Java Overview
CSE 422
Michigan State University
prepared by Philip K. McKinley
presented by SeyedMasoud Sadjadi
These materials are excerpted from a course created by Arthur Ryman
of IBM Toronto, and used at the University of Toronto. Thanks!
CSE422 Tuesday (Sept. 16, 2003)
Agenda

Introduction to Java (today)
– What is Java?
– Tools Overview
– Language Overview
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Advanced Topics (next session)
– Error Handling
– Multithreading
– Networking
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
2
What is Java?
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A concurrent, object-oriented programming
language (OOPL)
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A virtual machine (run-time environment)
– can be embedded in web browsers (e.g. Netscape
Navigator, Microsoft Internet Explorer and IBM
WebExplorer) and operating systems.
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Portable, Dynamic, and Extensible

A set of standardized packages (class libraries)
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
3
Java, A Concurrent OOPL
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Complete OOPL (not only structures into objects)
Characteristics of both C++ and Smalltalk
– C++
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Same syntax for expressions, statements and control flow
Similar OO syntax (classes, access, constructors, methods, ... )
– Smalltalk
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Similar object model (single-rooted inheritance hierarchy, access to
objects via reference only)
Compiled to a byte-code (initially interpreted)
Dynamic loading
Garbage collection
Concurrency and synchronization (threads)
– Objects can force mutual exclusion of threads running inside them
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
4
Java Virtual Machine
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Java is complied to byte-codes whose target
architecture is the Java Virtual Machine (JVM)
The virtual machine is embeddable within other
environments, e.g. web browser & operating sys.
Uses a byte-code verifier when reading in bytecodes.
The Class Loader for classes loaded over the
network (enhances security).
Java
Source
javac
.java
Java Overview by SeyedMasoud Sadjadi
Environment
Java
Byte-code
Java VM
.class
CSE 422 09/16/2003
5
Portable, Dynamic, and Extensible

Java runtime based on architecturally neutral byte-codes
(per class)
.class
files
interpret
load
Java Runtime
loaded classes
(byte-codes)
call
Native
.dll
Native
.dll
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
6
Standard Set of Packages
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Windowed GUIs
– Full set of standard window-based GUI classes
– Extremely easy to build GUI clients
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Images and audio
– Support for creating Image objects from .gif, .jpg, etc.
– Provides Image processing “filters”
– Applets can also play audio files
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Networking
– Library supports retrieving files, images, etc. via URL
– Clean support for sockets providing access to Internetbased services
– VM can dynamically load classes over the Internet
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
7
Agenda

Introduction to Java (today)
– What is Java?
– Tools Overview
– Language Overview

Advanced Topics (next session)
– Error Handling
– Multithreading
– Networking
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
8
JDK Tools
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Java Developer’s Kit’s (JDK) three main tools
are:
– javac the Java compiler
– java VM for running stand-alone Java applications
– appletviewer a utility to view applets
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Also included are:
–
–
–
–
javah Header file generator for interlanguage linking
javap A disassembler
javadoc HTML generator from Java source code
jdb a rudimentary Java debugger
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
9
JIT Compiler
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Although Java is interpreted, Just-In-Time
compilers provide “client-side” compilation of
byte-codes to machine code (native binary)
This provides:
– Improved performance
– Better match to specific hardware
JVM running
Applet or
Application
Java Overview by SeyedMasoud Sadjadi
.class
machine
code
J.I.T.
Compiler
CSE 422 09/16/2003
10
Eclipse
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jdt: java development tools subproject
– Plug-ins for Java development
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
11
Agenda

Introduction to Java (today)
– What is Java?
– Tools Overview
– Language Overview

