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The Java Programming Language
• Invented 1995 by James Gosling at Sun Microsystems.
• Based on previous languages: C, C++, Objective-C, …
• Intended to be a safe programming language for the
Internet
• Common choice for first programming language





Object oriented
Encourages good programming habits
Very popular commercially
Simpler and more elegant than C++, but similar to it
Cool
Lecture A
Slide 1 of 40.
Hello World Program
// My First Program!!
public class HelloWorld {
public static void main(String[] args){
System.out.println(“Hello World!”);
}
}
Lecture A
Slide 2 of 40.
Java Program Elements
• Words


Keywords
Identifiers
• Constants


Numbers
Strings
• Symbols

()[]{},.;+-*/
• Comments
Lecture A
Slide 3 of 40.
Java Elements
Comment
Identifier
Symbols
// My First Program!!
public class HelloWorld {
public static void main(String[] args){
System.out.println(“Hello World!”);
}
}
Keywords
String Constant
Lecture A
Slide 4 of 40.
Java Program Structure
• A program is composed of a collection of classes
• Each class contains a collection of methods
• Each method is composed of a sequence of instructions
(commands, statements)
Lecture A
Slide 5 of 40.
Java Program Structure
Class
Method
// My First Program!!
public class HelloWorld {
public static void main(String[] args){
System.out.println(“Hello World!”);
}
}
Instruction
Lecture A
Slide 6 of 40.
Running a Java Program
• Use an editor to enter (type) the program

Save to a file named HelloWorld.java
• Use the compiler to translate the program into bytecode


javac HelloWorld.java
This produces a file named HelloWorld.class
• Run the program using the Interpreter

java HelloWorld
Lecture A
Slide 7 of 40.
Java Compilation
Xxx.java
Compiler
Xxx.class
Interpreter
Output
Lecture A
Slide 8 of 40.
Entering a Java Program into an Editor
Lecture A
Slide 9 of 40.
Running a Java Program
Lecture A
Slide 10 of 40.