The Java Programming Language

Institute for Visualization and Perception ResearchI VPR 1

© Copyright 1998 Haim Levkowitz

The Java Programming Language

• A Quick Tour of Java …



Getting Started ...

• Why Java?

• What Is Java? ...

• Two Simple Examples ...

• Executing a Java Applet ...

• Java Virtual Machine ...

• Java and the Web ...

• Java vs. C ...



Properties of Java (claimed & desired) ...

• Simple ...• Object-oriented ...• Distributed ...• Interpreted ...• Robust ...• Secure ...• Architecture neutral ...• Portable ...• High-performance ...• Multithreaded ...• Dynamic ...



Simple ...• Small number of language constructs• Look familiar: like C / C++• No goto (break & continue instead)• No header files• No C preprocessor• No struct, union, operator overloading, multiple

inheritance• No pointers: auto handling of de/referencing• Auto garbage collection



Object-oriented ...• Data• Methods• Class: data + methods

• Describe state & behavior of object• Hierarchy: subclass inherit behavior from superclass

• Packages of classes• java.awt (Abstract Windowing Toolkit): create GUI components• java.io: I/O• java.net: Network functionality

• Object class in java.lang package• root of Java class hierarchy• most things are objects

• numeric, character, boolean types only exceptions



Distributed ...

• Network connectivity: classes in java.net

• E.g., URL class: open & access remote objects on Internet

• ==> Remote / local files same

• Socket class: stream network connections

• ==> Distributed clients & servers



Interpreted ...• Bytecodes (no machine code)• Java interpreter to execute compiled bytecodes• Architecture-neutral object file format• ==> Portable to multiple platforms• Java Virtual Machine

• Java interpreter• Run-time system

• Link: only to load new classes into environment• ==> Rapid prototyping / easy experimentation



Robust ...• Original design for consumer electronics• Strongly typed ==> compile-time check for type-mismatch • Requires explicit method declarations• Memory model

• No pointers ==> no memory overwriting / corrupting• Automatic garbage collection

• Run-time checks: e.g., array / string access within bounds• Exception handling: try/catch/finally statement

• ==> group all error handling in one place• ==> Simpler error handling & recovery



Secure …• Application Safety 4 Layers ...• Solid examination of security

• Bugs identified & corrected: http://java.sun.com/sfaq

• The Princeton Hacks• Three problems• Rogue classloader• IP Spoofing• Denial-of-service



Application Safety 4 Layers ...

• Layer 1: Language and Compiler ...

• Layer 2: Bytecode Verifier ...

• Layer 3: Classloader ...

• Layer 4: Interface-specific Security ...



Layer 1: Language and Compiler ...

• Memory allocation model• Compiler doesn't handle memory layout

• ==> Can't guess actual memory layout• No pointers ==> all memory references

via symbolic handles• Memory reference --> real memory at

run-time by interpreter



Layer 2: Bytecode Verifier ...• Java code can be loaded over untrusted

network

• ==> Take into account potential hostile Java compilers

• Runtime system: all code through theorem prover ...

• Checks also verify stack overflows ==> interpreter exec faster



Runtime system: all code through theorem prover ...• Verify that code• Doesn't forge pointers to illegally manipulate

objects outside VM• Doesn't violate access restrictions• Access objects according to correct type• Use correct type for all instruction parameters• Use legal object field accesses according to

their private, public, or protected def'n



Layer 3: Classloader ...

• Loaded classes in separate namespace than local

• Prevent malicious applet from replacing standard applet

• Single namespace for all classes from local file sys



Layer 4: Interface-specific Security ...• Interface to standard networking protocols

• HTTP, FTP, etc.• Network package configured to provide diff levels

of security• 1. Disallow all network accesses• 2. Allow network accesses to only hosts from

which code was imported• 3. Allow network accesses only outside firewall

if code came from outside• 4. Allow all network accesses



Architecture neutral ...

• Originally: Consumer electronics

• Next: network-based applications

• Also: cross platform

• With java.awt: appropriate behavior and appearance for each



Portable ...• Architecture neutral

• No implementation-dependent aspects of language spec's

• E.g., explicitly specify size of each primitive data type

• And arithmetic behavior

• Java compiler written in Java

• Java run-time system written in ANSI C.



High-performance ...

• Interpreted ==> avg. 20 time slower than C• Still adequate for interactive, GUI-, network-based

app's• For critical performance: "just in time" compilers

• Translate Java bytecodes --> machine code at run-time

• Performance nearly as good as native C / C++• Middle: between C / C++ and Tcl / UNIX shells• Better than Perl



Multithreaded ...

• Need locks to prevent deadlocks• Built-in support: Thread class (in java.lang

package)• Support methods to start / run / stop / check

thread• Synchronization primitives

• Prevents certain methods from running concurrently

• ==> Ensure consistent state of variables



Dynamic ...