Advanced Topics (next session)
– Error Handling
– Multithreading
– Networking
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
12
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
13
Java Programs
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Two broad categories of program can be written
– Applet
a
Java program that runs inside a Java-enabled
Web browser.
– Application
a
Java program run via the “java” command.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
A Simple Java Application
import java.io.*;
/** File: Count.java*/
public class Count {`
public static void main (String[] s) throws IOException {
int count = 0;
while (System.in.read() != -1)
count++;
System.out.println("Input has "+count+" chars");
}
}
• Compile the .java file to generate the .class file
cmd>javac Count.java
• Run the interpreter on the .class file
cmd> java Count
This is a test.
Input has 16 chars
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Analysis
import java.io.*;
/** File: Count.java*/
public class Count {`
public static void main (String[] s) throws IOException {
int count = 0;
while (System.in.read() != -1)
count++;
System.out.println("Input has "+count+" chars");
}
}
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All Java code is contained within classes.
Java classes consist of fields (variables) and methods.
A Java source file contains at most one public class.
Applications must provide a method called main. To be recognized,
the main method must have the correct method signature.
Java stores collections of classes in packages (class libraries). The
import keyword selects the packages available.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Comments
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There are different types of comments
// single line comment (until eol)
/* single/multi-line comment (do not nest) */
/** multi-line documentation comment.
Use immediately before class, method, and
variable declaration. The javadoc utility will
use this comment to automatically generate HTML.
May also include HTML and use optional tags:
<B> Here is a bolded comment <\B>
@author Neil Bartlett
@param d a number
@return sqrt of the number
*/
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
18
Variables and Identifiers
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Variable may be a primitive data type or reference to an
object
Unicode 1.1 character set used (16 bit international
character set encoding). This applies to the char data
type.
An identifier starts with:
– a letter (from any language encoded by Unicode)
– an underscore (_), or
– a dollar sign($)
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Subsequent characters may be letters or digits or both
Identifiers can be any length
Identifiers may not be a reserved word or a boolean literal
(true, false)
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Data Types - Primitive Types
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Primitive Type
Precision
Default Value
byte
8 bits
0
short
16 bits
0
int
32 bits
0
long
64 bits
0
char
16 bits
'\u0000'
float
32 bits
+0.0f
double
64 bits
+0.0d
boolean
false
No variable can have an undefined value
Class variables are implicitly initialized to the default
unless set explicitly
Local variables are not implicitly initialized to a default
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Scope of a Variable
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Scope is the block of code in which a variable is
accessible.
– member variable. Declared within a class but not within a method.
– local variable. Declared within a method or within a block.
– method parameter. Values passed into method (more later)
i
f
class MyClass {
float myMethod( float f ) {
f1
float f1;
f2
{ // define a block inside a method just for fun
float f2 = 10F;
f1 = f2;
}
f3
float f3 = f1;
return f*f3+i;
}
int i = 0;
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Access Specifiers
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Specifies who may access variables. Also applies
to classes, constructors, and methods.
public
– available everywhere
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protected
– available only to the current class and its subclasses

private
– available only to the class in which it is declared. This
is applied at the class not the object level.

package
– If no access specifier is explicitly, available only within
the current package
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
23
Flow of Control - Conditional
if (condition) {
statements;
} else {
statements;
}
switch (intVal) {
case intVal1:
statements;
break;
case intVal2:
statements;
return;
default:
statements;
break;
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Flow of Control - Looping
for (initialize; test; increment){
statements;
}
while (condition)
statements;
}
{
do {
statements;
} while (condition);
goto // reserved word that does nothing!
break label;
continue label;
restart:
for (int i = start; i < a.length ; ++i) {
// mess with start
if (a[i] == ';')
continue restart;
}
CSE 422
Java Overview by SeyedMasoud
Sadjadi
09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
26
Creating Objects
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Objects are instances of classes.
To declare an object, use the class name (the
type) and a identifier, e.g.
Date today;
A variables stores reference to an object. The
declaration does not create an object.
Objects are created with the new reserved word.
This will create the memory for the object and
return a reference to the object
today = new Date();
Or, using one step
Date today = new Date();
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
The new operator
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The new operator creates an object by allocating memory.
Takes one parameter - the class constructor. The class
constructor is a special method declared in the class. It is
responsible for initializing the object to a known state.
Rectangle r = new Rectangle(0, 0, 100, 200);
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Constructors have the same name as the class. A class
can have more than one constructor, e.g.
Rectangle r = new Rectangle(100, 200);
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Constructors typically set up the object's variables and
other initial state. They might also perform some initial
behavior.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Objects and References
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A variable stores a reference to an object. There is no equivalent of
C++ pointer.
Many objects references may refer to the same object
MyClass o1 = new MyClass();
MyClass o2 = o1;
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Both o1 and o2 now refer to the same object

