Core Java- An advanced review of features

The Global Open University
Nagaland
JAVA
Special Tips & Tricks

2
Java Virtual Machine
Program
in Java
Java
Compiler
Java
Bytecode
Java Virtual Machine
“WRITE ONCE, RUN ANYWHERE!”
debug
pretty portable

Web applciations!
3
Java Applet
Server
Java Applet

 Clients download applets via Web browser
 Browser runs applet in a Java Virtual Machine (JVM)
Interactive web, security, and client consistency
Slow to download, inconsistent VMs (besides, flash won
this war)
4
Applet
Client
Server

 Thin clients (minimize download)
 Java all “server side”
THIS IS WHAT YOU’LL BE DOING!!
5
Client
Server
JSPs
Servlets
EJB
JDBC

 Compared to C++:
 no header files, macros, pointers and references, unions, operator
overloading, templates, etc.
 Object-orientation: Classes + Inheritance
 Distributed: RMI, Servlet, Distributed object programming.
 Robust: Strong typing + no pointer + garbage collection
 Secure: Type-safety + access control
 Architecture neutral: architecture neutral representation
 Portable
 Interpreted
 High performance through Just in time compilation + runtime
modification of code
 Multi-threaded
6

 Well defined primitive data types: int, float, double, char, etc.
 int 4 bytes [–2,147,648, 2,147,483,647]
 Control statements similar to C++: if-then-else, switch, while, for
 Interfaces
 Exceptions
 Concurrency
 Packages
 Name spaces
 Reflection
 Applet model
7

 Java programming language specification
 Syntax of Java programs
 Defines different constructs and their semantics
 Java byte code: Intermediate representation for Java programs
 Java compiler: Transform Java programs into Java byte code
 Java interpreter: Read programs written in Java byte code and execute
them
 Java virtual machine: Runtime system that provides various services to
running programs
 Java programming environment: Set of libraries that provide services
such as GUI, data structures,etc.
 Java enabled browsers: Browsers that include a JVM + ability to load
programs from remote hosts
8

How are Java programs written?
How are variables declared?
How are expressions specified?
How are control structures defined?
How to define simple methods?
What are classes and objects?
What about exceptions?
9

 Define a class HelloWorld and store it into a file:
HelloWorld.java:
public class HelloWorld {
public static void main (String[] args) {
System.out.println(“Hello, World”);
}
}
 Compile HelloWorld.java
javac HelloWorld.java
Output: HelloWorld.class
 Run
java HelloWorld
Output: Hello, World
10

Fibonacci:
class Fibonacci {
public static void main(String[] arg) {
int lo = 1;
int hi = 1;
System.out.println(lo);
while (hi < 50) {
System.out.println(hi);
hi = lo + hi;
lo = hi – lo;
}
}
}
11

 Arithmetic: +, -, *,/, %, =
8 + 3 * 2 /4
Use standard precedence and associativity rules
 Predicates: ==, !=, >, <, >=, <=
public class Demo {
public static void main (String[] argv) {
boolean b;
b = (2 + 2 == 4);
System.out.println(b);
}
}
12

Every method is defined inside a Java class definition
public class Movie {
public static int movieRating(int s, int a, int d) {
return s+a+d;
}
}
public class Demo {
public static void main (String argv[]) {
int script = 6, acting = 9, directing = 8;
displayRating(script, acting, directing);
}
public static void displayRating(int s, int a, int d){
System.out.print(“The rating of this movie is”);
System.out.println(Movie.movieRating(s, a, d));
}
}
13

Typical flow of control statements: if-then-else, while, switch, do-while,
and blocks
class ImprovedFibo {
static final int MAX_INDEX = 10;
public static void main (String[] args) {
int lo = 1;
int hi = 1;
String mark = null;
for (int i = 2; i < MAX_INDEX; i++) {
if ((i % 2) == 0)
mark = " *";
else mark = "";
System.out.println(i+ ": " + hi + mark);
hi = lo + hi;
lo = hi - lo;
}}}
14

Classes: templates for constructing instances
 Fields
 Instance variables
 Static variables
 Methods
 Instance
 Static
class Point {
public double x, y;
}
Point lowerleft = new Point();
Point upperRight = new Point();
Point middlePoint = new Point();
lowerLeft.x = 0.0; lowerLeft.y = 0.0;
upperRight.x = 1280.0; upperRight.y = 1024.0
middlePoint.x = 640.0; middlePoint.y = 512.0
15

