1 Programming II (CS300) Chapter 02: Using Objects MOUNA KACEM [email protected] Fall 2018 Using Objects 2 u Introduction to Object Oriented Programming Paradigm u Objects and References u Memory Management in Java u Keep in Mind Object-Oriented Programming Paradigm 3 u Object-oriented (OO) design goals: u Improvement over the procedural/structured programming paradigm u Robustness u Adaptability u Reusability u Emphasis on data rather than algorithms u Data and associated operations are unified u Grouping objects with common attributes, operations and semantics Object-Oriented Programming Paradigm 4 Procedural/Structured Programming Object-Oriented Programming Paradigm Paradigm u Programs are designed around the u Programs are designed around the data operations u Algorithmic problem decomposition u Data-centric problem decomposition Object-Oriented Programming Paradigm 5 u Object-Oriented Thinking u Everything in the world is an object u Any system consists of a set of interacting objects u Interactions among the objects inside and outside the system Flower Tree Student House Object-Oriented Programming Paradigm 6 u Object Oriented Design Principles Abstraction Encapsulation Modularity Object-Oriented Programming Paradigm 7 u Object u The main “actor” in Object-oriented programming paradigm u Instance of exactly one class u Represents the properties of a single instance of a class u Class u A class represents a Data type (prototype or model) u A class represents a description of the common properties of a set of objects u A class can be viewed as a “recipe” on how to make or generate objects u The class should be defined before creating any instance (object) of that class u Properties u Data attributes u Methods u Sequence of instructions that a class or an object follows to perform a task (functions/capabilities) Objects and References 8 u Object Concept u An object is an instance of a Class u An object is an encapsulation of Data u An object has u Identity (a unique reference) u State (also called fields or variables) u Each object has its own variables u Behaviors u All objects instances of the same class share the methods defined by the class Objects and References 9 u Class Concept u A class is used as a template or pattern to create new objects u A class defines u the variables that hold the object’s state u the methods that describe the object’s behaviors Objects and References 10 Object-Oriented Abstraction Programming Class Properties Data (State) Entity Methods Operations (Behaviors) Instantiate Instantiate Instantiate … Object Object1 n Object2 Objects and References 11 u Example: Class definition and object instantiation public class Student { private String name; // Student’s name. private double test1, test2, test3; // Grades on three tests. public double getAverage(){ // compute average test grade // code goes here (method implementation) } } // end of class Student Objects and References 12 u Object Instantiation (Example) Objects and References 13 u Object Instantiation (Example) Objects and References 14 u Object Instantiation (Example): References illustration 864 Instance1of Student std Instance1 1000 Instance2of Student of Student (@ 864) 1024 std = 864 std1 std1 = 1000 Instance2 3200 of Student 5436 std2 = 1000 (@ 1000) std2 5508 std3 = null X std3 Objects and References 15 u Object Instantiation (Example) Objects and References 16 u Object Instantiation (Example) Object no longer accessed Garbage ! Objects and References 17 u There are only eight primitive types in Java u byte, short, int, long, float, double, char, and boolean. u Any other type including String is a reference type u All variables of a reference type (non-primitive type) are objects and are accessed via a reference u In Java, fields of classes and objects that do not have an explicit initializer and elements of arrays are automatically initialized with the default value for their type u false for boolean, u 0 for all numerical types, u null for all reference types Objects and References 18 u Object Behaviors u Constructors u Constructors are invoked only when the object is created u Constructors initialize the state of an object upon creation u Accessor Methods u An accessor method is used to return the value of a private field u Accessor or getter: the method name begins with the prefix “get” Objects and References 19 u Object Behaviors u Accessor Methods u An accessor method is used to return the value of a private field u Accessor or getter: the method name begins with the prefix “get” u Mutator Methods u A mutator method is used to set a value of a private field u A mutator method does not have a return type u Mutator or setter: the method name begins with the prefix “set” u Why it is recommended to use Accessors and Mutators to access to a private field? u One of the ways to enforce encapsulation Objects and References 20 u Constructor (Example) public class Person { // Defined Constructor public Person(String firstName, String lastName, String address) { //Private fields