CS2514

Introduction to Java

Dr Derek Bridge

School of Computer Science & Information Technology

University College Cork

Administrivia: Module delivery

Credit weighting:	5 credit module
Prerequisites:	CS1117
Lectures:	2 × 1 hr per week
Labs:	1 × 2 hr per week
Private study:	At least 2 hrs per week

Administrivia: Assessment

Examination:	1.5 hr written exam (80% of the marks)
Continuous assessment:	Programs (20% of the marks)

The usual finger-wagging shit

How to fail:	Skip lectures & labs; avoid private study; cram at Easter; expect the exam to be a memory test
How to pass:	Attend lectures & labs; summarize the notes; tackle the lab activities properly; expect a problem-solving exam

Resources

• Slides:
  • www.cs.ucc.ie/~dgb/courses/itj.html
  • NB deliberately incomplete!
• Numerous books and online tutorials, e.g.
  

Why learn Java?

• Consistently over its 20 year history, one of the top programming languages
  • E.g. the TIOBE index
• Official versions for
  • Java Standard Edition (Java SE) – desktop applications
  • Java Enterprise Edition (Java EE) – e.g. running on servers
  • Java Mobile Edition (Java ME) – for mobile applications
• But also unofficial versions, e.g. the Android SDK

Why object-oriented programming?

• Object-oriented programming helps us to
  • de-couple parts of our programs from each other
  • minimise redundancy
  • reuse and extend existing code
• But it needs more up-front planning (design!)
  • so it is more suitable for long-term, large-scale project development
  • it may be too much effort for short-term, small-scale scripting

What's an object?

• An object has
  • state: what the object 'knows'
  • behaviour: what the object can do
• An object in a program has
  • variables known as instance variables (aka fields, attributes)
  • functions known as methods or instance methods

Getting work done

• In a procedural program, you get work done by running functions
• In an object-oriented program, you get work done by asking objects to run functions (methods)

'Printing' to the screen

• There is a 'predefined' object which knows how to 'print' to the screen
• Its 'name' is java.lang.System.out
• You can use its short 'name': System.out
• It has several functions (methods) including:
  • println for printing some data and a new line character
  • print for printing some data (and remaining on the same line)
• E.g. System.out.println("Hello world!");

Where to write your code

Store this in HelloWorld.java:

class HelloWorld {
            
    public static void main(String[] args) {
    
        // For now, all your code goes here
        
    }
    
}

Reading input from the keyboard

• There is a 'predefined' object which knows how to read input from the keyboard
• Its 'name' is java.lang.System.in
• You can use its short 'name': System.in
• It has several functions (methods) including:
  • read for reading one byte
• Q: This is somewhat painful. Why?

Creating new objects

• When there's no suitable 'predefined' object, create one using new
• E.g. the following creates an object whose 'name' is sc:

  java.util.Scanner sc = new java.util.Scanner(System.in);

• sc has several functions (methods) including:
  • nextInt for reading an integer
  • nextDouble for reading a floating-point number
  • nextLine for reading a String

Exercise

Write a program that asks the user for his/her name and prints it out again, e.g.:

Halt! Who goes there? Ann
Hello, Ann. You may pass.

(User's input is shown in red.)