o1

MyClass
Object
o2
Comparing variables that refer to objects just compares the
references. It does not compare the objects.
Integer i1 = new Integer(10);
Integer i2 = new Integer(10);
if (i1 == i2) {
// not true
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
30
Arrays
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Arrays are objects in Java. Use new to create them.
Arrays are fixed length. Length cannot be changed once
created. Indexes start at zero. Indexes are bounds
checked.
Primitive Array
byte[] bArray = new byte[5];
for (int i=0; i < bArray.length; i++)
bArray[i] = 42; // value
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Object Array
Date dArray[]
for (int i=0;
// Must now
dArray[i] =
= new Date [5];
i < dArray.length; i++)
create the date objs
new Date (); //ref
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Multidimensional Arrays
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Implemented as arrays of arrays
int twoDArray[][] = new int[300][400];
– Declares a variable of type int[][]
– Dynamically allocates an array of with 300 elements
– Allocates arrays of 400 ints for each element of the 300 element
array
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Can provide partial sizing
int threeDArray[][][] = new int[10][][];
Multidimentional arrays need not be rectangular
int threeDArray[][][] = new int[10][][];
threeDArray[0] = new int[100][4];
threeDArray[1] = new int[3][5000];
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Initializing Arrays
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Arrays may be initialized with static initializers
int lookup_table[] = {1, 2, 3, 4, 5, 6, 7, 8};
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This is equivalent to
int lookup_table[] = new int[8];
lookup_table[0] = 1;
…
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Similarly for multidimentional arrays
String param_info[][] = {
{"fps", "1-10", "frames per second"},
{"repeat", "boolean", "repeat image loop"},
{"imgs","url","images directory"}
};
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Strings
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Strings are objects, not primitives
Not null-terminated, not array of char
Rich set of string manipulation methods
Initializing
– must construct a string object, String s does not create
an object
– String a = "abc" eqv. String a = new
String("abc")
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Concatenation operator, "abc"+"def"
String class is non-mutative
Use StringBuffer class for strings that change
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
35
Declaring Methods
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Must declare the data type of the value it returns. If no
value is returned, it must be declared as returning void.
Methods may take arguments. These are values that are
passed into the method when it is called. Arguments are
typed and named. If names conflict with the class level
variables, the argument names will hide the class level
variable names.
Methods are scoped for the whole class. No need for
forward references.
Java is very strongly typed. No equivalent of C variable
length argument list.
Cannot pass methods into methods. (Methods are not a
type)
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Passing Arguments to Methods
Arguments are passed by value. Changing the value inside the
method does not effect the value outside the method. This applies to
both primitive types and object references.
public class myClass {
int x;
void myMethod(myClass ac, int ay) {
ay = 10;
ac.x = 5;
ac = null;
}
public static void main (String args[]) {
myClass c = new myClass();
c.x = 1000;
int y = 2000;
c.myMethod(c, y);
System.out.println("c:"+c+" c.x:"+c.x+" y:"+y);
}
}
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Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
38
Declaring Classes
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A class is declared using the class keyword
class myClass {
// class body
}
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A class is by default accessible (e.g. can create
objects of the class) from any other classes in the
same package.
The public keyword can be used to create a class
that is available anywhere.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Constructors

Constructors are called by the new operator when a class
is created. The constructor initializes the new object.
class myClass {
int x;
String s;
myClass() {
x = 10;
S = "Hello";
}
}

Constructors have no return type, but they may take
arguments. The argument types must match those
provided by the new operator.
myClass(int x, String s) { … }
myClass c = new myClass(10, "Hello");
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Default Constructor
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If no constructor is present a default constructor
will be provided by the compiler.
The default constructor has no arguments and
just calls the super class's constructor.
class myClass {
myClass() {
super();
}
}

Does not provide default constructor with
arguments
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Class Variables and Methods
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Problem: If all method calls need an object, how to we provide global
constants and utility functions. Must we create an object just to use
them?
Classes can provide variables and methods that may be used with
out an object. There are called class variables and class methods.
To make a variable or method into a class variable, use the static
keyword, e.g.
static int count;
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In contrast variables and methods of objects are called instance
methods and instance variables.
Class methods may directly use other class methods and class
variables. If they want to use an instance methods or variables, they
must instantiate objects.
class method may not be overridden.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Examples of Class Variables and Methods
import java.util.*;
public class ClassMethodExample {
static String todaysDate() {
Date d = new Date();
return d.toString();
}
public static void main(String[] s) {
System.out.println( "The square root of pi is "+
Math.sqrt( Math.PI ));
System.out.println( "The date is "+todaysDate() );
}
}
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main method is a class method. It does not require an object.
Math and System are part of the core java packages.
– They provide useful math and system functions and constants. These are
implemented as class methods and class variables.
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todaysDate is a user-defined class method. It constructs a Date
object to do its job.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
44
Inheritance
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To inherit a class from another class use the
extends keyword
class SubClass extends SuperClass {
...
}
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The sub class inherits variables and methods
from its super class. It also inherits variables and
methods from the super class of the super class
and so on up the inheritance tree.
Java has single inheritance. A class can only
have one super class.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
The Object class
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Every class you define has a super class. There is a special class
called Object which is the implicit super class of any class which does
not explicitly descend from a class, so
class MyClass