Instance methods take an implicit parameter: instance on
which method is invoked
public class Movie {
public int script, acting, directing;
public int rating() {
return script + acting + directing;
}
}
public class Demo {
public static void main (String argv[]) {
Movie m = new Movie();
m.script = 6; m.acting = 9; m.directing = 8;
System.out.print(“The rating of this movie is”);
System.out.println(m.rating());
}
}
16

 Inheritance: mechanism for extending behavior of
classes; leads to construction of hierarchy of classes
[Note: no multiple inheritance]
 What happens when class C extends class D:
 Inherits instance variables
 Inherits static variables
 Inherits instance methods
 Inherits static methods
 C can:
 Add new instance variables
 Add new methods (static and dynamic)
 Modify methods (only implementation)
 Cannot delete anything
17

18
public class Attraction {
public int minutes;
public Attraction() {minutes = 75;}
public int getMinutes() {return minutes;}
public void setMinutes(int d) {minutes = d;}
}
public class Movie extends Attraction {
public int script, acting, directing;
public Movie() {script = 5; acting = 5; directing = 5;}
public Movie(int s, int a, int d) {
script = s; acting = a; directing = d;
}
public int rating() {return script + acting + directing;}
}
public class Symphony extends Attraction {
public int playing, music, conducting;
public Symphony() {playing = music = conducting = 5;}
public Symphony(int p, int m, int c) {
playing = p; music = m; conducting = c;
}
public int rating() {return playing + music + conducting;}
}

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 Abstract class: Merely a place holder for class
definitions; cannot be used to create
instances.;public abstract class Attraction {
public int minutes;
public Attraction() {minutes = 75;}
public int getMinutes() {return minutes;}
public void setMinutes(int d) {minutes = d;}
public abstract void m();
}
 Following is an error:
Attraction x;
x = new Attraction();
 Following is not an error:
public class Movie extends Attraction { … }
public class Symphony extends Attraction { … }
Attraction x;
x = new Movie ();
x = new Symphony();

20
Object
Attraction Auxiliaries Demonstration
Movie Symphony
extends
extends
• How do we organize above classes into a single unit? Put them in file?
However, only one public class per file (whose name is same as file’s)
• Solution: Place several files (compilation units) into a package

 units of organizing related Classes, Interfaces, Sub
packages
 Why?
 Reduce name clashing
 Limit visibility of names
 Java programs typically organized in terms of packages
and subpackages
 Each package may then be divided into several packages,
subpackages, and classes
 Each class can then be stored in a separate file
 Each source file starts with something like:
package mypackage;
 Code in source file is now part of mypackage
21

package onto.java.entertainment;
public class Movie extends class Attraction {…}
22
import java.io.*;
import java.util.*;
public class Auxiliaries { … }
public abstract class Attraction { … }
•Where to store packages?
•How does Java find packages?
•Export and Import
•Access control

public class A {
public void foo() throws MyException {
if(aBadThingHappened()) {
throw new MyException();
}
}
public void bar() {
try {
this.foo();
} catch (MyException e) {
e.printStackTrace();
}
}
}
public class MyException extends Exception {
public MyException() {}
public MyException(String message) {
super(String message);
}
}
23

public class A {
public void foo() throws MyException {
throw new MyException();
}
}
public void bar() {
try {
this.foo();
} catch (MyException e) {
} catch (YourException e) {
} finally {
... // always executed before leaving the try/catch
}
}
}
24

 http://java.sun.com/
 Java[tm] 2 Platform, Standard Edition v1.4.1
 java, javac, jar, jre, etc.
 Any platform... FREE!
 Online documentation and tutorials
 http://www.eclipse.org/
 Integrated development environment (IDE) for nothing in particular
 Java[tm] development tools (JDT) (comes with Eclips)
 Project management
 Editor
 Incremental compiler
 CVS support
 C/C++ extension in progress
 AspectJ support
 Windows, Linux, and Mac.... FREE!
25

public – any class* may access
(no qualifier) “package protected” – only the class*
and classes* in the same package may access
protected – only the class* and decendent classes*
may access
private – only the class* may access
The class or instances of the class (an object of the
class)
26

27
package edu.ucdavis;
public class A {
int x;
}
package edu.ucdavis;
public class B {
void foo(A a) { a.x; } // OK, same package
}
package edu.ucdavis.cs;
public class B {
void foo(A a) { a.x; } // Not OK, different package
}
package edu;
public class B {
}
package edu.ucdavis.cs;
public class B {
}
package org.omg;
public class B {
}

