Computer Science A, Spring 2007
Roskilde University

• Computers are programmed to perform tasks
• Different tasks = different programs
• Program
  • Sequence of basic operations executed in succession
  • Contains instruction sequences for all tasks it can execute
• Sophisticated programs require teams of highly skilled programmers and other professionals

• A programming language (such as Java) describes the legal syntax and the meaning of its programs.
• Analogy: A natural language (such as Chinese) describes the legal syntax and the meaning of its sentences.
  • But: Programming languages are not ambiguous

• A programming language consists of a small number of general building blocks
• These building blocks can be composed in a particular program in many different ways
• Knowing the building blocks is only one step to learning how to program.
  • What should the program do? (Ask, for example, "how would I solve this problem?" or "how would I tell a friend how to solve this problem?")
  • How should the program do it?
  • Which building blocks should be used?

• Find the way out of a maze
• Tell a friend how to find the way out of a maze
• Tell a computer how to find the way out of a maze

• Beyond programming, software development also involves
  • Analysis (What should the system accomplish)
  • Design (How is the implementation planned)
  • Testing
  • Deployment
• Programming may also involve
  • Designing datastructures and algorithms

• In this course you will become familiar with the most fundamental building blocks of the programming language Java
  • You will not learn how to design solutions to computational problems. (Algorithmics)
  • You will not learn how to analyze the requirement of a specific system. (System development)
• To explain building blocks in isolation, we will mostly write small and simplistic programs.

• Central processing unit
  • Chip
  • Transistors
• Storage
  • Primary storage: Random-access memory (RAM)
  • Secondary storage: e.g. hard disk
  • Removable storage devices: e.g.: floppy disks, tapes, CDs
• Peripherals
• Executes very simple instructions
• Executes instructions very rapidly
• General purpose device

• Java Virtual Machine (JVM) – a typical sequence of machine instructions is:
  
  1. Load the contents of memory location 40.
  2. Load the value 100.
  3. If the first value is greater than the second value, continue with the instruction that is stored in memory location 240.
  Machine instructions are encoded as numbers:
  21 40
  16 100
  163 240
• Compiler translates high-level language to machine code

6. What is the code for the Java virtual machine instruction "Load the contents of memory location 100"?
7. Does a person who uses a computer for office work ever run a compiler?

6. 21 100
7. No–a compiler is intended for programmers, to translate high-level programming instructions into machine code.

• Simple
• Safe
• Platform-independent ("write once, run anywhere")
• Rich library (packages)
• Designed for the internet

• Log in
• Locate the Java compiler
• Understand files and folders
  • Programs are kept in files
  • File: a collection of items of information that are kept together
  • Files have names, and the rules for legal names differ from one system to another
  • Files are stored in folders or directories; these file containers can be nested
• Write a simple program (later)
• Save your work
  • Develop a strategy for keeping backup copies of your work

1: public class HelloTester
2: {
3:    public static void main(String[] args)
4:    {
5:       // Display a greeting in the console window
6: 
7:       System.out.println("Hello, World!");
8:    }
9: }

Output

   Hello, World!

• public class ClassName (in a file called ClassName.java)
• public static void main(String[] args)
• // comment
  
• Method call
  
• System class
• System.out object
• println method

object.methodName(parameters) Example:   System.out.println("Hello, Dave!"); Purpose: To invoke a method of an object and supply any additional parameters

12. How would you modify the HelloTester program to print the words "Hello," and "World!" on two lines?
13. Would the program continue to work if you omitted the line starting with //?
14. What does the following set of statements print?

    System.out.print("My lucky number is");
    System.out.println(3 + 4 + 5);

12. System.out.println("Hello,"); System.out.println("World");
13. Yes–the line starting with // is a comment, intended for human readers. The compiler ignores comments.
14. The printout is My lucky number is12. It would be a good idea to add a space after the is.

• Syntax errors
  System.ouch.print(". . .");
  System.out.print("Hello);
• Detected by the compiler
• Logic errors
  System.out.print("Hell");
• Detected (hopefully) through testing

15. Suppose you omit the // characters from the HelloTester.java program but not the remainder of the comment. Will you get a compile-time error or a run-time error?
16. How can you find logic errors in a program?

15. A compile-time error. The compiler will not know what to do with the word display.
16. You need to run the program and observe its behavior.

17. What do you expect to see when you load a class file into your text editor?
18. Why can't you test a program for run-time errors when it has compiler errors?

17. A sequence of random characters, some funny-looking. Class files contain virtual machine instructions that are encoded as binary numbers.
18. When a program has compiler errors, no class file is produced, and there is nothing to run.