is equivalent to
class MyClass extends Object
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
Object is the root class of all classes.
The Object class provides generic methods for all objects. These
include:
– getClass. Returns an object that contains information about the class that
the object was created from.
– toString. Provides a generic string detailing the object.
– clone. A placeholder method to allow copying of an object
– equals. A method to compare objects.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
What's Inherited?
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When one class extends from another, the sub class inherits those
variables and methods that:
– are declared with the public or protected access specifiers.
– have no access specifier

But don't inherit those
– with the same name a one in the sub class.
– declared as private.
class SuperClass {
int x, y;
int methodA() {…}
int methodB() {}
}
class SubClass extends SuperClass {
int y; // hides SuperClass.y
int methodB() {} // overrides SuperClass.methodB
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
48
Polymorphism - Method Hiding
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A method with the same signature as a method in its
super class hides or overrides the method in the super
class.
An object of a sub class may be assigned to a reference
of a super class. In this case, these overridden methods
will be called
class SuperClass {
void aMethod() { … }
}
class SubClass {
void aMethod() {…}
}
SuperClass s = new SubClass();
s.aMethod(); // calls SubClass's aMethod
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
The final keyword
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The final keyword is used to limit what can be changed
when it is inherited It can be applied to:
classes. A final class cannot be extended from.
Generally you do this for security reasons , e.g. the
String class
final class MyClass

methods. This stops a method from being overridden in
a subclass. The method may still be called by the
subclass.
final double sqrt(double d)

variables. This declares a constant value. The value is
available to the subclass but it may not change or
shadow the value.
final int useful_constant=10;
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
this and super
In method body, this is a reference to the current object
and super is a reference to its parent.
class A {
Object x;
}
class B extends A {
float x;
float calculate(){ … }
}
class C extends B {
int x;
void m(char x) {
char cmx = x;
int cx = this.x;
float bx = super.x;
Object ax = A.x;
}
float calculate() {return
}
Java Overview by SeyedMasoud Sadjadi
//the
//C’s
//B’s
//A’s
method argument
member x
x, also B.x
x
super.calculate();}
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
52
Abstract Classes
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Super classes that define generic behaviours that must be
implemented by derived classes
abstract class DiscPlayer {
protected int track;
void setTrack() { /* cue the track to play*/ }
public abstract void play();
}
class VideoDiscPlayer extends DiscPlayer {
public void play() { /* play the video disk */ }
}
class MultiCDPlayer extends DiscPlayer {
protected int currentCD;
public void play() { /* play the current CD */ }
}
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DiscPlayer explicitly provides some responsibility while deferring
other responsibility to its subclasses.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Abstract Classes and Methods
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An abstract class may contain zero or more abstract methods. Any
class that contains an abstract method is implicitly abstract.
An abstract class cannot be instantiated.
An abstract method must be implemented in a subclass to instantiate
an object of the subclass.
Abstract methods can provide implementation. This is useful to
provide default processing:
class SuperClass {
abstract void aMethod() { /* something useful */ }
}
class SubClass extends SuperClass {
void aMethod() {
super.aMethod();
...
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Interfaces
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An interface specifies methods that must be implemented. The
interface does not implement the methods; The methods are
implemented by the class that implements the interface.
public interface Runnable {
public abstract void run ();
}
class A implements Runnable {
public void run() {
// do something
}
}
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All interfaces are public. All methods of interfaces are implicitly public
and abstract
Also permitted as part of an interface are public static final fields.
public interface myInterface {
public static final aConstant=100;
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Class Inheritance Vs. Interfaces
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Interfaces define only method signature. Inherited
classes can provide implementation.
Can only have one super class. Can implement a
number of interfaces
class A implements Runnable, Printable
{
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Choose class inheritance for strongest isA
relationship. Choose interfaces for behavior (Note
frequent use of the suffix -able for interface
names)
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
57
Garbage Collection
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The runtime reclaims storage asynchronously using a
garbage collector. The garbage collector frees the
memory associated with any objects that do not have
references.
The garbage collector runs in a low-priority thread. It is
also called if the memory system runs out of memory to
allocate.
Use System.gc() to explicitly force garbage collection
For variables that do not go out of scope, but that you still
want to be garbage collected, set the object reference
variable to null. This removes the reference to the
underlying object.
obj = null;
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Finalize
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A class may request finalization of its instances by
implementing a finalize() method
protected void finalize () throws Throwable {
/* cleanup */;
super.finalize();
}
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Note that this method may NOT have any other method
modifiers associated with it.
When an object is first detected to be unreferenced, the
finalize method is invoked (if present). If it is subsequently
determined to be unreferenced, the object is reclaimed.
All uncaught exceptions occurring during finalization are
ignored.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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
Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
60
The Class class
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The Class class contains information about a class. This
allows us to provide runtime type information (RTTI)
There is a Class object for each class that has been
loaded.
The Class class allows information about the class to be
inspected, e.g. getName, getInterfaces, getSuperClass.
The Object class has a method getClass which will return
a class object.
// print out the name of the class of an object
String objClassName = obj.getClass().getName();
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Class Loading
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Classes are loaded dynamically by the system from .class files.
When a class is first referenced, a store of classes is checked, if the
class has not been loaded, it is loaded.
Classes can be dynamically loaded under program control. The Class
class has a method forName which takes a String of the name of the
class and returns a Class object.
Any blocks inside the class that are marked as static are run when the
class first loads.
class myClass {
static {
System.loadLibrary("mylib.dll");
}
}
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Advanced Object Creation
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It is not necessary to use the new operator to create an
object.
Objects can be created from a just a name.
Objects of the Class class provide the newInstance
method to create an instance of the class, e.g.
Class aClass = Class.forName ("myClass");
Object o = aClass.newInstance ();

