Classes and Objects in Java_Object-oriented programming

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Classes and Objects in Java_Object-oriented programming

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Outline: Classes, Working with objects, Attributes, methods, and access control, Constructors. Readings: Java how to program, chapter 3, 8. A Java program is a collection of objects. Each class is specified in one source file (file name is the same with class name), Every line of code you write in Java must be inside a class (not counting import directives), Increase modularity, Easier to modified code, shorter compile time.

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Classes and Objects in Java Object-oriented programming
Outline  Classes  Working with objects  Attributes, methods, and access control  Constructors  Readings:  Java how to program, chapter 3, 8 Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 2
Java program  A Java program is a collection of objects  Each class is specified in one source file (file name is the same with class name)  Every line of code you write in Java must be inside a class (not counting import directives)  Increase modularity  Easier to modified code, shorter compile time Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 3
class declared as public must be // GradeBook.java stored in a file with the same name class declaration public class GradeBook begins / ends { // display a welcome message to the GradeBook user 02 classes public void displayMessage() in 02 files { System.out.println( "Welcome to the Grade Book!" ); } } // end class GradeBook main() is called automatically by // GradeBookTest.java the Java Virtual Machine when public class GradeBookTest the program is executed { // main method begins program execution public static void main( String args[] ) { // create a GradeBook object and assign it to myGradeBook GradeBook myGradeBook = new GradeBook(); // call myGradeBook's displayMessage method myGradeBook.displayMessage(); } creates an instant / object of the class } // end class GradeBookTest myGradeBook is the reference to it Welcome to the Grade Book! Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 4
Objects  Objects are manipulated via references  Object references play the roles similar to pointers  Objects must be explicitly created by new operator public class GradeBookTest { // main method begins program execution public static void main( String args[] ) { // create a GradeBook object and assign it to myGradeBook GradeBook myGradeBook = new GradeBook(); // call myGradeBook's displayMessage method myGradeBook.displayMessage(); } } // end class GradeBookTest Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 5
Objects and Object references ... // create a GradeBook object and assign it to myGradeBook GradeBook myGradeBook = new GradeBook(); ... Heap memory myGradeBook The object GradeBook the object reference created by new GradeBook() Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 6
Attributes, methods, and access control  Access modifiers:  Public  Accessible anywhere by anyone  Protected  Accessible only to the class itself and to its subclasses or other classes in the same “package”  Private  Only accessible within this class Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 7
GradeBook - courseName : String + setCourseName( name : String ) + getCourseName() : String private: // GradeBook.java + displayMessage() accessed by public class GradeBook the class’s { methods only private String courseName; // course name for this GradeBook attribute. // method to set the course name Each Gradebook object has its public void setCourseName( String name ) own instant variable named { courseName courseName = name; // store the course name access } modifiers // method to retrieve the course name public String getCourseName() { return courseName; methods } // display a welcome message to the GradeBook user public void displayMessage() { System.out.println( "Welcome to the Grade Book!" ); } } // end class GradeBook Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 8
Overloading methods  Methods can have the same name but different argument lists. class MyDate { … public boolean setMonth(int m) { …} public boolean setMonth(String s) { …} } … d.setMonth(9); d.setMonth(”September”); Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 9
Constructors  Every class has a default “method” called a Constructor  Invoked when the object is to be “created” / “allocated” by using “new”  Main purposes:  Initialise object’s attributes / data members  A class may have multiple constructors  Distinguished at compile time by having different arguments  The default constructor takes no arguments and is implicit when no other constructors are specified Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 10
GradeBook - courseName : String // GradeBook.java «constructor» GradeBook( name: public class GradeBook String ) { + setCourseName( name: String ) private String courseName; // course name for this GradeBook + getCourseName() : String // constructor initializes courseName + displayMessage() public GradeBook( String name ) { courseName = name; // initializes courseName 12 } // method to set the course name public void setCourseName( String name ) { courseName = name; // store the course name } // method to retrieve the course name public String getCourseName() { return courseName; // GradeBookTest.java } … // display a welcome message to the GradeBook user // create a GradeBook object and assign it to myGradeBook public void displayMessage() GradeBook myGradeBook = { new GradeBook( "CS101 Introduction to Java Programming“ ); System.out.println( "Welcome to the Grade Book!" ); … } } // end class GradeBook Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 11
Implementation vs. Interface  GradeBookTest: a “client” of GradeBook  Implementation  Data structures and code that implement the features (variables and methods)  Usually more involved and may have complex inner workings  Clients don’t need to know  Interface  The controls exposed to the “client” by the implementation  The knobs on the block box Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 12
Encapsulation / information hiding  “Don’t expose internal data structures!”  Objects hold data and code  Neither is exposed to the end user  Objects expose an interface  Anthropomorphic nature of objects  Think of objects and people who have specialized roles!  Lawyer, Mechanic, Doctor  Complexity is hidden inside the object  Make life easier for clients  More modular approach  Implementation changes in one component doesn’t affect others  Less error-prone Đại học Công nghệ. ĐHQG Hà Nội Classes and objects in Java 13