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public class A {
protected int x;
}
public class B extends A {
void foo(A a) { this.x; a.x; } // OK, B is a decendent of A
}
public class C extends B {
void foo(A a) { this.x; a.x; } // OK, C is a decendent of A through
B
}package edu; // Uh oh!
public class D extends C {
void foo(A a) { this.x; a.x; } // OK, D is a decendent of A
}
public class E {
void foo(A a) { this.x; a.x; } // NOT OK, E is NOT a decendent of A
}

 Multiple “threads” of execution within the same
program, share the same memory space ->
“lightweight”.
 Perform multiple tasks at the same time.
 Work on the same task in parallel.
 Heavily used in user interfaces.
 Web browsers: load web pages while the user can still
scroll, go back, open a new window, etc.
 Web servers: serve multiple requests in parallel.
 Can take advantage of multiple processors.
 Threads in Java
 Java manages and schedules threads
 Java provides “synchronize” to help coordinate
multiple threads
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public class MyThread extends Thread {
public MyThread(String threadName) {
super(threadName);
}
public void run() {
for(int i = 0; i < 10; i++) {
System.out.println(i + “ “ + getName());
try {
sleep((long)(Math.random() * 1000));
} catch(InterruptedException e) {}
}
}
}
30

public class ThreadTest {
public static void main(String[] args) {
for(int i = 0; i < args.length; i++) {
MyThread t = new MyThread(args[i]);
t.start();
}
}
}
> java ThreadTest Bob Frank
0 Bob
0 Frank
1 Bob
2 Bob
1 Frank
3 Bob
2 Frank
3 Frank
4 Frank
...
31

public class MyRunnable implements Runnable {
String name;
public MyRunnable(String name) {
this.name = name;
}
public void run() {
for(int i; i < 10; i++) {
System.out.println(i + “ “ + name());
try {
sleep((long)(Math.random() * 1000));
} catch(InterruptedException e) {}
}
}
}
public class ThreadTest {
public static void main(String[] args) {
for(int i = 0; i < args.length; i++) {
Thread t = new Thread(new MyRunnable(args[i]), args[i]);
t.start();
}
}
}
32

33
public class Producer
extends Thread {
private Share shared;
public Producer(Share s) {
shared = s;
}
public void run() {
for(int i = 0; i < 10; i++){
shared.put(i);
}
}
}
shared.put(0)
shared.get() // 0 gotten
shared.get() // 0 gotten again!!
shared.put(0)
shared.put(1)
shared.get() // 0 never gotten!!
public class Consumer
extends Thread {
private Share shared;
public Consumer(Share s) {
shared = s;
}
public void run() {
int value;
for(int i = 0; i < 10; i++) {
value = shared.get();
}
}
}
// what about simultaneous
// access?!
shared.put(0) shared.get()
RACE CONDITIONS!

public class Share {
private int s;
public synchronized int get() { ... }
public synchronized void put(int s) { ... }
}
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 Synchronized provides mutual exclusion on an
object
 For any object, only one thread may execute
inside any of that object’s synchronized
methods
Share s1 = new Share();
Share s2 = new Share();
Thread t1 = ...;
Thread t2 = ...;
t1 -> s1.get() // gets in
t2 -> s1.put(32) // blocks
t1 -> s1.get() // gets in
t2 -> s2.put(4) // gets in

public class Share {
private int s;
private boolean empty = true;
public synchronized int get() {
while (empty == true) {
try {
wait(); // nothing to get, wait
} catch (InterruptedException e) {}
}
empty = true;
notifyAll(); // wakeup waiting Consumers/Producers
return s;
}
public synchronized void put(int s) {
while (empty == false) {
try {
wait(); // no room
} catch (InterruptedException e) {}
}
this.s = s;
empty = false;
notifyAll(); // wakeup waiting Consumers/Producers
}
}
35

This material has been taken from Online
Certificate course on JAVA from Global Open
University Online certification programme. For
complete course material visit:
http://tgouwp.eduhttp://tgouwp.edu
About Global Open University :
The global open university is now offering certification courses in
various fields. Even you can study, give exam from comfort of your
home. These are short term and totally online courses. For more
details you can visit:
Email id: info@tgouwp.edu

Core Java- An advanced review of features

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