1. EEC 484 Computer Networks Java Tutorial #1 Wenbing Zhao
Cleveland State University

2. Outline Dissect a simple Java program
Object-oriented programming concepts
Exceptions handling
Packages
Materials taken from Sun’s Java Tutorial:
Another good Java tutorial:
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EEC484 Computer Networks

3. public class Calculator { private int m_num1; / / first number
private int m_num2; // second number
private char m_op; // operator: add, sub, mul, div
private int m_result; // result of the calculation
public Calculator() {
m_num1 = 0; m_num2 = 0; m_op = ' '; m_result = 0; }
public void enter1stNumber(int num1) {
m_num1 = num1;
public void enter2ndNumber(int num2) {
m_num2 = num2;
public void enterOperator(char op) {
m_op = op;
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4. /* return the result of the calculation */ public int calculate() {
switch(m_op) {
case '+':
return m_num1 + m_num2;
case ' - ':
return m_num1
m_num2;
case '*':
return m_num1 * m_num2;
case '/':
return m_num1 / m_num2;
default:
return 0; }
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5. ic void main(String[] args) { Calculator c = new Calculator();
public stat
ic void main(String[] args) {
Calculator c = new Calculator();
c.enter1stNumber(1);
c.enterOperator('+');
c.enter2ndNumber(1);
int result = c.calculate();
System.out.println("1+1="+result); }
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6. How to learn a programming language quick and dirty?
Search for APIs and examples on the Web!
Tasks:
get input from standard input (stdin)
Make sure it is not empty
For number input, make sure it is an integer
For operator input, make sure it is a valid operator
Deal with exceptions
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7. Object Concept Real-world objects: state and behavior
State of a bicycle: current gear, current pedal cadence, current speed
Behavior of a bicycle: changing gear, changing pedal cadence, applying brakes
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8. Software Object A software object mimics a real-world object
Fields (state)
A software object mimics a real-world object
Member variables (or fields): store the state of the object
Member functions (or Methods): expose the object behavior
Methods operate on an object's internal state
Methods (behavior)
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9. Object Oriented Concept
Object-oriented communication: Different objects communicate (interact) with each other by calling methods defined in the objects
Data encapsulation: Hiding internal state and requiring all interaction to be performed through an object's methods
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10. Class
A class is the blueprint from which individual objects are created
public
class Bicycle {
private
int speed = 0;
int gear = 1;
public Bicycle() {
gear = 1;
speed = 0; }
void changeGear(int newValue) {
gear = newValue;
void spe
edUp(int increment) {
speed = speed + increment;
void applyBrakes(int decrement) {
speed = speed -
decrement;
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11. Access Level Modifiers
At the top level—public, or package-private (no explicit modifier)
At the member level—public, private, protected, or package-private (no explicit modifier)
Access Levels
Modifier
Class
Package
Subclass
World
public Y
protected N
no modifier
private
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12. Creating Objects Bicycle b = new Bicycle()
Declaration: associate a variable name with an object type
Instantiation: The new keyword is a Java operator that creates the object
Initialization: The new operator is followed by a call to a constructor, which initializes the new object
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13. Using Objects Calling an object’s methods:
Referencing an object’s fields:
Bicycle b = new Bicycle()
b.speedup(10);
Calling an object’s methods: append the method's name to the object reference, with an intervening dot operator (.). Also, you provide, within enclosing parentheses, any arguments to the method. If the method does not require any arguments, use empty parentheses.
objectReference.fieldName
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14. Returning a Value from a Method
A method returns to the code that invoked it when it
completes all the statements in the method,
reaches a return statement, or
throws an exception (covered later),
whichever occurs first
return returnValue;
return;
If no return value is needed
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15. Inheritance
Different kinds of objects often have a certain amount in common with each other
Mountain bikes, road bikes, all share the characteristics of bicycles (current speed, current pedal cadence, current gear)
Yet each also defines additional features that make them different
Road bikes have drop handlebars; some mountain bikes have an additional chain ring, giving them a lower gear ratio
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16. Inheritance
Classes can inherit commonly used state and behavior from other classes Bicycle is the superclass of MountainBike, RoadBike, and TandemBike
In the Java, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses
class MountainBike
extends
Bicycle { //
new fields and methods defining a moun
tain bike would go here }
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17. Inheritance
Super class
Subclasses
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18. Inheritance
A class that is derived from another class is called a subclass (also a derived class, extended class, or child class)
The class from which the subclass is derived is called a superclass (also a base class or a parent class)
A subclass inherits all the members (fields, methods, and nested classes) from its superclass
Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass
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19. Inheritance
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it
public class MountainBike extends Bicycle {
// the MountainBike subclass adds one field
publi c
int seatHeight;
// the MountainBike subclass has one constructor
public MountainBike(int startHeight) {
super();
seatHeight = startHeight; }
// the MountainBike subclass adds one method
public void setHeight(int newVal
ue) {
seatHeight = newValue;
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20. Inheritance You can assign a subclass to a superclass variable
Bicycle b = new MountainBike(10);
But not the other way around. You have to cast a superclass object to a sublass object
MountainBike mb = (MountainBike)b;
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21. Abstract Class
An abstract class is a class that is declared abstract: it may or may not include abstract methods
public abstract class GraphicObject {
// declare fields
// declare non -
abstract methods
abstract void draw(); }
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22. Abstract Class
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon
If a class includes abstract methods, the class itself must be declared abstract
Abstract classes cannot be instantiated, but they can be subclassed
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23. Abstract Class: Example
abstract class GraphicObject {
int x, y;
...
void moveTo(int newX, int newY
) { }
abstract void draw();
abstract void resize();
class Circle extends GraphicObject {
void draw() {
... }
void resize() {
class Rectangle extends GraphicObject {
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24. Interface interface Bicycle { void changeCadence(int newValue);
Objects define their interaction with the outside world through the methods that they expose.
