Description

Inheritance, Hash, Design Pattern and Big Number Objectives

1.1 Inheritance in Java

Person

Different kinds of objects often have a certain amount in common with each other. Employees, customers, and managers, for example, all share the characteristics of a person (name, etc.). Yet each also defines additional features that make it different. Object-oriented programming allows such different classes to inherit commonly used state and behavior from other classes. In this example,  is the superclass of

Employee and Customer; Employee is the super class ofSwEngineer and HwEngineer; SwEngineer is the superclass of _{ProjectManager}.

In Java, each class is allowed to have and only have one direct superclass, and each superclass has the potential for an unlimited number of subclasses. A subclass contains all the methods (functions) and attributes (variables) defined in the superclass plus the subclass’s own methods and variables.

extends

Java implements inheritance through the  keyword.

Example: a software engineer has all the attributes of an employee plus attributes picked up from a person, which means the software engineer is an employee, and an employee is a person, and therefore the software engineer is also a person. In Java we would represent this as:

class SwEngineer extends Employee class Employee extends Person

SwEngineer

gets all the attributes and methods defined in:

SwEngineer

,

Employee

and

Person

By doing so, .

public

/

private

. An attribute is usually

private

NOTE: A method is usually , meaning that it cannot be

directly accessed outside of the class. A “getter” and “setter” method enables such access. For example:

Person person = new Person(); person.setName(“John Doe”); int age = person.getAge();

1.2  Interface

Interfaces define a standardized set of commands that a class will obey. The commands are a set of methods. The interface definition states: the names of the methods; the return types; and the signatures (argument lists). There is no executable body for any method. The body is left to each class to implement.

A class can implement multiple interfaces. Once a class implements an interface, you are able to invoke the corresponding method. That is, implementing an interface enables a class to be “plugged in” to any situation that requires a specific behavior (manifested through the set of methods).

Java implements interface through the	implements	keyword.
interface SalaryRaisable {     public double RaiseSalary(); } class Employee implements SalaryRaisable {     double baseSalary; @Override public double RaiseSalary() {         return baseSalary *= 1.2; } }

2  Deliverable 1 — Inheritance and Interfaces

In this part you will write a program to represent people who are involved in a software production company (name, department, related projects…), as mentioned at the beginning of this lab.

Step 1: create classes

Please refer to the following UML class diagram. Your program will only have the classes and interfaces

public static void main(String[] args)

mentioned in the diagram (you can put the main entry  and test

cases anywhere you like), where the two interfaces Also, your classes will only have the attributes and methods menti	Printable	and	SalaryRaisable	are already provided.
			oned in the diagram.

super

HINT: You can use  to call the constructor of the super class.

RaiseSalary

The  method raises the salary in a certain rate and returns the raised salary amount:

Role                                                                                   Rate

HwEngineer

0.18

ProjManager

0.24

HwEngineer

with base salary of $3000, and an instance of

ProjManager

Create an instance of  with base salary of $6000. Display their final (raised) salaries — they should be different.

 

————————– |        Person          | ————————– | – name: String         | |————————- | + Person(name: String) | | + getName(): String    | ————————– ^     ^ extends  |     |   extends                 +————-     ————-+ |                               | —————————–    —————————-  implements   ——————– |         Employee          |    |         Customer         | ————> |   <<interface>>  | —————————–    —————————-               |    Printable     | | – baseSalary: double      |    | – projPrice: double      |   +———> ——————– |—————————|    —————————-   |           | + PrintInfo():   | | + Employee(               |    | + Customer(              |   |           |     String       | |     name: String,         |    |     name: String,        |   |           ——————– |     baseSalary: double)   |    |     projPrice: double)   |   | | + getBaseSalary(): double |    | + getProjPrice(): double |   | —————————–    —————————-   |             ^     ^                                               |     extends |     |          extends                              |             |     +—————————–+                 | |                                   |                 | —————————–    —————————-   | |         SwEngineer        |    |        HwEngineer        | ——+ —————————–    —————————-   |   | | – projName: String        |    |                          |   |   | |—————————|    —————————-   |   | | + SwEngineer(             |    | + HwEngineer(            |   |   | |     name: String,         |    |     name: String,        |   |   | implements |     baseSalary: double,   |    |     baseSalary: double)  |   |   | |     projName: String)     |    |                          |   |   | | + getProjName(): String   |    |                          |   |   | —————————–    —————————-   |   | ^                                                     |   | | extends                                             |   | —————————–             implements            |   |       ——————– |        ProjManager        | ———————————-+   +—–> |  <<interface>>   | —————————–                                               |  SalaryRaisable  | | – projDeadline: Date      | ——————————————–> ——————– |—————————|             implements                        | + RaiseSalary(): | | + ProjManager(            |                                               |     double       |   |     name: String,         |                                               ——————– |     baseSalary: double,   | |     projName: String,     | |     projDeadline: Date)   | | + getProjDeadline(): Date | —————————–

PrintInfo

Step 3: implement 

The printed information shall be:

Customer	:
ProjManager

• name + project price;
• : name + project name + final salary + project deadline.

DEMO this deliverable to the lab instructor (10 points).

3  Deliverable 2 — equals and hashCode

Comparison is a common activity, hence nearly every class has its own definition of equals and hashCode.

SwEngineer

and

ProjManager

Override the two methods for . Your implementations must follow the best practice as discussed in the lecture.

equals

, you still need to override

hashCode

Note 1: even if you only need a customized  — this is important.

equals

and

hashCode

Note 2: if you override  of a subclass, you may need to override the superclass, too.

This is because the hash code of the subclass relies on that of its superclass.

DEMO this deliverable to the lab instructor (5 points).

4  Deliverable 3 — Java Design Pattern

Consider the code from JavaDPExample.java and provide answers to these questions (google useful clues if you need):

1. Why do we use a static method in this situation?
2. The code implements a class-level (involving multiple classes) programming “good practice”, commonly these practices are called design patterns in Java. Which design pattern is implemented?
3. Explain why this is considered a good practice.

DISCUSS this deliverable with the lab instructor (5 points).

5  Deliverable 4 — Big Numbers

Consider the following piece of C code. What is it doing? Convert it into Java codes.

uint64_t fnv(void *b, int c) {     unsigned char *p = b;

uint64_t h = 14695981039346656037;     int i;

for (i = 0; i < c; i++)         h = (h * 1099511628211) ^ p[i];     return h; }

long

, because Java doesn’t have unsigned version of

int

/

long

BigInteger

Hint 1: _h is too big even for . So you may need .

Hint 2: ^ in C is “bitwise XOR”, not Math.pow.

Hint 3: Don’t forget the overflow of higher bits, or you may get incredibly large number. Such number is incorrect, and what’s more, it may crash Eclipse.