1. Introduction to Java
OOSSE Programming with Java
Lecture 1
Dec 21, 2012 OOSSE - Java Lecture 1 1

2. Objectives
In this lecture, we will:
• Introduce Java and the Java Virtual Machine
• Discuss the basic object oriented concepts
• Define classes and instances
• Review the structure and syntax of a Java program
• Introduce the Scanner class and simple I/O
Dec 21, 2012 OOSSE - Java Lecture 1 2

3. Introduction to Java
• Java is a high level object oriented programming language
• Java was designed to be:
– Simple
– Object oriented
– Distributed
– Robust
– Secure
– Architecture-Neutral
– Portable
– Multithreaded
Dec 21, 2012 OOSSE - Java Lecture 1 3

4. A Simple Java Application
// A simple Hello World Java Application
public class Hello
{
public static void main (String[] args)
{
System.out.println ("Hello World");
System.out.println ("Welcome to Java");
}
}
Dec 21, 2012 OOSSE - Java Lecture 1 4

5. The Java Source File
• A Java program can be developed using any simple editor to
generate the Java source file
• For example Notepad could be used
• The source file is simple text but must be saved with an
extension of java
Dec 21, 2012 OOSSE - Java Lecture 1 5

6. Compiling a Java Application
• The Java source file is in a suitable format for humans to
read but must be compiled into bytecode before it can be
used
• Sun provide a compiler called javac that is used to
compile the source code into bytecode
• The compiler can be invoked from a command prompt in a
windows environment
– javac Hello.java
• The result of a successful compilation is a class file
containing the bytecode
– Hello.class
Dec 21, 2012 OOSSE - Java Lecture 1 6

7. Executing a Java Application
• The bytecode can be executed on the Java Virtual Machine
using the interpreter provided by Sun: java Hello
• Note the bytecode is held in a file called Hello.class but the
extension class is not included in the call to the interpeter:
java Hello
Dec 21, 2012 OOSSE - Java Lecture 1 7

8. Java Application Development Process
Create/ Modify Source Code notepad
Source Code Hello.java
Compile Source Code javac Hello.java
Bytecode Hello.class
Execute Bytecode java Hello
Dec 21, 2012 OOSSE - Java Lecture 1 8

9. The Java Virtual Machine
• The bytecode produced by the Java compiler is not
targeted at a specific machine
– It is targeted at the Java Virtual Machine
• The Java Runtime Environment executes the virtual
machine
– The Java Bytecode is executed on the virtual machine
• Hence the bytecode is portable
– It will execute on any machine that is running the Java
Virtual Machine
– The JVM needs to be target specific
Dec 21, 2012 OOSSE - Java Lecture 1 9

10. Pitfalls
• Java is case sensitive
– Hello and hello are not the same
• All Java programs must have the java extension
• The extension is specified when the compiler is used
– javac Hello.java
• The bytecode produced by the compiler is held in a file
with a .class extension but the extension is NOT specified
when the interpreter is used
– java Hello
Dec 21, 2012 OOSSE - Java Lecture 1 10

11. Java and Classes
• Java is an object oriented programming language
• All code is wrapped in the form of a class:
public class Hello
{
…
}
• Note the keywords public and class
• The class is given a name, Hello in this case, and must be
saved in a file called Hello.java
• Code similar to this will appear in each of your
applications
– The name of the class will change
Dec 21, 2012 OOSSE - Java Lecture 1 11

12. The Method main
• Classes use methods to specify what can be done
• A main method is required in a Java application and
defines where the application begins
• The structure of main is fixed and will be the same in all
applications:
public static void main (String[] args)
{
…
}
• The syntax will be discussed in detail later
– For now please just accept that it must be as it is
Dec 21, 2012 OOSSE - Java Lecture 1 12

13. The Body of main
• The code that you write to specify what the application
should do makes up the body of main
• A block of code; that is one or more program statements
wrapped in braces { }
• For example:
{
System.out.println ("Hello World");
System.out.println ("Welcome to Java");
}
• Note that each statement is terminated by a semicolon
Dec 21, 2012 OOSSE - Java Lecture 1 13

14. Simple Input – The Scanner Class
Dec 21, 2012 OOSSE - Java Lecture 1 14

15. Input in Java – the Scanner class
• Java 1.5 introduces adequate support for input
– Prior to this input from the keyboard was not trivial
• The Scanner class can be used for keyboard input
• Consider the following code extract:
// build an object that knows how to obtain keyboard data
Scanner kybd = new Scanner(System.in);
int num1;
// input the next integer and assign to num1
num1 = kybd.nextInt();
• The object kybd knows how to obtain the next integer from
the keyboard that is identified by System.in
Dec 21, 2012 OOSSE - Java Lecture 1 15

16. Objects and Classes
• Using classes makes performing complex tasks simple
• In order to use a class you need to know:
– Where the class is located
– How to build an instance of the class – an object
– What instances of the class can do
– How to ask the instance to do something
• A class has a set of methods that define the functionality
that it can provide
– For example nextInt is a method of the Scanner class
• A method is invoked by sending a message to an object
– num1 = kybd.nextInt();
Dec 21, 2012 OOSSE - Java Lecture 1 16

17. Using the Scanner Class
• The Scanner class is contained in a Java package
– A package is a library of classes
• The package must be imported into an application that
uses the Scanner class
– So that the compiler can find it
import java.util.*; // the package containing Scanner
public class TestScan
{
public static void main (String [] args)
{
// build an instance of Scanner, that is an object
Scanner kybd = new Scanner(System.in);
Dec 21, 2012 OOSSE - Java Lecture 1 17

18. Methods of the Scanner Class
• Some of the methods of the Scanner class are:
– nextInt() reads an integer
– nextDouble() reads a double
– next() reads a word
– nextLine() reads the rest of the current input line
• The two methods next and nextLine both return a value of
type String
– String name;
– name = kybd.next(); // assumes kybd is an instance of
// the Scanner class
Dec 21, 2012 OOSSE - Java Lecture 1 18

19. Pitfalls
• The nextLine method inputs the REST of a line of text
– It starts wherever the last input finished
• Consider the following section of code:
Scanner kybd = new Scanner(System.in);
String s1, s2;
int num1 = kybd.nextInt();
s1 = kybd.nextLine();
s2 = kybd.nextLine();
• What would s1 and s2 be if you entered the following?
42
The answer to
Life the Universe and everything
Dec 21, 2012 OOSSE - Java Lecture 1 19

20. Pitfalls
• The variable s1 would be set to an empty string
• The variable s2 would be set to “The answer to”
• Why?
• What would s1 and s2 be if you entered the following?
42 The answer to
Life the Universe and everything
Dec 21, 2012 OOSSE - Java Lecture 1 20

21. Coding Style Guidelines
• Java is a free format language but layout makes a huge
difference to understanding
• Adopt a good program layout to improve readability
– Generous use of space
– Vertical alignment of keywords
– Indentation as appropriate
• Use meaningful comments
– Level of intent
– Particularly where the code is not obvious
• Use meaningful identifiers
• Avoid complex program structures where possible
• Try not to sacrifice clarity and simplicity for efficiency
Dec 21, 2012 OOSSE - Java Lecture 1 21

22. Summary
In this lecture we have:
• Introduced Java and the Java Virtual Machine
• Discussed the basic object oriented concepts
• Defined classes and instances
• Reviewed the structure and syntax of a Java program
• Introduced the Scanner class and simple I/O
Dec 21, 2012 OOSSE - Java Lecture 1 22