Object-Oriented Programming in Java: Defining and Using Classes, Slides of Web Design and Development

Download Object-Oriented Programming in Java: Defining and Using Classes and more Slides Web Design and Development in PDF only on Docsity! Web Design & Development Lecture 4 Docsity.com OOP in Java Docsity.com Defining Classes Docsity.com The Structure of Classes class name { declarations constructor definitions method definitions } instance variables and symbolic constants how to create and initialize objects how to manipulate those objects (may or may not include its own “driver”, i.e., main( )) These parts of a class can actually be in any order Docsity.com Defining a Class Comparison with C++  Java gives you the ability to write classes or user-defined data types similar to the way C++ does, with a few differences  Points to consider when defining a class  There are no global variables or functions. Everything resides inside a class. Remember we wrote our main method inside a class  Specify access modifiers (public, private or protected ) for each member method or data members at every line.  No semicolon (;) at the end of class  All methods (functions) are written inline. There are no separate header and implementation files. Docsity.com  Points to consider when defining a class (cont)  Access Modifiers  public : Accessible anywhere by anyone  Private : Only accessible within this class  Protected : Accessible only to the class itself and to it’s subclasses or other classes in the same “package”  Package : Default access if no access modifier is provided. Accessible to all classes in the same package  Constructor  Same name as class name  Does not have a return type  No initialization list  JVM provides a zero-argument constructor only if a class doesn’t define it’s own constructor  Destructor  Destructors are not required in a java class Defining a Class Comparison with C++ (cont) Docsity.com Example Docsity.com Task - Defining a Class  Create a class for Student  should be able to store the following characteristics of student  Roll No  Name  Provide default, parameterized and copy constructors  Provide standard getters/setters for instance variables  Make sure, roll no has never assigned a negative value i.e. ensuring the correct state of the object  Provide print method capable of printing student object on console Attributes: Roll NO Name Methods: constructors getters/setters print Student Docsity.com // Standard Getters public String getName ( ) { return name; } public int getRollNo ( ) { return rollNo; } Student Implementation Code cont. Docsity.com // Constructor that uses a default value instead of taking an argument. public Student() { name = “not set”; rollNo = 100; } // parameterized Constructor for a new student public Student(String name, int rollNo) { setName(name); //call to setter of name setRollNo(rollNo); //call to setter of rollNo } // Copy Constructor for a new student public Student(Student s) { name = s.name; rollNo = s.rollNo; } Student Implementation Code cont. Docsity.com // method used to display method on console public void print () { System.out.println("Student name:" +name+ ", roll no:" +rollNo); } } // end of class Student Implementation Code cont. Docsity.com Using a class Comparison with C++  Differences from C++ (cont)  Objects are always passed by reference whereas primitive data types are passed by value.  All methods use the run-time, not compile-time, types (i.e. all Java methods are like C++ virtual functions)  The types of all objects are known at run-time  All objects are allocated on the heap (always safe to return objects from methods) Docsity.com Task - Using Student Class  Create objects of Student class by calling default, parameterized and copy constructors.  Call Students class various methods on objects ali Attributes: Roll NO Name Methods: constructors getters/setters print Student Attributes: Roll NO: 89 Name: ali raza Methods: getters/setters print class object Docsity.com Student Client Code public class Test{ public static void main (String args[]){ // Make two students Student s1 = new Student("ali", 15); Student s2 = new Student(); //call to default costructor s1.print(); s2.print(); s2.setName("usman"); s2.setRollNo(20); System.out.print("Student name:" + s2.getName()); System.out.println(" rollNo:" + s2.getRollNo()); //continue…. Docsity.com More on Classes Docsity.com Static  A class can have static  Variables  Methods  Static variables and methods  Are associated with the class itself!!  Not associated with the object  Therefore Statics can be accessed without instantiating an object!  Generally accessed by class name  Cannot refer to a non-static instance variable in a static method  No this reference Docsity.com Static Variable & Methods  Occurs as a single copy in the class  For example;  System.out is a static variable  JOptionPane.showInputDialog(String) Docsity.com Garbage collection and finalize  Java performs garbage collection and eliminates the need to free objects explicitly.  When an object has no references to it anywhere, except in other objects that are also unreferenced, its space can be reclaimed.  Before the object is destroyed, it might be necessary for the object to perform some actions.  For example closing an open file. In such a case define a finalize() method with the actions to be performed before the object is destroyed. Docsity.com finalize  When a finalize method is defined in a class, Java run time calls finalize() whenever it is about to recycle an object of that class. protected void finalize() { // code }  A garbage collector reclaims objects in any order or never reclaim them.  System.gc()  Request the JVM to run the garbage collector  Not necessary it will run Docsity.com Memory Mangement public class Test{ public static void main|(String args[]){ Student s1 = new Student(“ali”); Student s2 = new Student(“raza”); s1= s2; } } No Memory leakage in Java, Automatic Garbage Collection will take care of such scenarios s1 s2 0F59 0F59 03D2 name ali Stack Heap 03D2 name raza Docsity.com // Constructor that uses a default value instead of taking an argument. public Student() { name = “not set”; rollNo = 100; countStudents += 1; } // parameterized Constructor for a new student public Student(String name, int rollNo) { setName(name); //call to setter of name setRollNo(rollNo); //call to setter of rollNo countStudents += 1; } // Copy Constructor for a new student public Student(Student s) { name = s.name; rollNo = s.rollNo; countStudents += 1; } Modify Student Class Docsity.com // Overridden methods // Overriding toString method of class java.lang.Object public String toString () { return ("name: "+name + "RollNo: " + rollNo); } //Overriding finalize method of Object class protected void finalize () { countStudents -= 1; } } // end of class Modify Student Class Docsity.com Student Client Code public class Test{ public static void main (String args[]){ int numObjs; numObjs = Student.getCountStudents(); System.out.println("Students Objects:"+numObjs); Student s1 = new Student("ali", 15); System.out.println("Student:" + s1.toString() ); numObjs = Student.getCountStudents(); System.out.println("Students Objects:"+numObjs); Docsity.com