• E.g., load classes across network as needed

• Classes have run-time representation

• Object can check which is its class

• Can dynamically link classes into a running system



Two Simple Examples ...

• Hello, world ...

• A scribble applet ...



Hello, world ...

class HelloWorld {public static void main(String[] args) {

System.out.println("Hello, world");}

}javac HelloWorld.java ==>

HelloWorld.classjava HelloWorld



A scribble applet ...

• 1-2-Scribble.java



Executing a Java Applet ...• Compile ==>

stand-alone code

• Distributed applets

Java source

Java compiler

Java bytecode

Bytecode loader

Bytecode verifier

Bytecode interpreterJust-in-time

compilerJava run-time

Hardware

Network or file system

or



Java Virtual Machine ...• SW simulation of idealistic HW architecture

• Execute Java bytecodes• Java instruction ...• Virtual machine architecture

specification ...• Current implementation (Sun) ...• Bytecode execution ...



Java instruction ...

• One-byte opcode: operation• 0 or more operands• Inner loop of the JVM

do {fetch an opcode byteexecute an action depending on

value of opcode} while (there is more to do);



Virtual machine architecture specification ...• Basic set of supported data types

• No specific internal structure of objects



Current implementation (Sun) ...• Object reference as handle w / pair of pointers

• 1 --> object's method table• 2 --> data allocated by object

• (another option: in-line caching rather than method table dispatch)

• Written in C• Pointers don't violate security model / lack of

pointers• Can't access VM pointers directly from Java

source code / compiled Java bytecode



Bytecode execution ...

• Like simple RISC microprocessor• Program counter: address of current

bytecode• VM executes single method at a time ...• Add'l registers: info on current exec.

method ...• Java VM instructions ...



VM executes single method at a time ...

• Multithreading through built-in threads lib

• SW construct

• Doesn't depend on specific HW support



Add'l registers: info on current exec. method ...

• vars: reference memory space allocated for method's local variable

• optop: point to method's operand stack• frame: point to method's execution

environment structure• Size of memory spaces pointed by

registers well def'd at compile time



Java VM instructions ...

• Take operands from operand stack• Operate on them• Return results to stack• Stack organization easy to emulate

efficiently on machines w/ limited # of registers• E.g., Intel 486



Instruction categories ...

• Stack manipulation (load, store, move)• Array management• Arithmetic (integer, floating point)• Logical• Control transfer• Method invocation and return• Exception handling• Monitors

• Implement locking mechanisms• Provide exclusive access



Java and the Web …

• Simple Applet

• Embedding in Web page ...



Simple Applet

import java.awt.*;import java.applet.*;public class Wave extends Applet {

int n = 1public void paint(Graphics g) {

double y = 0.0, oy = 0.0;for (int x = 0; x < size().width; oy = y, y = 0, x++) {

for (int j = 0; j < n; j++)y += Math.sin((2 * j + 1) * x / 15.0) / ( 2 * j + 1);

Cont. ...



Cont. ...y = 0.47 * y + 0.5;

if (x > 0) g.drawLine(x, (int)(oy * size().height), x + 1, (int)(y * size().height));

}}public boolean mouseDown (java.awt.Event evt.

int x, int y){ n = n < 15 ? n + 1 : 1 ; repaint (); return true; }

}



Embedding in Web page ...<html>

This simple applet example draws a sine wave.

<hr>

<applet codebase="classes"

code="Wave.class"

width=600 height=100>

</applet>

</html>



Java vs. C ... • Program Structure and Environment ...• The Name Space: Packages, Classes, & Fields ...• Comments ...• No Preprocessor ...• Unicode and Character Escapes ...• Primitive Data Types ...• Reference (Non-primitive) Data Types ...• Objects ...• Arrays ...• Strings ...• Operators ...• Statements ...• Exceptions and Exception Handling ...• Miscellaneous Differences ...



Program Structure and Environment ...

• Command-line arguments ...

• Program exit value ...

• Environment ...



Command-line arguments ...

• Single argument to main(): array of strings, argv[]

• argv.length



Program exit value ...

• main() must be declared to return void

• Can't return in main()

• Use System.exit() to return value

• Interpreter exits immediately

• OS dependent



Environment ...

• No reading OS env var's (OS dependent)

• Instead, system properties (OS independent)

• Lookup with System.getProperty() method

• Set of standard sys prop's

• Can insert add'l (-D option to interpreter)



The Name Space: Packages, Classes, & Fields ...• No global variables ...

• Packages, classes, and directory structure ...• Packages of the Java API ...• The Java class path ...• Globally unique package names ...• The package statement ...• The import statement ...• Access to packages, classes, and fields ...