This is often used to load subclasses of superclass, e.g. a
game player might be allowed to upload new monsters. In
this case it is necessary to cast the returned class
String monsterClassName = getNameFromUser();
Class aClass = Class.forName (monsterClassName);
Monster m = (Monster) aClass.newInstance ();
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi
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Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
64
Compilation Units
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Classes and interface are defined in a
compilation unit (a file)
A compilation unit declares zero or more classes.
At most, one declared type (class or interface)
may be declared public.
For a compilation unit which declares a public
type ClassName, the file must be named
ClassName.java.
Multiple classes in a compilation unit will result in
multiple .class files after compilation
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Packages
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Packages are used to logically group together classes.
Each .class file is part of a package. Package is declared using the
package operator. This must form the first statement in the source.
package mypackage;
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If no package is explicitly stated, then the package is unnamed. All
'unpackaged' classes in the directory in which the .class file resides
are part of this package.
Package names have a one-to-one correspondence to a directory.
Package names are dot separated (e.g., java.lang)
Packages can be imported by other source files:
– Example:
import packagename.*;
import packagename.classname;
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
The CLASSPATH
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The CLASSPATH is an environment variable
used to locate packages.
The CLASSPATH consists of a series of
directories separated by semi-colons (Windows)
and colons (UNIX).
set CLASSPATH = d:\mydir;c:\java

Each directory forms the root directory to search
for a package. Thus if the package were
java.lang, there must be a directory called java
under one of the directories in the CLASSPATH
and there must be a directory call lang under that,
for the package to be found.
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Language Overview
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Java Programs
Variables
Flow Control
Objects
Arrays
Methods
Classes
Inheritance
Polymorphism
Abstract Classes
Java Overview by SeyedMasoud Sadjadi




Garbage Collection
Reflection
Packaging
Java vs. C++
CSE 422 09/16/2003
68
Java vs. C++
Java does not:
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have pointers and pointer
arithmetic
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have structs, unions, enums
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have templates
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Java does have:

different compilation model
(compiles to byte-codes per class)
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single-rooted class inheritance
hierarchy
support operator overloading
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multiple interface inheritance
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support multiple inheritance
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strings and arrays are true objects
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have any standalone functions
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garbage collection
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support default arguments for
methods
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support for concurrency via
Threads
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have a delete operator
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have variable arguments
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make use of a preprocessor
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synchronous destructors
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
Agenda

Introduction to Java (today)
– What is Java?
– Tools Overview
– Language Overview

Advanced Topics (next session)
– Error Handling
– Multithreading
– Networking
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003
70
Summary

Java is a full-featured OOP language
– Single-implementation inheritance
– Multiple-interface inheritance
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Java has a similar syntax to C++ but different
semantics
Portable
Garbage Collection
Dynamic Loading
Reflection
Java Overview by SeyedMasoud Sadjadi
CSE 422 09/16/2003