Lab 3: Inheritance, Hash, Design Pattern and Big Number Solved

Description

Objectives

• Learn how to use inheritance concept in Java programming

• Learn how to use interface concept in Java programming

• Learn the importance of equals and hashCode

• Get familiar with Java design patterns

• Learn how to calculate with big numbers in Java

• Full mark: 25 points

Source files

• Printable.java

• SalaryRaisable.java

• JavaDPExample.java

1 Introduction

1.1 Inheritance in Java

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, Person is the superclass of Employee and Customer ; Employee is the super class of SwEngineer 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.

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).





	————————–
	|	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 |

—————————–

Step 3: implement PrintInfo

The printed information shall be:

Customer : name + project price;

ProjManager : 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 .

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

Note 1: even if you only need a customized equals , you still need to override hashCode — this is important.

Note 2: if you override equals and hashCode 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;

}

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.

Demo and discuss this deliverable with the lab instructor (5 points).