No global variables ...

• Variables, methods within class

• Class part of package

• ==> Var's, methods ref'd by fully qualified name

• Package_name.Class_name.Field_name (field: var / method



Packages, classes, and directory structure ...

• Compiled class in separate file ...

• Source code: .java ...



Compiled class in separate file ...

• Same name as class, w/ .class extension

• Stored in directory w / same components as package name

• E.g., Pkg.Sub.Subsub.class_name in Pkg/sub/subsub/class_name.class



Source code: .java ...• One or more class def'ns• If more than one, only one declared public

• I.e., available outside package• Must have same name as source file

(wo .java)• Multiple classes in source compiled to

multiple .class files



Packages of the Java API ...

• java.applet: implementing applets• java.awt: graphics, text, windows, GUIs• java.awt.image: image processing• java.awt.peer: platform-independent GUI toolkit• java.io: input / output• java.lang: core language• java.net: networking• java.util: useful data types



The Java class path ...• Interpreter looks up class

• System classes in• Platform-dependent default location, or• Relative to dir. spec'd by -classpath arg

• User defined classes in• Current directory, and• Relative to dir's spec'd by CLASSPATH env. var.

• E.g., on UNIX: setenv CLASSPATH .:~/classes:/usr/local/classes



Globally unique package names ...

• Internet-wide unique package naming scheme• Based on domain names

java.lang.Sring.substring()

package name class method

package name class

domain prefix



The package statement ...• First text other than comments, whitespace

• Which package this code is part of

• ==> Access to all classes in package

• ==> Access to all non-private methods & fields in classes

• Compiled class must be placed in appropriate position in CLASSPATH dir. hierarchy

• If omitted ==> code is part on unnamed default package

• ==> Can be interpreted from current dir

• ==> Convenient for small test programs



The import statement ...• Make classes available to current class under

abbreviated name

• Public classes always available by fully qualified name

• Any number, but must appear

• After package statement

• Before first class / interface def

• Three forms ...



Three forms ...

• import package; ...

• import package.class; ...

• import package.*; ...



import package; ...

• Spec'd package by name of its last component

• E.g., import java.awt.image;

• ==> Can call java.awt.image.ImageFilter just ImageFilter



importpackage.class; ...

• Spec'd class in package by its class name alone

• E.g., import java.util.Hashtable

• ==> Can use Hashtable instead of java.util.Hashtable



import package.*; ...• All classes in package available by their

class name• E.g., import java.lang.*;

• Implicit (no need to specify)• ==> Make core classes available by

unqualified class names• Ambiguous class names ==> must use full

name



Access to packages, classes, and fields ...• Package accessible if files/dir's are accessible• All accessible to all others in same package ...• Public class in pkg accessible within another

pkg ...• All fields (var's, methods) of a class are accessible

within class• Fields in class accessible from diff class in same

pkg ...• Fields in class accessible from diff package if ...



All accessible to all others in same package ...

• Can't define classes visible only in single file



Public class in pkg accessible within another pkg ...

• Non-public class not accessible outside package



Fields in class accessible from diff class in same pkg ...• ... if not

• private, or

• accessible only within own class

• private protected

• accessible only within own class and within subclass



Fields in class accessible from diff package if ...

• Class is accessible

• Field declared public, or

• Field declared protected or private protected and access is from subclass of class



Comments ...• 1. /* Standard C-style */

• Cannot be nested• 2. // C++-style

• Use if want to comment out blocks• No preprocessor #if 0 to comment out block

• 3. /** Special "doc comment" */• Processed by javadoc ==> simple online doc• Cannot be nested



No Preprocessor ...

• Defining constants ...

• Defining macros ...

• Including files ...

• Conditional compilation ...



Defining constants ...

• Declare variable final ...• E.g., java.lang.Math.PI ...• Note• C convention of CAPITAL• Globally unique hierarchical names• ==> No name collision• Strongly typed ==> better type-checking by

compiler



Declare variable final ...

• Must specify initializer when declared

• Equivalent of C #defined: static final

• Within class definition

• If compiler can compute value, uses value to pre-compute other constants referring value



E.g., java.lang.Math.PI ...

public final class Math {

...

public static final double PI = 3.14159.....;

...

}



Defining macros ...

• C preprocessor

• Looks like function invocation

• Replaced directly with C code

• Save overhead of function call

• No equivalent in Java

• Compiler might help



Including files ...

• No #include; no need• Mapping of fully qualified class names to directory

& file structure• E.g., java.lang.Math and java/lang/Math.class

• Compiler knows exactly where to find what needs• No diff between declaring var / proc & defining

• ==> No need for header files, function prototypes• ==> Single object file: def'n & implementation

• Import ==> shorter names (e.g., Math.PI)



Conditional compilation ...

