Enhancing class Enhancing class Chapter 4 Chapter 4 1 Out lin e • Method parameters • The static modifier • Nested class and Inner classes • Interface • Polymorphism 2 Method parameters Method parameters  The Java programming language always uses call by value • This means that the method gets a copy of all parameter values • In particular, the method cannot modify the contents of any parameter variables that are passed to it  parameter passing is like an assignment statement  There are two kinds of method parameters: • Primitive types (numbers, boolean values) • Object references 3 Method parameters Method parameters (cont.) (cont.)  A method cannot modify a parameter of primitive type (that is, numbers or Boolean values)  A method can change the state of an object parameter • The method gets a copy of the object reference, and both the original and the copy refer to the same object (aliases)  A method cannot make an object parameter refer to a new object  See examples next… 4 Example: Pass primitive parameter Example: Pass primitive parameter class MyClass{ public static void setNum( int n ) { n = 5; System.out.println(“Trong hàm setNum, num= ” + n); } public static void main(String[] agrs) { int num = 10; System.out.println(“Truoc khi goi ham setNum, num= ” + num); setNum(num); // n=num System.out.println(“Sau khi goi ham setNum, num= ” + num); } } n = 1 0 5 Example: Pass object reference parameter Example: Pass object reference parameter  Change state of the object 6 public class RectangularTest { static void modifyRec( Rectangular r ) { r.setWidth(50); } public static void main( String[] args ) { Rectangular rect = new Rectangular(10, 5); double area = rect.getArea(); System.out.println("rect after created has area is "+ area); modifyRec( rect ); // r = rect System.out.print("After calling modifyRect, new width = " + rect.getWidth()); area = rect.getArea(); System.out.println(", new area is "+ area); } } rect r (10, 5)(50, 5) Example: Pass object reference parameter Example: Pass object reference parameter  Chang the object that the reference points to 7 public class RectangularTest { static void modifyRec( Rectangular r ) { r = new Rectangular(50, 5); } public static void main( String[] args ) { Rectangular rect = new Rectangular(10, 5); double area = rect.getArea(); System.out.println("rect after created has area is "+ area); modifyRec( rect ); // r = rect System.out.print("After calling modifyRect, new width = " + rect.getWidth()); area = rect.getArea(); System.out.println(", new area is "+ area); } } rect r (10, 5) (50, 5) Example: Swap objects in the method Example: Swap objects in the method  Chang the object that the reference points to 8 public class RectangularTest { static void swap( Rectangular x, Rectangular y ) { Rectangular temp = x; x = y; y = temp; } public static void main( String[] args ) { Rectangular a = new Rectangular( 5, 5 ); Rectangular b = new Rectangular( 10, 10 ); System.out.println("Before: a has width, height =" + a.getWidth() + " " + a.getHeight()); System.out.println("Before: b has width, height =" + b.getWidth() + " " + b.getHeight()); swap( a, b ); System.out.println("After: a has width, height =" + a.getWidth() + " " + a.getHeight()); System.out.println("After: b has width, height =" + b.getWidth() + " " + b.getHeight()); } } The x and y parameters of the swap method are initialized with copies of these references. The method then proceeds to swap these copies Out lin e • Method parameters • The static modifier • Nested class and Inner classes • Interface • Polymorphism 9 The The static static modifier modifier  In Chapter 2 we discussed static methods, for example, the methods of the Math class are static: double result = Math.sqrt(25);  Variables can be static as well  To declare static methods and static variables, using the static keyword public static double getID() private static int k;  Static methods and static variables are invoked through its class name  Static methods and static variables often work together 10 [...]... • Classes within another class is called a nested classes • A nested class produces a separate bytecode file class Outside { … public class Inside_1 { … } public static class Inside_2 { … } Outside .class Outside$Inside_1 .class Outside$Inside_2 .class } 17 Inner Class  Nested classes are divided into two categories: • Static nested class • Non-static nested class: is called an inner class – Inner class. .. RectangleTest.java class RectangleTest { public static void main(String[] agrs) { Rectangle n1 = new Rectangle(); Rectangle n2 = new Rectangle(8,10); Rectangle.printHeader(); n1.printInfo(); n2.printInfo(); } } 15 Outline • Method parameters • The static modifier • Nested class and Inner classes • Interface • Polymorphism 16 Nested Class  In addition to containing data and methods, a class can also contain other classes... that any class that implements the Comparable interface is required to have a compareTo method  How to declare a class that implements an interface? 22 Implement Interface  To declare that a class implements interfaces, use the implements keyword • A class can implement multiple interfaces (separated by commas) • The class must implement all methods in all interfaces listed in the header • A class that... implement other methods as well public class ClassName implements Interface1, Interface2 { // all methods of all interfaces } 23 Example  Shape interface public interface Shape { double getArea(); }  Circle class public class Circle implements Shape { private double radius; … public double getArea() { return Math.PI * radius * radius;} }  Rectangle class public class Rectangle implements Shape { private... access the data from the scope in which they are defined, even if they are declared private – Inner classes can be hidden from other classes in the same package – See more in Core Java 1, chapter 6 18 Example class Outer { int outer_x = 100; void test() { Inner inner = new Inner(); inner.display_x(); } class Inner { // có thể truy xuất trực tiếp đến biến của lớp Outer int inner_y = 10; void display_x()... field per class class Employee { • Example: class Employee private int id; private String name; private static int nextId = 1; } – Every employee object now has its own id and name field, but there is only one nextId field  Static fields are shared among all instances of a class • Changing the value of a static variable in one object changes it for all others  Static fields are called class fields... their class  Example: class Employee public static int getNextId() { return nextId; // returns static field } To call this method, you supply the name of the class: int n = Employee.getNextId();  It is legal to use an object to call a static method (not recommended) 12 Ex 1: StaticDemo.java e1 name=“John” e2 name=“Wayn” address=“Hollywood Blvd.” address=“WallStreet Blvd.” class Employee SSN=12 345 6789... không thể truy xuất biến của lớp Inner System.out.println(“display : inner_y = “ + inner_y); // Error } } class InnerClassDemo { public static void main(String args[]) { Outer outer = new Outer(); outer.test(); } } 19 Outline • Method parameters • The static modifier • Nested class and Inner classes • Interface • Polymorphism 20 Interface  An Interface is a collection of constants and abstract methods... positive if obj1 is greater than obj2 • The String class implements Comparable, giving us the ability to put strings in lexicographic order • To use the sort method of the Arrays class to sort an array of objects, the objects must belong to classes that implement the Comparable interface 25 Example: EmployeeSortTest.java import java.util.*; public class EmployeeSortTest { public static void main(String[]... } // No body // No body  An Interface is not a class, so it cannot be instantiated  By default, methods in an interface have public visibility 21 Interface (cont.)  Why we use interface? Let's look at a concrete example: • The sort method of the Arrays class promises to sort an array of objects, but under one condition: The objects must belong to classes that implement the Comparable interface • . { … } } Outside .class Outside$Inside_1 .class Outside$Inside_2 .class Inner Class Inner Class  Nested classes are divided into two categories: • Static nested class • Non-static nested class: is called an inner class – Inner class. another class is called a nested classes • A nested class produces a separate bytecode file 17 class Outside { … public class Inside_1 { … } public static class Inside_2 { … } } Outside .class Outside$Inside_1 .class Outside$Inside_2 .class Inner. modifier • Nested class and Inner classes • Interface • Polymorphism 16 Nested Class Nested Class  In addition to containing data and methods, a class can also contain other classes • Classes within