this.firstName = firstName; private String firstName; this.lastName = lastName; private String lastName; this.address = address; private String address; } // Default Constructor } public Person(){ firstName = " "; // Declare two variables of type Person lastName = " "; Person person1, person2; //references to objects of type Person address = " "; } // Create two Person objects and store their references in person1 and person2 variables Person person1 = new Person(); Person person2 = new Person(“Mouna”, “KACEM”, “Madison”); Objects and References 21 u Accessors and Mutators Examples //Accessor for firstName //Mutator for firstname public String getFirstName(){ public void setFirstname(String firstname) { return firstName; this.firstname = firstname; } } //Accessor for lastName //Mutator for address public String getLastName(){ public void setAddress(String address){ return lastName; this.address = address; } } Objects and References 22 u Message Passing u An Object-Oriented application is a set of cooperating objects that communicate with each other u In Java, "message passing" is performed by invoking (calling) methods u A message is sent to an object using the “dot notation”: u objectReference.methodX(); u objectReference.fieldY; u When an object receives a message, it looks for the corresponding method (ie. with the same signature) in its class. Then, it invokes and executes that method. Using Objects 23 u Introduction to Object Oriented Programming Paradigm u Objects and References u Memory Management in Java u Keep in Mind Memory Management in Java 24 u What’s Memory Management? u Memory management is the process of managing the memory space (of the RAM) allocated to a program while it is running u A Java virtual machine (JVM) is an abstract computing machine that enables a computer to run a Java program (i.e. the JVM represents the Java application) u Managing the memory space used by a JVM includes u Allocating Java code, called methods and their local variables in the memory u Allocating new created objects u Removing unused objects Memory Management in Java 25 u Java Memory Model (1) Code u The Java memory model specifies how the Java Heap Virtual Machine (JVM) uses the computer memory (RAM) (memory Allocation) Static A computer allocates mainly two dynamic areas of Memory memory for a program: u Stack to store information about method calls Stack u When a piece of code calls a method, information (method call) about the call is placed on the stack. u When the method returns, that information is popped off the stack console.log() u Heap contains all objects created by the application main() JVM Memory Management in Java 26 Thread1 Stack Thread2 Stack Heap JVM u Java Memory Model (2) u Each thread running in the JVM has its own stack (Thread Stack) u All threads running in the JVM share the heap Memory Management in Java 27 u Java Memory Model (3) u A thread stack contains information about Thread1 Stack Thread2 Stack u The methods called by a thread to reach method_f() method_A() the current point of execution (call stack) Local variable1 Local variable1 Local variable2 u All local variables for each method being Local variable2 executed Local variable3 method_g() method_B() u Local variables related to one thread are not visible to the all other threads including Local variableX Local variableY u local variables of primitive data types (boolean, char, byte, short, int, long, float, double) Heap u If two threads are executing the exact same code (multi-thread applications), u Each thread will create its own local variables of JVM that code in its own thread stack Memory Management in Java 28 u Java Memory Model (2) u The heap contains all objects Thread1 Stack Thread2 Stack created in the Java application method_f() method_A() u Including the object version of Local variable1 Local variable1 primitives types (Boolean, Byte, Local variable2 Local variable2 Character, Double, Float, Integer, Local variable3 Long, Short, and Void) method_g() method_B() Local variableX Local variableY u Objects on the heap are visible Heap Object1 Object4 to all threads Object3 Object2 Object5 JVM Memory Management in Java 29 u Garbage Collection u Java Garbage Collector is responsible for freeing objects when they are no longer accessed u Reclaims heap space u No manual garbage collection in Java u Fully automatic process which runs periodically and performs the real destruction of the objects Keep in Mind 30 u A reference is a variable that stores the memory address where an object is temporarily stored in the stack or the special reference null. u An object can be referenced by multiple references u “==“ operator is used to compare two references u It returns true if the two references refer to the same object u “=“ operator makes a reference variable refer to another object u “.” operator allows the selection of object’s method or access to its fields Keep in Mind 31 u Reference types u For reference types and arrays, “=“ is a reference assignment rather than an object copy.