To Lecture Notes

IT 313 -- Mar 5, 2020

Review Exercises

1. Write the getCheckedOut method of the LibraryManager for Project 6b. Ans:
   

   public ArrayList<LibraryItem> getCheckedOut( ) {
       ArrayList<LibraryItem> output = new ArrayList<LibraryItem>(20);
       for(LibraryItem item : output.values( )) {
           if(item.isCheckedOut( )) {
               output.add(item);
           }
       }
       Collections.sort(output);
       return output;
   }
   

2. Explain how the State pattern works. Give an example.
   Ans: The State pattern creates one object in the state diagram. For example, for the Gumball State Diagram, the classes are
   

   HasQuarterState  NoQuarterState  SoldState  SoldOutState
   

   A simpler example is this Two Floor Elevator State Diagram. The states could be represented by the classes FirstFloorState and SecondFloorState. The two actions "Press Button 1" and "Press Button 2" shown on the state diagram are the meaningful actions. Other actions such as "Press Button 1" if the elevator is already at Floor 1 or "Press Button 2" if the elevator is already at Floor 2 are not meaningful and do not result in a state transition. Of course the controller for an elevator is probably not implemented with Java classes.
3. In the observer pattern, what is the difference between the observable and the observer? Look them up in the Observer Example, list the methods for each.
   Ans: Observable is what is being observed, such as a Twitter Feed. Observer is what is observing the Observable, for example the Twitter followers. Observable is a base class; Observer is an interface. The Observable methods are addObserver, clearChanged, countObservers, deleteObserver, hasChanged, notifyObservers, setChanged. The required method of Observable is update.
4. Explain the similarities and differences between the classical Observer pattern and a dentist's office and its patients being notified of upcoming appointments.
   Ans: The dentist's office is the Observable and the patients are the Observer objects. For a patient to become an Observer, the receptionist must record the patient in the appointment book. Then the day before the appointment, the patient gets a call that he or she has an appointment the next day. The analogy breaks down a little because an Observable notifies all the patients at the same time when notifyObservers gets called. However, the receptionist does not notify all patients at the same time. They are notified individually the day before their appointment occurs.
5. Explain how the Strategy Pattern works. Give an example.

SQL (Standard Query Language)

• SQL was originally developed at IBM in the early 1970s.
• It was originally called SEQUEL (Structured English Query Language). It was later changed to SQL because SEQUEL was already trademarked by the Hawker Siddeley British aircraft company.
• SQL was based on relational database concepts of Codd, Chamberlin, and Boyce.
• List some examples of SQL statements.

The SQLite Relational Database Management System

• SQLite was created by D. Richard Hipp in 2000. It was part of the software he was developing for onboard guided missle destroyers.
• It is a self-contained, serverless, zero-configuration, and transactional database system.
• It is designed to be operated without an installed database management system and without a database administrator.
• SQLite was designed for simplicity. Because of this, it has some deficiencies:
  
  1. It stores the entire database in one file, whose maximum size is 140 TBytes.
  2. It does not do well with many concurrent users; it only allows one writer at a time.
• SQLite1 was released in 2000, SQLite2 in 2001, and SQLite3 in 2004. Here is a History of SQLite releases.
• You don't need to install the SQLite3 to complete Project 8.
• However, if you want to install it, download and unzip the files sqlite-tools-win32-*.zip and sqlite-dll-win32-*.zip or sqlite-dll-win64-x64-*.zip from the website
        https://www.sqlite.org/download.html
  This gives you sqlite3.def, sqlite3.dll and sqlite3.exe files, which can be placed into any folder, for example c:\sqlite3. Then add c:\sqlite3 to your path.
• To run SQLite3 using the Command Prompt user interface, type
  

  sqlite> sqlite3 mydatabase.db
  

  mydatabase.db is the database name, which is automatically created if it does not exist.
• The file students.sql contains SQL statements that create and populate the students table.
• Copy the file students.sql into the current folder.  Then use this sqlite statement to execute the SQL statements that it contains:
  

  sqlite> .read students.sql
  

• Notice that SQLite3 commands start with a period. Here are some useful commants:
  

  .bail .databases .exit .headers  .help
   
  .mode (csv column html insert line list 
  
  tabs tcl) .open .output .quit .read  .save
  
  .sep .schema .show .tables .width .timer
  

Java and SQL

• The Horstmann textbook has a supplementary web chapter, Chapter 22, on Relational Databases.
  This chapter has a good introduction to SQL and to the Java classes that are needed for SQL database programming.
• See the Tutorials Point: SQLite Java Tutorial.
• Look at the JDBC Example. This example contains a link to the JAR file that you must use to
• Java uses JDBC (Java Database Connectivity), an API for accessing relational databases. A list of available JDBC drivers. Interestingly, this list does not include SQLite3.
• JDBC uses these classes and interfaces (marked with *) to facilitate database connectivity:
  DriverManager   Manages a variety of JDBC drivers.
  *Connection   A connection with a specific database. SQL statements are executed and results are returned within the context of a connection.
  
  *Statement   Contains one or more SQL statements.
  
  *ResultSet   A table of data returned by executing one or more SQL statements.
  
  *ResultSetMetaData   Contains information about a result set, for example, the number of columns in the result set and the column labels.
• Practice Problem:
  
  1. Modify the LoadKids and DisplayKids classes in the JDBC Example to work with the fields of the Transaction class of Project 7.