CSE422 Tuesday (Sept. 16, 2003)
Java OverviewJava Overview
CSE 422CSE 422
Michigan State Universityprepared 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!
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AgendaAgenda
Introduction to Java (today)– What is Java?– Tools Overview– Language Overview
Advanced Topics (next session)– Error Handling– Multithreading– Networking
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What is Java?What is Java?
A concurrent, object-oriented programming language (OOPL)
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.
Portable, Dynamic, and Extensible
A set of standardized packages (class libraries)
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Java, A Concurrent OOPLJava, A Concurrent OOPL
Complete OOPL (not only structures into objects) Characteristics of both C++ and Smalltalk
– C++ Same syntax for expressions, statements and control flow Similar OO syntax (classes, access, constructors, methods, ... )
– Smalltalk 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
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Java Virtual MachineJava Virtual Machine
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 byte-codes.
The Class Loader for classes loaded over the network (enhances security).
Java VM
EnvironmentJava
Source
.java
Java Byte-code
.class
javac
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Portable, Dynamic, and ExtensiblePortable, Dynamic, and Extensible
Java runtime based on architecturally neutral byte-codes (per class)
Java Runtime
Native.dll
Native.dll
interpret
loaded classes (byte-codes)
.class files
call
load
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Standard Set of PackagesStandard Set of Packages
Windowed GUIs– Full set of standard window-based GUI classes– Extremely easy to build GUI clients
Images and audio– Support for creating Image objects from .gif, .jpg, etc.– Provides Image processing “filters”– Applets can also play audio files
Networking– Library supports retrieving files, images, etc. via URL– Clean support for sockets providing access to Internet-
based services– VM can dynamically load classes over the Internet
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AgendaAgenda
Introduction to Java (today)– What is Java?– Tools Overview– Language Overview
Advanced Topics (next session)– Error Handling– Multithreading– Networking
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9Java Overview by SeyedMasoud Sadjadi
JDK ToolsJDK Tools
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
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
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JIT CompilerJIT Compiler
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
J.I.T.Compiler
.class
machine code
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EclipseEclipse
jdt: java development tools subproject – Plug-ins for Java development
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AgendaAgenda
Introduction to Java (today)– What is Java?– Tools Overview– Language Overview
Advanced Topics (next session)– Error Handling– Multithreading– Networking
CSE 422 09/16/2003
13Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Java ProgramsJava Programs
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.
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Java Overview by SeyedMasoud Sadjadi
A Simple Java ApplicationA 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 filecmd>javac Count.java
• Run the interpreter on the .class filecmd> java CountThis is a test.Input has 16 chars
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Java Overview by SeyedMasoud Sadjadi
AnalysisAnalysis
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.
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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Java Overview by SeyedMasoud Sadjadi
CommentsComments
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*/
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18Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Variables and IdentifiersVariables and Identifiers
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($)
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)
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Java Overview by SeyedMasoud Sadjadi
Data Types - Primitive TypesData Types - Primitive Types
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
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Scope of a VariableScope of a Variable 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)
class MyClass {
float myMethod( float f ) {
float f1;
{ // define a block inside a method just for fun
float f2 = 10F;
f1 = f2;
}
float f3 = f1;
return f*f3+i;
}
int i = 0;
}
i
ff1
f2
f3
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Java Overview by SeyedMasoud Sadjadi
Access SpecifiersAccess Specifiers
Specifies who may access variables. Also applies to classes, constructors, and methods.
public– available everywhere
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
CSE 422 09/16/2003
23Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Flow of Control - ConditionalFlow of Control - Conditional
if (condition) { statements;} else { statements;}
switch (intVal) {case intVal1:
statements;break;
case intVal2:statements;return;
default:statements;break;
}
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
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; }
Flow of Control - LoopingFlow of Control - Looping
CSE 422 09/16/2003
26Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Creating ObjectsCreating Objects
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();
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
The new operatorThe new operator
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);
Constructors have the same name as the class. A class can have more than one constructor, e.g.
