Introduction to Programming

1. Introduction to Programming Lecturer: Steve Maybank Department of Computer Science and Information Systems sjmaybank@dcs.bbk.ac.uk Spring 2014 Week 1: First Program Birkbeck College, U. London

2. Module Information • Time: 18.00-21.00 on Fridays in the spring term. • A-K: lectures 18.00 to 19.20 lab sessions 19.40 to 21.00 • L-Z: lab sessions 18.00 to 19.20 lectures 19.40 to 21.00 • Week 11: in lab test. Birkbeck College, U. London

3. Assessment • In Lab test: 30% • Two hour examination in summer 2014: 70% • The mock examination and the mock in Lab test do not contribute to the marks. Birkbeck College, U. London

4. Essential Text Book • Cay Horstmann (2013) Java for Everyone, Wiley, 2nd edition. • This module draws on the first six chapters of JFE. • The lab classes are based on exercises suggested in JFE. Birkbeck College, U. London

5. Syllabus • First program • Safe operation of computing equipment • Variables and number types • String types • Arithmetic and Boolean operations • If statement • Loops • Methods • Arrays Birkbeck College, U. London

6. This Lecture • Based on Ch. 1 of JFE • Aim 1: provide background information on computing • Aim 2: provide enough information to write a first Java program. Birkbeck College, U. London

7. Structure of a Computer • Central Processing Unit: executes instructions. • Main memory (= primary storage): stores programs and data ready for the CPU. • Bus: connects the CPU and the main memory. Main memory CPU Birkbeck College, U. London

8. Problem • The CPU of a computer is designed to carry out a fixed set of simple instructions. • It takes too much time to write programs using these instructions. Birkbeck College, U. London

9. Solution • Write programs in a special high level programming language. • Then use another program (a compiler) to convert each high level program into a list of instructions for the CPU. Birkbeck College, U. London

10. Java • First version developed in 1991 by James Gosling and Patrick Naughton for use in consumer products, eg. set top boxes. • Java based browser presented in 1995 at the SunWorld exhibition. • A browser can run Java code (applets) obtained from anywhere on the Internet. Birkbeck College, U. London

11. Advantages of Java • Portable: the same Java program can be run without change on Windows, UNIX, Linux or Macintosh. • Safe: a browser can run a Java program without endangering the PC running the browser. • Very large libraries for graphics, user interface, cryptography, networking, etc. Birkbeck College, U. London

12. Java Portability • The Java compiler converts a Java program to Java byte code. • The Java byte code is executed by a Java Virtual Machine. • The JVM is not portable: different JVMs are needed for different platforms. • The Java program is portable given the right JVM. Birkbeck College, U. London

13. Integrated Development Environment • Our IDE for Java is BlueJ • BlueJ facilities for Java programs: Edit Compile Run • See practical session. Birkbeck College, U. London

14. BlueJ Console Window See http://en.wikipedia.org/wiki/File:BlueJ_screenshot.png Birkbeck College, U. London

15. Classes, Objects and Methods • A class describes a set of objects with the same behaviour. • An object consists of data, together with methods which act on the data in that object. • A method is a sequence of instructions with a name. Birkbeck College, U. London

16. The String Class • Objects (string literals): “Hello”, “World”. • Methods: the object “Hello” contains methods length() and substring() which act on the data in “Hello”: “Hello”.length(); // 5 “Hello”.substring(0, 2); // “He” Birkbeck College, U. London

17. Java Programs • Java programs consist of classes. • When a Java program runs, objects are created and the methods act on the data in the objects. • Exception: static methods can be defined on their own, i.e. not as part of an object. Birkbeck College, U. London

18. My First Program public class HelloPrinter { public static void main(String[] args) { System.out.println(“Hello, World!”); } } Birkbeck College, U. London

19. Commentary 1 • MFP contains a single class: HelloPrinter • The class has no associated objects. • The class contains one method, main(), which can be invoked without first creating any object. Birkbeck College, U. London

20. Commentary 2 • public class: HelloPrinter and main() are available to any user • static: main() can be invoked without any associated object. • void: no value is returned by main(). • public, class, static, void are reserved words. Birkbeck College, U. London

21. System.out.println(“Hello”) • System.out is an object • println() is a method which is applied to data in System.out. • The string “Hello” is a parameter for the method println(). • Note how the details of System.out are hidden from the user. This is an example of encapsulation. Birkbeck College, U. London

22. Java Syntax • Braces (brackets) occur in matching pairs, {…}. • Every statement must end in a semicolon. • Upper case and lower case are distinguished. • Every Java application must have a main method. • Reserved words cannot be used for other purposes. • A method can have any number of parameters, including none. Birkbeck College, U. London

23. String Concatenation • “Hello, ”+”World!” produces “Hello, World!” • “Hello”+7 produces “Hello7” (The number 7 is converted to a string.) • Similarly, 7+”Hello” produces “7Hello” • 7+7 produces 14. Birkbeck College, U. London

24. Escape Sequences • On printing “He said \”Hello\”” the result is He said “Hello” The pair \” is an escape sequence. • The escape sequence \n denotes new line, for example System.out.print(“*\n**\n”); produces * ** Birkbeck College, U. London