Java I Lecture 1 UPD1

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Chapter 1
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• It all started with an angry programmer.
1990
Sun Microsystems Software Engineer Patrick Naughton, age 25, was disgusted with his job at Sun. He had the impossible job of making different software APIs--from dozens of languages, platform OS’s and vendors--all work as one. It was impossible.
The Genesis of Java
The Genesis of Java
Naughton announced to CEO Scott McNealy that he was quitting Sun.
Pat was going to join NeXT, because Sun was a mess.
McNealy asked Pat to write a memo outlining his gripes.
The CEO asked Pat to suggest a solution, “As if you were God.”
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The Genesis of Java
Jan 1991
The array of standards spurred the formation of the “Green Project.” Its goal was making Consumer Electronics devices talk to each other.
Since VCRs, Laser Disc Players and Stereos were all made with different CPUs, they all needed special programming.
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Java I--Copyright © 2000-2004 Tom Hunter
• James Gosling, then age 36, was asked to find a programming language for the project. Gosling, who had left IBM in 1984 to join Sun, first chose C++. But he soon gave up on C++, which was incapable of doing what he wanted. So, he started to modify C++, (which is a direct descendant of the C programming language).
The Genesis of Java
• Soon, Gosling was writing a new language, which he named “Oak” after the tree outside his window.
Oak to had to be: • Small to work on Consumer electronics, • Platform independent, to avoid hassles like the ones Naughton encountered, • an Interpreted language, • Object Oriented,
The Genesis of Java
The Genesis of Java
• Reliable--which made him remove aspects of C++: i.) No Multiple Inheritance--he used interfaces instead ii.) No Operator Overloading iii.) No Manual Memory allocation and dealloc iv.) No Pointers--no pointer arithmetic v.) No assignment in conditionals (== vs = ) and add things C++ lacked: i.) Implicit Garbage Collection--no memory leaks ii.) Data Structures only in Objects iii.) Built in Security.
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• Demo of *7, Programmed in Oak
3 Sept 1992
This was the prototype of the first device to use the Oak programming language. The “Star7” also featured the debut of “Duke,” the Java mascot. An early applet showed Duke doing cartwheels across the screen.
The Genesis of Java
The Genesis of Java
Jan 1995
By this time, the Internet had taken off. Bill Joy, one of Sun’s founders, realized that the needs of the Web [ reliability, platform independence, security ] exactly matched the characteristics of Oak, which had just been renamed Java.
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The Genesis of Java
• Java Catches Fire
23 Mar 1995
Although Java had not yet been officially released, it was spreading like wildfire among developers.
Then, something very lucky happened...
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The Genesis of Java
•Netscape Navigator 2.0
23 May 1995
Two months later, at the SunWorld conference, Marc Andreessen stepped on stage and announced that “Java is real, and it will be incorporated into Netscape Navigator 2.0.”
At this moment, Sun’s entire Java team only numbered 30 people.
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Java’s
Major Advantage
• Because pointers were a major source of bugs in C and C++, Gosling omitted pointers entirely from Java.
• Actually, pointers are still an important part of the language--all objects are referenced by pointers--but the language handles them, not the programmer.
Java’s Major Advantage over C & C++
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“Java is C without the Guns and Knives.”
Thus, it has been said that...
Java’s Origins in C & C++
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• By now, Java itself has matured into its 3rd version, named Java 2. This course is based on Java 2. The most current is Java 2 (1.5.1)
• Java is Object-Oriented--that means everything in the language behaves like an object.
• What exactly that means will be explained in the coming weeks.
Java Architecture
Java Architecture
Java’s Architecture comes from four separate but intertwined technologies:
• the Java Programming Language • the Java class file format • the Java API, or Application Programming Interface • the Java Virtual Machine
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Java Architecture
All procedural code falls within methods.
Programs are compiled into Java class files.
Classes run in the Java Virtual Machine.
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Java Architecture
• When a Java program runs, it is assisted by other classes in the Java the Application Programming Interface, or API.
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Java Architecture
Example Java API class files
Combined, the Java Virtual Machine and the Java API form a “Platform.”
The Java Platform
• The Java Platform is unique, because it can work without modification on any platform, on any operating system, if that platform has a “Java Virtual Machine.”
Java Architecture
Comparison of a typical Procedural Program with a Java Program:
• In a typical C program, the source code is compiled into a native machine language module that consists of 1’s and 0’s.
What is the ?
C object module
C Source Code
• The machine language is specifically tailored to one OS, be it Wintel, Mac, UNIX or MVS. • Therefore, it is impossible for one object module to be portable between platforms.
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In contrast to conventional programming languages, a Java program is not compiled into native machine language.
• Instead, Java makes bytecode.