• No #ifdef, #if• Good compiler won't compile code known to

never be executed• ==> if (false) block equiv. to #if 0 #endif in C• Works with constants; E.g.,

• private static final boolean DEBUG = false;• if (DEBUG) block not compiled• Change DEBUG to true to activate

debugging



Unicode and Character Escapes ...• Characters, strings, identifiers in 16-bit Unicode

characters• ==> Easier to internationalize

• Compatible w/ ASCII• First 256 chars (0x0000 – 0x00FF) identical to

ISO8859-1 (Latin-1) 0x00 – 0xFF• String API makes char rep'n transparent• Unicode escape sequence: \uxxxx (if platform can't

display all 34,000 Unicode char's)• Also full support of C char escape sequences



Primitive Data Types ...

• byte and boolean primitive type + standard set of C types

• Strictly defined size and signdness of types• All variables have guaranteed default values• The boolean type ...• The char type ...• Integral types ...• Floating-point types ...



The boolean type ...

• Contains: true or false

• Default: false

• Size: 1 bit

• Min value: NA

• Max value: NA



The char type ...

• Contains: Unicode character

• Default: \u0000

• Size: 16 bits

• Min value: \u0000

• Max value: \uFFFF



Integral types ...

• All are signed (except char)• Division by 0 ==> ArithmeticException

thrown• byte ...• short (NOT short int!) ...• int ...• long (NOT long int!) ...



byte ...

• Contains: signed integer

• Default: 0

• Size: 8 bits

• Min value: -128

• Max value: 127



short (NOT short int!) ...


• Default: 0

• Size: 16 bits

• Min value: -32768

• Max value: 32767



int ...


• Default: 0

• Size: 32 bits

• Min value: -2147483648

• Max value: 2147483647



long (NOT long int!) ...


• Default: 0

• Size: 64 bits

• Min value: -9.223372036854775808

• Max value: 9.223372036854775807

• Can append L or l to long constants



Floating-point types ...

• float ...

• double ...

• Special values ...



float ...

• Contains: IEEE 754 floating-point

• Default: 0.0

• Size: 32 bits

• Min value: ±3.40282347E+38

• Max value: ±1.40239846E-45

• Can append F or f to value



double ...

• Contains: IEEE 754 floating-point

• Default: 0.0

• Size: 64 bits

• Min value: ±1.79769313486231579E+308

• Max value: ±4.94065645841246544E-324

• Can append D or d to value



Special values ...

• In java.lang.Float, java.lang.Double• POSITIVE_INFINITY• NEGATIVE_INFINITY• NaN

• Unorderd ==> comparison yields false• Use Float.isNaN(), Double.isNaN() to test

• Negative zero compares equal to positive zero ...



Negative zero compares equal to positive zero ...

• 1/-0 = NEGATIVE_INFINITY

• 1/0 = POSITIVE_INFINITY

• Floating point arithmetic never causes exception



Reference (Non-primitive) Data Types ...• Handled by reference (primitive types handled by value)• No &, *, -> operators• Two diff vars may refer to same object ...• Not true of primitive types ...• Copying objects• Checking objects for equality ...• Java has no pointers ...• null ...• Reference type summary ...



Two diff vars may refer to same object ...Button a, b;

p = newButton(); // p refs to Button object

q = p; // q refs to same Button

p.setLabel("OK"); // Change object through p ...

String s = q.getLabel(); // ... also visible through q

// s now contains "OK"



Not true of primitive types ...

int i = 3; // i contains value 3

int j = i; // j contains copy of value in i

i = 2; // changing i doesn't change j

// Now, i == 2; j == 3



Copying objects

• Button a = newButton("OK");

• Button b = newButton("Cancel");

• a = b;

• a contains ref to object that b ref's to

• Object that a used to ref to is lost

• To copy, use clone() method ...

• To copy array values ...



To copy, use clone() method ...

• Vector b = newVector;

• c = b.clone();

• Not all types support clone()

• Only classes that implement Cloneable interface may be cloned



To copy array values ...

• Assign each value individually, or

• Use System.arraycopy() method



Checking objects for equality ...

• == : 2 vars ref same object (not 2 objects have same value)

• To compare values, use method for object type

• equals() in some classes



Java has no pointers ...

• Auto ref, deref of objects• Can't cast object / array ref --> int (or vice versa)• No pointer arithmetic• Can't compute size in bytes of primitive type /

object• ==> Avoid pointer bugs• ==> Better security



null ...• Reserved value: “absence of reference”

• Var doesn't refer to any object / array• Default valued of all reference types

• Reserved keyword• May be assigned to any variable of reference

type• Cannot be cast to any primitive type• Not equal zero

Documents

The Java Programming Language