This document provides an introduction and overview of the Java programming language. It discusses key Java concepts like classes, objects, inheritance, packages, exceptions, threads, and the producer-consumer problem. It explains how Java programs are written, variables declared, methods defined, and control structures specified. It also covers Java features, environments, compilers, virtual machines, and resources for learning more. The document is intended to give readers a basic understanding of the Java language.

1. Intro to Java
for ECS 160
Stoney Jackson
jacksoni@cs.ucdavis.edu
wwwcsif.cs.ucdavis.edu/~jacksoni
What’s the first question you’ve got to
ask about a language named Java?

2. Can you make coffee with it?
2

3. It was meant to!!
A programming language for appliances!
3

4. Must Run on Any Architecture
debug
“WRITE ONCE, RUN ANYWHERE!”
Program
in Java
Java
Compiler
Java Virtual Machine
pretty portable
Java
Bytecode
Java Virtual Machine
4

5. Doesn’t Make Coffee Yet
5

6. So What’s Java Good For?
Web applications!
Java Applet
Java Applet
Server
6

7. Java on the Web: Java Applets


Clients download applets via Web browser
Browser runs applet in a Java Virtual Machine
(JVM)
Applet
Client
Server
Interactive web, security, and client consistency
Slow to download, inconsistent VMs (besides,
flash won this war)
7

8. Java on the Web: J2EE


Thin clients (minimize download)
Java all “server side”
JSPs
Servlets
Client
Server
EJB
THIS IS WHAT YOU’LL BE DOING!!
JDBC
8

9. The Java programming environment

Compared to C++:








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


no header files, macros, pointers and references, unions,
operator overloading, templates, etc.
High performance through Just in time compilation +
runtime modification of code
Multi-threaded
9

10. Java Features

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
10

11. The Java programming environment

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
11

12. Java: A tiny intro

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?
12

13. How are Java programs written?

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
13

14. How are variables declared?
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;
}
}
}
14

15. How to define expressions?

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);
}
}
15

16. How are simple methods defined?
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));
}
}
16

17. How are control structures specified?
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;
}}}
17

18. What are classes and objects?
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
18

19. How are instance methods defined?
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());
}
}
19

20. How to extend classes?


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
Inherits
Inherits
Inherits
C can:




instance variables
static variables
instance methods
static methods
Add new instance variables
Add new methods (static and dynamic)
Modify methods (only implementation)
Cannot delete anything
20

21. How to extend classes?
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;}
}
21

22. What are abstract classes?

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();
22

23. Packages
Object
extends
Attraction
Auxiliaries
Demonstration
extends
Movie
Symphony
• 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
23

24. Packages – cont’d.


units of organizing related Classes, Interfaces,
Sub packages
Why?



Java programs typically organized in terms of
packages and subpackages



Reduce name clashing
Limit visibility of names
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
24

25. Packages – cont’d.
package onto.java.entertainment;
public abstract class Attraction { … }
package onto.java.entertainment;
public class Movie extends class Attraction {…}
package onto.java.entertainment;
import java.io.*;
import java.util.*;
public class Auxiliaries { … }
•Where to store packages?
•How does Java find packages?
•Export and Import
•Access control
25

26. Exceptions
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);
}
}
26

27. Finally
public class A {
public void foo() throws MyException {
throw new MyException();
}
}
public void bar() {
try {
this.foo();
} catch (MyException e) {
e.printStackTrace();
} catch (YourException e) {
e.printStackTrace();
} finally {
... // always executed before leaving the try/catch
}
}
}
27

28. Resources

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!

28

29. Qualifiers




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)
29

30. Package Protected
package edu.ucdavis;
public class A {
int x;
}
package edu.ucdavis;
public class B {
void foo(A a) { a.x; } // OK, same package
}
package org.omg;
public class B {
void foo(A a) { a.x; } // Not OK, different package
}
package edu.ucdavis.cs;
public class B {
void foo(A a) { a.x; } // Not OK, different package
}
package edu.ucdavis.cs;
public class B {
void foo(A a) { a.x; } // Not OK, different package
}
package edu;
public class B {
void foo(A a) { a.x; } // Not OK, different package
}
30

31. Protected
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
}
31

32. Threads




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
32

33. Creating a Thread in Java
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) {}
}
}
}
33

34. Creating a Thread in Java
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
...
34

35. Creating a Thread in Java via Interface
public class MyRunnable implements
String name;
public MyRunnable(String name) {
this.name = name;
}
public void run() {
for(int i; i < 10; i++) {
System.out.println(i + “ “ +
try {
sleep((long)(Math.random()
} catch(InterruptedException
}
}
}
Runnable {
name());
* 1000));
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();
}
}
}
35

36. Producer Consumer Problem
public class Producer
extends Thread {
private Share shared;
public class Consumer
extends Thread {
private Share shared;
public Producer(Share s) {
shared = s;
}
public Consumer(Share s) {
shared = s;
}
public void run() {
for(int i = 0; i < 10; i++){
shared.put(i);
}
}
public void run() {
int value;
for(int i = 0; i < 10; i++) {
value = shared.get();
}
}
}
}
shared.put(0)
shared.get() // 0 gotten
shared.get() // 0 gotten again!!
shared.put(0)
shared.put(1)
shared.get() // 0 never gotten!!
// what about simultaneous
// access?!
shared.put(0) shared.get()
RACE CONDITIONS!
36

37. Synchronized
public class Share {
private int s;
public synchronized int get() { ... }
public synchronized void put(int s) { ... }
}
Synchronized provides mutual exclusion on an
object
 For any object, only one thread may execute
inside any of that object’s synchronized
Share s1 = new Share();
t1 -> s1.get() // gets in
methods
Share s2 = new Share();
t2 -> s1.put(32) // blocks

Thread t1 = ...;
Thread t2 = ...;
t1 -> s1.get() // gets in
t2 -> s2.put(4) // gets in
37

38. Producer Consumer Coordination
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
}
38