• Bytecode is the result of a “compile”, but the compile results in an intermediate form that stops short of native machine-specific code.
Java “bytecode”
Java Architecture
• Instead of making a machine language native code for each particular OS, Java makes a single, universal bytecode module that feeds into any Java Virtual Machine (JVM).
• Each OS has its own different implementation of the
Java Virtual Machine.
• The JVM sets up its own world within your RAM.
• The JVM creates an internal
software-only sub-computer within the OS.
• The bytecode talks to the JVM, and the JVM talks to the Operating System.
Java Architecture
• Thus, you get the Holy Grail of software reuse:
“Write Once,
Run Anywhere”.
Java Architecture
Java Source
Java Bytecode
Wintel
Mac
UNIX
MVS
JVM-Win
JVM-Mac
JVM-Unix
JVM-IBM
You can easily see why Bill Gates isn’t in love with Java!
The bytecode is met half-way by the JVM.
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• The Virtual Machine interprets the bytecode one instruction at a time, and translates it into native machine code.
• You compile your program once into bytecode, but it is interpreted anew every time it runs.
Java Architecture
Security and the “Sandbox”
C and C++ are famous for speed.
• One reason they are fast is because C and C++ don’t do things like checking the bounds of arrays. • In C or C++, a program can walk off the edge of an array and invade the memory space beyond.
• Hackers love that about C and C++.
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• Another weakness of C/C++, that is a favorite among Hackers, is the Buffer Overflow.
• In this attack, the Hacker floods too much data into a buffer and whatever overflows it is turned loose on the system.
• Java solves these problems
Java checks array boundaries
Java halts Buffer Overflows
Java has Garbage collection to get rid
of objects that are no longer used.
Java’s compiler checks to make sure
the code is safe before it runs.
• Gosling built security into Java, using a concept known as the “Sandbox.”
JDK 1.2 Security Model
SANDBOX
All Code, both Local and Remote, Must Pass Security Policy
Local Code
2.) Compiler creates bytecode
3.) The “Class loader” places the .class file in memory.
4.) The “Bytecode Verifier” makes sure the code adheres to Java’s security rules.
5.) The JVM Interpreter reads bytecode and makes platform native code.
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• You see, preventing problems is a major design consideration in Java.
• This idea led to the most import aspect of Java: Object Orientation.
• Object Orientation protects data and lets a program do only what is explicitly permitted.
• You could say Java is pessimistic.
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Objects in Java
• In Java, Object Orientation is so pervasive that it’s nearly impossible to write a strictly procedural program in the language.
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Objects in Java
• Objects are reusable components.
• In Java, everything must be run from a “class” file. This “class” contains bytecode.
• Java source code has the extension Xxx.java
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Objects in Java
Hello.java
Hello.class
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Objects in Java
• A class object is compiled Java code that contains its own data variables, called members, and sections of procedural code called methods.
If you have programmed in COBOL, a method is like a paragraph you perform.
If you have programmed in C or C++, a method is like a function your program calls.
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Objects in Java
and the methods
write
Objects in Java
• Java has a rich collection of Class Libraries.
• These are also known as the Java API or Application Programming Interface.
• To program in Java, you must
i.) Learn the Language, and
ii.) Learn the Class Libraries.
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Objects in Java
• These class libraries greatly simplify your job as a Java programmer.
• They help you to write complex programs quickly.
• To master Java, you must master these class libraries.
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Compiling A Java Program
• To compile it, you run the JDK supplied utility called:
javac
Hello.class will be produced.
• The two largest varieties of Java programs:
Applications
Applets
First Java Program
• A Java Application is a free-standing program that is capable of running directly in the Java Virtual Machine.
• A Java Applet is a mini-program that is much more limited in its abilities. An Applet can only run within the context of an HTML browser.
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The double slashes denote a “C++”-style comment. Everything on the line after the double slashes is ignored by the compiler.
This is the class name. Every single bit of code in Java must sit in curly brackets. Class names are capitalized. Words within the name are also capitalized. This Java program must be saved in a file with the exact same name--matching the upper case--as you see in blue above: HelloWorld.java
Now our Application is complete. We have added the method “main”. All methods are lower case. main is a special method--it actually runs the program.
In any application, you are always guaranteed that method main will run.
A Java Application
public class HelloWorld
public class HelloWorld
{
A Java Application
C:\>javac HelloWorld.java
C:\>
• A successful compile of your java program will return to a bare cursor, as you see here.
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C:\>javac HelloWorld.java
C:\>java HelloWorld
A Java Application
• To run your compiled Application, you enter lowercase java HelloWorld on the command line. • Notice, the “.class” extension is omitted.
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Now load the JDK1.4.1,
In Class Activity
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Java I Lecture 1 UPD1