Content of Test 3

Format of the test:

• not multiple choice
• focus on chapters 10, 11, 13, 15
• using constructs from previous chapters cannot be avoided

Event Handling:

• You have to understand the example programs discussed in class and in the textbook.
• You have to know the concepts
• related to event handling (such as event source, event, event listener)
• commonly used in the context of event handling (such as interfaces, adapters, inner classes).

Arrays / Vectors

• how many values are larger / smaller than ...
• what is the largest / smallest value
• what is the sum / average of the values
• which element(s) have the value ...
• add / remove an element to an array

Exceptions

• how to catch exceptions
• how to throw exceptions
• how to delegate exceptions to the calling method
• the difference between checked and unchecked exceptions
• that IOExceptions are checked (not programmer's fault)
• and most other common ones unchecked (programmer's fault)

File I/O

• difference between text files and binary files
• difference between sequential and random access
• you won't have to write code to open / close files or read from / write to files

Algorithms

• suggest an algorithm
• determine the order O(...) for an algorithm
• implement an algorithm

Review

Events

• event source
• generates and manages event
• event
• event listener
• implements a specific listener interface (to guarantee methods to be called by the event source)
• must be registered with the event source

Callback

• when you register a listener with an event source you tell the source that it can call the methods guaranteed by the interface

Interfaces

• only definitions, no implementations
• advantage: a class can only extend one super class but may implement many interfaces
• all methods that are promised in the interface have to be implemented!

Adapter

• replaces listener interface with a class
• adapter implements interface and provides methods with empty bodies
• be careful to spell all methods correctly!

Inner Classes

• class within another class

Arrays

• all elements of equal type (fundamental datatype or class)
• extend class Object
• object references are passed to methods
• syntax is different from other Objects
• double[] data = new double[10];
• BankAccount[] accountArray = new BankAccount[50];
• this does not create any BankAccount objects!!!
• fixed length
• int len = data.length;
• length is a public instance variable of the array!
• you commonly access data using an index
• data[3] = 123.4;
• value = data[7];
• if you don't know the number of data items in advance
• create a larger array
• keep track of actual number of data items with a separate variable
• check if array is full when adding elements
• use arrayCopy otherwise
• adding to / deleting from unsorted arrays
• add at the end
• deleting: replace element by the last one in the array
• adding to / deleting from sorted arrays
• elements have to be moved
• 2 or more dimensional arrays:
• arrays of arrays

Vectors

• properly object oriented
• Vector points = new Vector();
• Point2D.Double p = new Point2D.Double(100,200)
• points.add(p);  // adds at the end
• variable length
• int noOfElements = points.size();
• all elements of type "Object" => any combination of objects possible
• special methods for retrieval of data

methods return objects of class Object => downcasting necessary
Point2D.Double pOut = (Point2D.Double) points.get(23);

Exceptions

• may be caught
• program stops otherwise

Catching Exceptions

• try block can contain more than one statement
• you can either catch an Exception exactly of the class thrown or any superclass of it.
• catching IOException catches also EOFException, etc.
• catching the generic "Exception" is not recommended
• you can add more than one catch block

Delegating Exceptions

• Add "throws IOException" (or whichever exception applies) to the definition of the method

public void readFromFile() throws IOException

• used if the method that calls your method can better handle the exception (e.g., if you don't know if your method will be called by an application or applet)
• do not use try / catch blocks in that case!
• not recommended for main

Throwing exceptions

• E.g. you expect to read 20 lines but the file only contains 10

Checked vs. unchecked exceptions

• Checked exceptions have to be caught
• usually not due to programming mistake
• e.g., IOException
• Unchecked exceptions don't have to be caught
• usually due to programming mistake
• e.g., NullPointerExceptions (and all others you have so far seen in labs)
• you can catch them but you are not encouraged to
• sometimes you should check "unchecked exceptions": e.g. Integer.parseInt(String) if input is provided by the user

Random Access

• method of accessing any file (other than keyboard input or screen output)
• file pointer can be moved to specific location
• usually record lengths should be constant
• you can use random access for text, or any other data type
• (methods for random access of text files in java use Unicode, which means that you cannot open other text files for random access)

Binary <=> text files

• binary format corresponds to the way data types / objects are stored internally
• text files contain humanly readable text
• numbers are converted to strings
• potential of wasting a lot of space

Algorithms

• can be written in any language (e.g., natural languages)
• unambiguous, executable, terminating

Analysing Performance

• scientists like power laws and orders of magnitude!
• don't waste effort on prefactors
• find some basic unit: data visits

O(...)

• O(...) describes how the fastest growing term depends on the relevant number(s)

Searching algorithms

• linear search for unsorted data
• binary search for sorted data

Sorting

• selection sort requires two loops through data => O(n^2)
• merge sort works on the basis of merging two sorted sets
• performance O(n log n)