Methods form the object's interface with outside world
An interface is a group of related methods with empty bodies
interface Bicycle {
void changeCadence(int newValue);
void changeGear(int newValue);
void speedUp(int increment);
void applyBrakes(int decrement); }
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25. Interface
To implement this interface, use the implements keyword in the class declaration:
class ACMEBicycle
implements
Bicycle {
// remainder of t
his class implemented as before }
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26. Interface
Interfaces form a contract between the class and the outside world
This contract is enforced at build time by the compiler
If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile
Implementing an interface allows a class to become more formal about the behavior it promises to provide
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27. Exception
An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions
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28. Exception
Code that might throw certain exceptions must be enclosed by either of the following:
A try statement that catches the exception. The try must provide a handler for the exception
A method that specifies that it can throw the exception. The method must provide a throws clause that lists the exception
public void writeList() throws IOException { … }
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29. Exception Try-Catch-Finally try { } catch (FileNotFoundException e) {
System.err.println("FileNotFoundException: "
+ e.getMessage());
throw new SampleException(e);
} catch (IOException e) {
System.err.println("Caught IOException: "
} finally {
if (out != null) {
System.out.println("Closing PrintWriter");
out.close(); }
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30. How to Throw an Exception
All methods use the throw statement to throw an exception. The throw statement requires a single argument: a throwable object
public Object pop() {
Object obj;
if (size
== 0) {
throw new EmptyStackException(); }
obj = objectAt(size -
1);
setObjectAt(size
1, null);
size -- ;
return obj;
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31. Throwable Class and Its Subclasses
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32. Package
A package is a namespace that organizes a set of related classes and interfaces
Think of packages as being similar to different folders
The Java platform provides an enormous class library (a set of packages)
This library is known as the "Application Programming Interface", or "API" for short
Java platform API specification:
Example packages: java.lang (Fundamental classes), java.io (Classes for reading and writing)
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33. Benefits of Creating a Package
You and other programmers can easily determine that these types are related
You and other programmers know where to find types that can provide graphics-related functions
The names of your types won't conflict with the type names in other packages because the package creates a new namespace
You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package
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34. Creating a Package
To create a package, you choose a name for the package and put a package statement with that name at the top of every source file that contains the types (classes, interfaces, enumerations, and annotation types) that you want to include in the package
The package statement (for example, package graphics;) must be the first line in the source file
There can be only one package statement in each source file
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35. in the Draggable.java file package graphics; //
in the Draggable.java file
package graphics;
public interface Draggable {
. . . }
in the Graphic.java file
public abstract class Graphic {
in the Circle.java file
public class Circle extends Gra
phic implements Draggable {
in the Rectangle.java file
public class Rectangle extends Graphic implements Draggable {
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36. Managing Source and Class Files
Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java.
For example:
// in the Rectangle.java file
package graphics;
public class Rectangle() {
. . . }
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37. Managing Source and Class Files
Then, put the source file in a directory whose name reflects the name of the package:
.....\graphics\Rectangle.java
The qualified name of the package member and the path name to the file are parallel
class name graphics.Rectangle pathname to file graphics/Rectangle.java
Produce a jar file for your classes
Under the parent directory of graphics
jar –cf graphics.jar graphics
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38. Using a Package
To use a public package member from outside its package, you must do one of the following:
Refer to the member by its fully qualified name: System.out.println(“Hello World”);
Import the package member
import graphics.Rectange;
Rectangle r = new Rectangle();
Import the member's entire package
import graphics.*;
Circle c = new Circle();
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39. Accessing a Package (Compilation and Execution)
For compilation:
javac –classpath graphics.jar YourClass.java
For execution:
java –classpath graphics.jar YourClass
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40. Questions Real-world objects contain ___ and ___.
A software object's state is stored in ___.
A software object's behavior is exposed through ___.
Hiding internal data from the outside world, and accessing it only through publicly-exposed methods is known as data ___.
A blueprint for a software object is called a ___.
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41. Questions
Common behavior can be defined in a ___ and inherited into a ___ using the ___ keyword.
A collection of methods with no implementation is called an ___.
A namespace that organizes classes and interfaces by functionality is called a ___.
The term API stands for ___?
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42. Questions What is wrong with the following interface?
Is the following interface valid?
public interface SomethingIsWrong {
void aMethod(int aValue){
System.out.println("Hi Mom"); }
public interface Marker { }
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43. Questions Is the following code legal? try { } finally { } 9/15/2018
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