An introduction to java programming language forbeginners(java programming tutorials)
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![http://aiti.mit.eduAfrica Information Technology Initiative © 2009
Example Code: Comments
/* The HelloWorld class prints “Hello,
World!” to the screen */
public class HelloWorld {
public static void main(String[] args) {
// Prints “Hello, World!”
System.out.println("Hello, World!");
// Exit the program
System.exit(0);
}
}](https://image.slidesharecdn.com/l01-140727143902-phpapp01/85/An-introduction-to-java-programming-language-forbeginners-java-programming-tutorials-12-320.jpg)
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An introduction to java programming language forbeginners(java programming tutorials)
- 1. Africa Information Technology Initiative Lecture 1: Introduction to Java AITI 2009
- 2. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Agenda • What makes Java special? • Advantages and disadvantages to using Java. • Methodology for developing applications. 2
- 3. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Compiler … a = b + c … … ld $r1, a ld $r2, b add $r3, $r1, $r2 st a, $r3 … Compiler High-Level Code Machine Code • A program that translates a programming language into machine code is called a compiler • Typically, we must have a compiler for each operating system/machine combination (platform)
- 4. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Compiling Computer Programs • Because different platforms require different machine code, you must compile programs separately for each platform, then execute the machine code. program compiler compiler compiler Win Mac Unix machine codemachine code machine code
- 5. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 The Java Compiler is Different! • The Java compiler produces an intermediate format called bytecode. • Bytecode is not machine code for any real computer. • Bytecode is machine code for a model computer. – This model computer is called the Java Virtual Machine. Java Program compiler Java Bytecode
- 6. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Java Interpreter • A Java Interpreter is required to execute the bytecode on a real computer. • A Java Interpreter converts the bytecode into machine code. – As the program executes – Simulate the execution of the Java Virtual Machine on the real computer • You can run bytecode on any computer that has a Java Interpreter (JRE) installed! – Only have to compile once – Can distribute the same bytecode to everyone
- 7. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 The Java Approach Java Program compiler Java bytecode Win Mac Unix Interpreter Interpreter Interpreter
- 8. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Advantages of Using Java • Once a Java program is compiled you can run the bytecode on any device with a Java Interpreter. – Because you do not have to recompile the program for each machine, Java is device independent. • Java is safe. The Java language and compiler restrict certain operations to prevent errors. – Would you want an application to have total control of your phone? • Make calls, send SMS messages? • Java standardizes many useful structures and operations such as lists, managing network connections, and providing graphical user interfaces
- 9. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Disadvantages of Using Java • Running bytecode through an interpreter is not as fast as running machine code – But this disadvantage is slowly disappearing • Using device specific features (e.g., bluetooth) is difficult sometimes because Java is device- independent. • In order to run a Java program on multiple devices, each must have a Java Interpreter – Ex: most Nokia phones come with Java Interpreter
- 10. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Programming Methodology 1. Specify and analyze the problem • Remove ambiguity • Decide on inputs/outputs and algorithms 1. Design the program solution • Organize problem into smaller pieces • Identify existing code to reuse! 1. Implementation (programming) 2. Test and verify implementation 3. Maintain and update program
- 11. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Writing Good Code • A program that meets specification is not necessarily good. • Will you be able to make changes to it? – Will you understand it after some time? • Others might need to look at your code – Can they understand it? • Write your program so that is easy to understand and extend! – Spend extra time thinking about these issues.
- 12. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Example Code: Comments /* The HelloWorld class prints “Hello, World!” to the screen */ public class HelloWorld { public static void main(String[] args) { // Prints “Hello, World!” System.out.println("Hello, World!"); // Exit the program System.exit(0); } }
- 13. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Comments • Comments are used to describe what your code does as an aid for you or others reading your code. The Java compiler ignores them. • Comments are made using //, which comments to the end of the line, or /* */, which comments everything inside of it (including multiple lines) • Two example comments: – /* The HelloWorld class prints “Hello, World!” to the screen */ – // Prints “Hello, World!”
- 14. http://aiti.mit.eduAfrica Information Technology Initiative © 2009 Comments on Commenting • You may collaborate on software projects with people around the world who you’ll never meet • Should be able to figure out how code works by reading comments alone • Anything that is not self-evident needs a comment • 50% of your code might be comments • Coding is easy, commenting is not
Editor's Notes
- #5: two step process!
- #7: the interpreter is called the JRE
- #8: much easier to standardize the interpreter, this is a piece of software that Sun releases for different platforms and programmers do not have to worry about the specifics of the machines they are targeting or distributing multiple machine codes. Can have a single compiled program (bytecode) run on anything there is an interpreter for. For example, a website with java code does not have to worry about the specific machine that is being used by the viewer. It is up to the user to get the interpreter.
- #11: very important to think about the problem and solution and create a design document… program is not static! it is always evolving. must make the code as understandable as possible, to yourselves and to others… very important that code is well designed