1. Chapter 6
Reusing Classes

2. off the shelf:现货供应、非定制
from scratch: from the very beginning

4. “has a” relationship Example: public class Car {
Engine engine = new Engine();
Wheel[] wheel = new Wheel[4];
Door left = new Door();
Door right = new Door(); … }
“has a” relationship

5. Types of Attributes Door Car Window window char color int price
Engine engine
Wheel wheel
Door left
Door right
Window

16. *参见程序运行

17. *参见程序运行

20. Reference:

21. “is a” relationship Please:
Person、 Employee、Customer、 Manager、 CSR 的继承树
“is a” relationship

22. Test Composition

28. Composition & Inheritance
Composition- ”has a”
Inheritance -”is a”

44. 覆盖所得的方法的可访问级别，不能够比它所覆盖的方法访问级别更低。

45. private void doSomething() 方法将出现错误信息：---------- javac ----------
UseBoth.java:5: doSomething() in Child cannot override doSomething() in Parent; attempting to assign weaker access privileges; was public
private void doSomething() {} ^
1 error
Normal Termination
Output completed (0 sec consumed).

46. Polymorphism & Overriding

47. class OrderTaker{
private String name;
private int basePay;
private int ordersTaken;
public void setName(String newValue){
name = newValue; }
public void setBasePay(int newValue) {
basePay = newValue;
public void incrementOrdersTaken() {
ordersTaken++;
public String getName() {
return name;
public int getBasePay(){
return basePay;
public int getOrdersTaken(){
return ordersTaken;
public double calculatePay() {
//Generic order takers get a weekly portion of their salary
return getBasePay() / 52;

48. public double calculatePay(){
class CSR extends OrderTaker {
public double calculatePay(){
//CSRs get their weekly pay plus 10 cents per order they take
return getBasePay() / 52 + getOrdersTaken() * .1; }
class OEC extends OrderTaker {
//OECs get their weekly pay plus 1 cent per order they take
return getBasePay() / 52 + getOrdersTaken() * .01;

49. private int peopleHarassed; public void incrementPeopleHarassed(){
class Telemarketer extends OrderTaker{
private int peopleHarassed;
public void incrementPeopleHarassed(){
peopleHarassed++; }
public int getpeopleHarassed() {
return peopleHarassed;
public double calculatePay(){
/*Telemarketers get 10 cents for every order
they take and 1cent per person they harass (call)*/
return getBasePay() / 52 + peopleHarassed * getOrdersTaken() * .1;

50. public class TestPolymorphism{
//class Clerks
public static void main(String args[]) {
OrderTaker first, second, third;
first = new OEC();
second = new CSR();
third = new Telemarketer();
first.setBasePay(52000);
second.setBasePay(52000);
third.setBasePay(52000);
first.incrementOrdersTaken();
second.incrementOrdersTaken();
third.incrementOrdersTaken();
((Telemarketer) third).incrementPeopleHarassed();
System.out.println("First made " + first.calculatePay());
System.out.println("Second made " + second.calculatePay());
System.out.println("Third made " + third.calculatePay()); }

51. Result:
java
First made
Second made
Third made
Normal Termination
Output completed (0 sec consumed).

56. Casts up implicitly :向上转型隐式进行的，会引起方法丢失。子类自动具有父类类型，始终允许向概括类、抽象类转型。
Downward casts :向下转型必须是子类，并由编译器检查。
RTTI(运行期类型识别)：运行期类型检查：对象的类型在运行时检查，若类型不匹配，将报错误信息。

58. //: c07:RTTI. java :From 'Thinking in Java, 3rd ed
//: c07:RTTI.java :From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// Downcasting & Run-Time Type Identification (RTTI) : {ThrowsException}
class Useful {
public void f() {}
public void g() {} }
class MoreUseful extends Useful {
public void u() {}
public void v() {}
public void w() {}
public class RTTI {
public static void main(String[] args) {
Useful[ ] x = { new Useful(), new MoreUseful() };
x[0].f();
x[1].g();
// x[1].u(); Compile time: method not found in Useful:
((MoreUseful)x[1]).u(); // Downcast/RTTI
((MoreUseful)x[0]).u(); // Exception thrown

61. Overriding & Overloading
Overriding Methods--- same method
Overloading Methods---many methods

65. 构造器不存在覆盖的问题，因父子类不同。

70. Constructor and Initialization Example