Classes and Objects in Java Object-oriented programming Classes and objects in Java 2 Đại học Công nghệ. ĐHQG Hà Nội Outline  Classes  Working with objects  Attributes, methods, and access control  Constructors  Readings:  Java how to program, chapter 3, 8 Classes and objects in Java 3 Đại học Công nghệ. ĐHQG Hà Nội 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 3 Classes and objects in Java 4 Đại học Công nghệ. ĐHQG Hà Nội // GradeBook.java public class GradeBook { // display a welcome message to the GradeBook user public void displayMessage() { System.out.println( "Welcome to the Grade Book!" ); } } // end class GradeBook // GradeBookTest.java 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 Welcome to the Grade Book! class declared as public must be stored in a file with the same name class declaration begins / ends main() is called automatically by the Java Virtual Machine when the program is executed 02 classes in 02 files creates an instant / object of the class myGradeBook is the reference to it Classes and objects in Java 5 Đại học Công nghệ. ĐHQG Hà Nội 5 Objects  Objects are manipulated via references  Object references play the roles similar to pointers  Objects must be explicitly created by new operator 5 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 Classes and objects in Java 6 Đại học Công nghệ. ĐHQG Hà Nội Objects and Object references myGradeBook GradeBook Heap memory // create a GradeBook object and assign it to myGradeBook GradeBook myGradeBook = new GradeBook(); The object reference the object created by new GradeBook() Classes and objects in Java 7 Đại học Công nghệ. ĐHQG Hà Nội 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 Classes and objects in Java 8 Đại học Công nghệ. ĐHQG Hà Nội // GradeBook.java public class GradeBook { private String courseName; // course name for this GradeBook // 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; } // display a welcome message to the GradeBook user public void displayMessage() { System.out.println( "Welcome to the Grade Book!" ); } } // end class GradeBook attribute. Each Gradeb ook object has its own instant variable named courseName private: accessed by the class’s methods only access modifiers GradeBook - courseName : String + setCourseName( name : String ) + getCourseName() : String + displayMessage() methods Classes and objects in Java 9 Đại học Công nghệ. ĐHQG Hà Nội 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”); Classes and objects in Java 10 Đại học Công nghệ. ĐHQG Hà Nội 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 [...]... 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... 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...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... 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 );...  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 . Classes and Objects in Java Object-oriented programming Classes and objects in Java 2 Đại học Công nghệ. ĐHQG Hà Nội Outline  Classes  Working with objects  Attributes, methods, and. control  Constructors  Readings:  Java how to program, chapter 3, 8 Classes and objects in Java 3 Đại học Công nghệ. ĐHQG Hà Nội Java program  A Java program is a collection of objects  Each class is specified in. 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 3 Classes and objects in Java 4 Đại