method The call value = addition (4f,6f); executes the second method Restriction: Single return value, e.g., int x = Input.readInt(); // Integer read function boolean y = Input.readBoolean(); // Boolean read function String z = Input.readLine(); // String read function This would be invalid: int x = Input.readNumber(); float y = Input.readNumber(); String z = Input.readNumber(); Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 18

18 I Classes and Objects: Generating Objects Constructor concatenation Constructors in Java can be concatenated (chained up), i.e., a constructor can call another constructor of that class. The called constructor is interpreted as normal method call; the reference to the constructor is established using this() Differentiation (compiler) to the this-pointer used above: Parentheses Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 19

19 I Classes and Objects: Generating Objects Constructor concatenation: Example public class Car { private String color; // paint private int hp; // horsepower private int regdate; // first registration public class Car { private String color; private int hp; private int regdate; public Car(String color, int hp) { this.color = color; if (hp<0) hp = 0; this.hp = hp; public Car(String color, int hp, int regdate){ this.color = color; if (hp<0) hp = 0; this.hp = hp; this.regdate = regdate; public Car(String color, int hp) { this.color = color; if (hp<0) hp = 0; this.hp = hp; public Car(String color, int hp, int regdate){ this (color, hp); this.regdate = regdate; Avoid duplicate code! Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 20

20 Classes and Objects: Generating Objects Reasons for declaration of (several) special constructors Simple use in programs to define the initial state of objects Most often it makes no sense to generate objects without assignment of initial values, e.g., each person has a name, maybe also an age no default constructor for the person class, i.e., Person(); but one or more special constructors such as Person (String name); Person (String name, int age); Person (String name, String phone, int age); Person (Person p); Generation of objects is not for free... (re)assignments at a later time are also costly therefore: try to assign everything (already known) at time of creation Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 21

21 Classes and Objects: Generating Objects Constructor used to modify (static) class variables For example, static variable 'nrpersons' is used to count the actually allocated person objects public class Person { private static int nrpersons = 0; // number of allocated person objects private String name; private int age; // fields/attributes of a person object public Person() {... // initialize person object nrpersons++; // increase number of person objects public static int getnrpersons () { // class method; 'getter' return nrpersons; Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 22

22 Classes and Objects: Generating and Releasing Objects Constructor is used to modify (static) class variables Create a few person objects... Person welma = new Person(); Person michael = new Person(); Person andreas = new Person(); nrpersons = Some objects are no longer needed; destroyed (garbage collector) michael = null; welma = null; // 'andreas' still exist nrpersons = 3 3 3? Class variable 'nrpersons' (number of allocated objects) is correctly increased (in the constructor), but will not be decreased... is there a concept similar to constructors (object creation) for destroying objects, i.e., that allows to execute concluding instructions? Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 23

23 Classes and Objects: Generating and Releasing Objects One step back... In Java, objects are dynamically allocated by using the new-operator How are objects destroyed (and their memory released for later reallocation) if they are no longer required? In some languages, such as C++, dynamically allocated objects must be manually released by use of a special delete -operator (malloc free) In Java: De-allocation of objects is handled automatically garbage collector Still, you might specify a destructor method to link the release of an object with additional statements or actions...but there is no explicit need to destroy objects as in C++ Some words about the operation principle of the garbage collector When no references to an object exist, that object is assumed to be no longer needed, and the memory occupied by the object can be reclaimed Garbage collection only occurs sporadically (if at all) during the execution of your program; it will not occur simply because one or more objects exist that are no longer used... Furthermore, different Java run-time implementations will take varying approaches to garbage collection, but for the most part, you should not have to think about it while writing your programs Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 24

24 Classes and Objects: Releasing Objects Destructors in Java: The 'finalize' method 'finalize()' is working similar to the constructor concept Destructors are used to link the release of an object with additional actions and they get called by the garbage collector (not by the user program) immediately before an object is destroyed Definition: like a ordinary method, named always 'finalize', without parameters, return type 'void' Example The specifier 'protected' prevents access to finalize()by code defined outside its class // called when the object is released protected void finalize() { nrpersons--; // decrease number of allocated person objects 'object = null;' object gets collected when GC runs the next time 'finalize()' is only called during garbage collection and it is not called when an object goes, e.g., out-of-scope a program should provide other means of releasing system resources used by the object it must not rely on finalize() for normal program operation Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 25

25 I Classes and Objects: Generating and Releasing Objects Some important points about finalizers If an object has a finalizer, the finalizer method is invoked sometime after the object becomes unused (or unreachable), but before the garbage collector reclaims the object Java makes no guarantees about when garbage collection will occur or in what order objects will be collected. Java can make no guarantees about when (or even whether) a finalizer will be invoked, in what order finalizers will be invoked, or what thread will execute finalizers The Java interpreter can exit without garbage collecting all outstanding objects, so some finalizers may never be invoked. In this case, though, any outstanding resources are usually freed by the operating system the method addshutdownhook() can safely execute arbitrary code before the Java interpreter exits After a finalizer is invoked, objects are not freed right away. This is because a finalizer method can resurrect an object by storing the thisreference somewhere so that the object once again has references In practice, it is relatively rare to use (or require) a finalize()-method Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 26

26 I SPECIAL KNOWLEDGE Classes and Objects: Generating and Releasing Objects Object cloning: The clone()-method The class Object (super class of every class) specifies a method to create and return a copy of the object the method is called on: protected Object clone() The precise meaning of copy may depend on the class of the object the general intent is that, for any object x, the following expressions will be true (but this is not an absolute requirement) 1) cloned object different from original object x.clone()!= x 2) but both objects are from same class x.clone().getclass() == x.getclass() 3) both objects are similar x.clone().equals(x) == true // no, two different objects! Using clone() requires to call super.clone() and to implement the interface Cloneable (for more information see Java API on class Object) Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 27

27 I SPECIAL KNOWLEDGE Classes and Objects: Generating and Releasing Objects Example: class person with cloneable person objects public class PersonClone implements Cloneable { // fields/attributes of a person object private String name; private int age; public PersonClone () { name = ""; age = 0; // overwritten default public PersonClone clone() { try { return (PersonClone) super.clone(); catch ( CloneNotSupportedException e ) { throw new InternalError(); // this should never happen... public static void main (String arguments[]) { PersonClone p1 = new PersonClone(); p1.name = "andreas"; p1.age = 32; PersonClone p2 = p1.clone(); System.out.println(p2.name + " is " + p2.age); p2.name = "susi"; System.out.println(p1.name + " is " + p1.age); System.out.println(p2.name + " is " + p2.age); Software Development I // Exercises // 08 Objects II (Generating and Releasing Objects) // 28 Independent objects p1, p2 no side effect andreas is 32 andreas is 32 susi is 32

28 Exercises Software Development I 08 Objects II Generating and Releasing Objects (Constructors/Destructors, this, Object cloning) December 3rd, 2014 Software Development I Winter term 2014/2015 Priv.-Doz. Dipl.-Ing. Dr. Andreas Riener Institute for Pervasive Computing Johannes Kepler University Linz riener@pervasive.jku.at