Rectangle r = new Rectangle(100, 200);
Constructors typically set up the object's variables and other initial state. They might also perform some initial behavior.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Objects and ReferencesObjects and References
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; Both o1 and o2 now refer to the same object
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
}
o1 o2MyClass Object
CSE 422 09/16/2003
30Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
ArraysArrays
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 Arraybyte[] bArray = new byte[5];
for (int i=0; i < bArray.length; i++)
bArray[i] = 42; // value Object Array
Date dArray[] = new Date [5];
for (int i=0; i < dArray.length; i++)
// Must now create the date objs
dArray[i] = new Date (); //ref
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Multidimensional ArraysMultidimensional Arrays
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
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];
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Initializing ArraysInitializing Arrays
Arrays may be initialized with static initializersint lookup_table[] = {1, 2, 3, 4, 5, 6, 7, 8};
This is equivalent toint lookup_table[] = new int[8];
lookup_table[0] = 1;
…
Similarly for multidimentional arraysString param_info[][] = {
{"fps", "1-10", "frames per second"},{"repeat", "boolean", "repeat image loop"},{"imgs","url","images directory"} };
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
StringsStrings
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")
Concatenation operator, "abc"+"def" String class is non-mutative Use StringBuffer class for strings that change
CSE 422 09/16/2003
35Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Declaring MethodsDeclaring Methods
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)
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Passing Arguments to MethodsPassing 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);}
}
CSE 422 09/16/2003
38Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Declaring ClassesDeclaring Classes
A class is declared using the class keywordclass myClass {
// class body
}
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.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
ConstructorsConstructors
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");
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Default ConstructorDefault Constructor
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
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Class Variables and MethodsClass Variables and Methods
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;
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.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Examples of Class Variables and Examples of Class Variables and MethodsMethods
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() );
}
}
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.
todaysDate is a user-defined class method. It constructs a Date object to do its job.
CSE 422 09/16/2003
44Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
InheritanceInheritance
To inherit a class from another class use the extends keyword
class SubClass extends SuperClass {
...
}
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.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
The Object classThe Object class
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
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.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
What's Inherited?What's Inherited?
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
}
CSE 422 09/16/2003
48Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Polymorphism - Method HidingPolymorphism - Method Hiding
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 calledclass SuperClass {
void aMethod() { … }
}
class SubClass {
void aMethod() {…}
}
SuperClass s = new SubClass();
s.aMethod(); // calls SubClass's aMethod
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
The final keywordThe final keyword
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;
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
this and superthis 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; //the method argumentint cx = this.x; //C’s member xfloat bx = super.x; //B’s x, also B.xObject ax = A.x; //A’s x
} float calculate() {return super.calculate();}}
CSE 422 09/16/2003
52Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Abstract ClassesAbstract Classes
Super classes that define generic behaviours that must be implemented by derived classesabstract 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 */ }}
DiscPlayer explicitly provides some responsibility while deferring other responsibility to its subclasses.
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Abstract Classes and MethodsAbstract Classes and Methods
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();
...
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
InterfacesInterfaces
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
}}
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;}
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Class Inheritance Vs. InterfacesClass Inheritance Vs. Interfaces
Interfaces define only method signature. Inherited classes can provide implementation.
Can only have one super class. Can implement a number of interfacesclass A implements Runnable, Printable
{
Choose class inheritance for strongest isA relationship. Choose interfaces for behavior (Note frequent use of the suffix -able for interface names)
CSE 422 09/16/2003
57Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Garbage CollectionGarbage Collection
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;
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
FinalizeFinalize
A class may request finalization of its instances by implementing a finalize() methodprotected void finalize () throws Throwable {
/* cleanup */;
super.finalize();
}
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.
CSE 422 09/16/2003
60Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
The Class classThe Class class
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();
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Class LoadingClass Loading
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");
}
}
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Advanced Object CreationAdvanced Object Creation
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 classString monsterClassName = getNameFromUser();
Class aClass = Class.forName (monsterClassName);
Monster m = (Monster) aClass.newInstance ();
CSE 422 09/16/2003
64Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Compilation UnitsCompilation Units
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
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
PackagesPackages
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; 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;
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
The CLASSPATHThe CLASSPATH
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.
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68Java Overview by SeyedMasoud Sadjadi
Language OverviewLanguage Overview
Java Programs Variables Flow Control Objects Arrays Methods Classes Inheritance Polymorphism Abstract Classes
Garbage Collection Reflection Packaging Java vs. C++
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
Java vs. C++Java vs. C++
Java does not: have pointers and pointer
arithmetic
have structs, unions, enums
have templates
support operator overloading
support multiple inheritance
have any standalone functions
support default arguments for methods
have a delete operator
have variable arguments
make use of a preprocessor
synchronous destructors
Java does have:
different compilation model (compiles to byte-codes per class)
single-rooted class inheritance hierarchy
multiple interface inheritance
strings and arrays are true objects
garbage collection
support for concurrency via Threads
CSE 422 09/16/2003
70Java Overview by SeyedMasoud Sadjadi
AgendaAgenda
Introduction to Java (today)– What is Java?– Tools Overview– Language Overview
Advanced Topics (next session)– Error Handling– Multithreading– Networking
CSE 422 09/16/2003
Java Overview by SeyedMasoud Sadjadi
SummarySummary
Java is a full-featured OOP language– Single-implementation inheritance– Multiple-interface inheritance
Java has a similar syntax to C++ but different semantics
Portable Garbage Collection Dynamic Loading Reflection