Topics for today 2499

1
Topics for today (2/4/99)

• Todays buzzwords
• Objects and methods (from prev. lecture)
• Exercise - playing with Strings
• Intro to Applets - the ccj way
• Syntax summary

2
Todays buzzwords

• Objects and Classes
• Methods
• Abstraction
• Keeping the internals hidden from outside users
• Escape characters
• Special characters that cannot be used otherwise
• Debugging
• Getting rid of those darn bugs!

3
Objects

• Like strings, there are many other types (or
  classes) of objects in Java.
• These classes can be built-in, or they can be in
  libraries that you can use.
• With classes, you can
• You can define variables of these class types
• Assign values to them
• Invoke methods on them
• Access their data (if allowed)

4
More on objects
We use a fictitious class Automobile for these
examples.

• Declaring a variable of type a class
• Automobile myCar
• Assigning a value (constructing or instantiating)
  to a class variable
• myCar new Automobile(Toyota)
• myCar new Automobile(Toyota, 1992)
• Invoking a method on a class variable
• miles myCar.odometerReading()

5
Back in Time()

• Declaring a Time object
• Time flies
• Creating a new Time object
• Time now new Time()
• Time yesterday new Time(1999,2,3,0,0,0)
• Running methods on the Time object
• int hourNow now.getHours()
• int secondElapsed
• now.secondsFrom(yesterday)

6
Why do we use objects?

• Users dont need to worry about how something is
  implemented (abstraction)
• How does the Time object find out what time it
  is?
• How does it calculate the number of seconds
  between two times?
• As long as the methods stay the same, someone can
  change object implementations without problems.
• Keeps the code clean and easy to understand

7
A little more on String

• What if you want to have a newline inside a
  string?
• String weird "I really need two lines to write
  this long sentence!"
• What if you need to place quotes inside a string?
• String talk
• "I said "hello" to him." / right? /
• Answer Escape characters
• Characters with a backslash ( \ )in front of them

8
Special Characters

• The following can be used in a string
• \n newline
• \t tab
• \" a double quote inside a string
• \\ the backslash itself
• "to type a long string without using another\
• string, use a \\ before the new line

9
Class exercise

• Given a one word name written in any way,
  generate the name in a proper way.
• e.g., generate "Shakespeare"
• for all the following inputs
• Shakespeare
• shakespeare
• sHAkeSPeaRE
• SHAKESPEARE

10
Debugging programs

• Using a debugger
• system dependent
• often confusing and takes time to learn
• Staring at the program
• you may need to stare quite long!
• Using temporary printlns
• If you suspect a problem somewhere, put a println
  before that to see if it comes out okay.

11
Other debugging techniques

• Trying out the program using constants instead of
  variables
• Commenting out problem areas
• Writing the program in stages that you know works
• Trying a different approach

12
Syntax summary

• Creating new objects
• myCar new Automobile("Toyota")
• Running methods on objects
• myEff myCar.fuelEfficiency()
• Escaping special characters
• \n \t \ \\
• \ followed by end of line, continued in next line