Chapter 4 Enhancing class

The static modifier  In Chapter 2 we discussed static methods, for example, the methods of the Math class are static: double result = Math.sqrt25;  Variables can be static as well  To

Enhancing class

Chapter 4

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• Method parameters

• The static modifier

• Nested class and Inner classes

• Interface

• Polymorphism

• In particular, the method cannot modify the contents of any

parameter variables that are passed to it

parameter passing is like an assignment statement

• Primitive types (numbers, boolean values)

• Object references

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Method parameters (cont.)

 A method cannot modify a parameter of primitive type

(that is, numbers or Boolean values)

• 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

Example: Pass primitive parameter

Example: Pass object reference parameter

 Change state of the object

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 = " +

(10, 5)

Example: Pass object reference parameter

 Chang the object that the reference points to

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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 = " +

(10, 5)

(50, 5)

Example: Swap objects in the method

 Chang the object that the reference points to

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() + " " +

• Method parameters

• The static modifier

• Nested class and Inner classes

• Interface

• Polymorphism

9

The static 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

static fields

 Each object has its own copy of all instance fields, but if

you define a field as static, then there is only one

such field per class

– 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

private String name;

private static int nextId = 1;

}

static methods

 Static methods are methods that do not operate on

objects

• You cannot access instance fields from a static method

• Static methods can access the static fields in their class

 It is legal to use an object to call a static method (not

public static int getNextId() {

return nextId; // returns static field

}

To call this method, you supply the name of the class:

int n = Employee.getNextId();

Ex 1: StaticDemo.java

class Employee

{

private String name, address;

private int SSN, number;

private static int counter;

public Employee( String name, String address, int

Employee

0 counter 1

number=2

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} }

Ex 2: RectangleTest.java

class RectangleTest {

public static void main( String [] agrs)

{

Rectangle n1 = new Rectangle();

Rectangle n2 = new Rectangle(8,10);

• Method parameters

• The static modifier

• Nested class and Inner classes

• Interface

• Polymorphism

Nested Class

 In addition to containing data and methods, a class

can also contain other classes

• Classes within another class is called a nested classes

public static class Inside_2 {

… }

}

Outside.class Outside$Inside_1.class Outside$Inside_2.class

Inner Class

 Nested classes are divided into two categories:

• Static nested class

• Non-static nested class: is called an inner class

– Inner class methods can 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

void display_y() { // 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();

} }

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• Method parameters

• The static modifier

• Nested class and Inner classes

• Interface

• Polymorphism

 An Interface is a collection of constants and

abstract methods

• An abstract method is a method header without a method

body (it is followed by a semicolon)

 An Interface is not a class, so it cannot be instantiated

 By default, methods in an interface have public visibility

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public interface InterfaceName {

SomeType method1(…); // No body

SomeType method2(…); // No body

}

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

• Comparable interface looks like:

• This means that any class that implements the Comparable

interface is required to have a compareTo method

public interface Comparable {

int compareTo( Object other );

}

public class Circle implements Shape {

private double radius; … public double getArea() {

return Math.PI * radius * radius;}

}

 Rectangle class

public class Rectangle implements Shape {

private double width, length; … public double getArea() {

return width * length;}

The Comparable interface

 The Java standard class library contains many helpful

interfaces

method called compareTo, which is used to compare

two objects

• 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

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Example: EmployeeSortTest.java

import java.util.*;

public class EmployeeSortTest {

public static void main( String [] args) {

Employee[] staff = new Employee[3];

staff[0] = new Employee(" Harry Hacker ", 35000);

staff[1] = new Employee(" Carl Cracker ", 75000);

staff[2] = new Employee(" Tony Tester ", 38000);

Arrays.sort(staff); // sắp xếp theo lương

for ( Employee e : staff)

System out.println(" name = " + e.getName() + " , salary = " + e.getSalary()); }

Example Employee.java

class Employee

implements Comparable<Employee>

{

private String name;

private double salary;

public Employee( String n, double s)

}

{ salary = s;

if (salary < other.salary) return -1;

if (salary > other.salary) return 1;

return 0;

}

}

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The Iterator interface

• Method parameters

• The static modifier

• Nested class and Inner classes

• Interface

• Polymorphism

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Polymorphism via Interface

 An interface name can be used as the type of an object

reference variable

• Example:

but: Shape x = new Shape(); //ERROR

 An interface reference variable can be used to refer to

any object of any class that implements that interface

• Example:

x = new Circle( .); // if Circle implement Comparable

x = new Rectangle( .); // if Rectangle implement

Comparable

of object that x is referring to

• That reference is polymorphism , which can be defined as

public interface Speaker {

public void speak();

public void announce ( String str); }

public class Philosopher implements Speaker

That line of code might execute different methods at different times if the object that obj points to changes

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Self-review questions

 What is the difference between a static variable and an

instance variable?

 What is the difference between a class and an interface?

 How to declare a class that implements an interface?