CS2514

Introduction to Java

Dr Derek Bridge

School of Computer Science & Information Technology

University College Cork

Summary of `Ticket` so far

class Ticket {

    private String occasion;
    private String date;
    private int ticketNumber;
    
    private static int nextTicketNumber = 1;
    
    public Ticket(String occasion, String date) {
        this.occasion = occasion;
        this.date = date;
        ticketNumber = nextTicketNumber;
        nextTicketNumber = nextTicketNumber + 1;
    }
    
    public String toString() {
        // not yet written
    }
       
}

`Ticket`'s `toString` method

public String toString() {
    int ticketWidth = Math.max(occasion.length(), date.length());
    String str = "";
    str += horizontalLine(ticketWidth);
    str += padString(occasion, ticketWidth);
    str += padString(date, ticketWidth);
    str += horizontalLine(ticketWidth);
    return str;
}

private static String horizontalLine(int width) {
    String str = "+";
    for (int i = 0; i < width; i = i + 1) {
        str = str + "-";
    }
    str = str + "+\n";
    return str;
}
    
private static String padString(String text, int width) {
    int amountOfPadding = width - text.length();
    int amountOfLeftPadding = amountOfPadding / 2;
    int amountOfRightPadding = (int) (Math.round(amountOfPadding / 2.0));
    String str = "|";
    for (int i = 0; i < amountOfLeftPadding; i = i + 1) {
        str = str + " ";
    }
    str = str + text;
    for (int i = 0; i < amountOfRightPadding; i = i + 1) {
        str = str + " ";
    }
    str = str + "|\n";
    return str;
}

Q: Think back to lecture 3. Why are these methods inefficient, and how would you rewrite them to make them more efficient?

Class methods

The signature of a class method includes the keyword static
To call a class method, e.g. horizontalLine:
- everywhere in Ticket, just use, e.g.:
```
horizontalLine(10);
```
- in other class definitions, you should prefix with the class name, e.g.:
```
Ticket.horizontalLine(10);
```
  (but whether this works depends on its visibility)

When do we use class methods?
Rule of thumb

If you are organizing the computation of a class method into smaller, helper methods,
- the helper methods will also be class methods
If you are organizing the computation of an instance method into smaller, helper methods,
- the helper methods can be instance methods (this will compile and run)
- but it misses the fact that some of these helper methods could actually be class methods (which run more efficiently)

When do we use class methods?
A more accurate rule

If a method
- does not call any instance methods in this class, and
- does not access any instance variables in this class
then it can be a class method. (Otherwise, it's an instance method)
Alternative way of saying the same thing:
- if a method does not use this (either explicitly or implicitly), then it can be a class method. (Otherwise, it's an instance method)

Examples of class methods

There are many class methods in the standard Java library
- Think about why they are class methods and not instance methods
In java.lang.Math:
- max, round, pow, sqrt, ...
In java.lang.Integer:
- parseInt, toBinaryString, toHexString,...
In java.lang.String:
- format,...
In java.util.Arrays:
- copyOf, binarySearch, fill,...
...and many more!

The `main` method

We can now understand main!
It's a method:
- Q: What kind of method is it?
- Q: How many parameters and what type?

To understand its parameter, we will run this program

public static void main(String[] args) {
    for (int i = 0; i < args.length; i = i + 1) {
        System.out.println(args[i] + " ");
    }
}

Every class definition can have a main method
- the main method can be the test-driver for the class
- other programmers prefer to put the test-driver's main method into a separate class definition (like we have done), to reduce clutter

What next for you?

CS2514 is just an introduction to Java, and so there are many features we have not studied fully or at all
Exceptions
Files and databases
Using an Integrated Development Environment (IDE), e.g. NetBeans or Eclipse
Testing with JUnit
Graphical User Interfaces (and event handlers, and nested classes and anonymous classes)
Packages
Deployment
Dates, times and localization/internationalization
Design patterns (these are not specific to Java)
Lambda expressions and aggregation operators
Concurrency
Generics

CS2514

Introduction to Java

Dr Derek Bridge

School of Computer Science & Information Technology

University College Cork

Summary of Ticket so far

Ticket's toString method

Class methods

When do we use class methods?Rule of thumb

When do we use class methods?A more accurate rule

Examples of class methods

The main method

What next for you?

Summary of `Ticket` so far

`Ticket`'s `toString` method

When do we use class methods?
Rule of thumb

When do we use class methods?
A more accurate rule

The `main` method