JETT Workshop - Java Basics

Computer Science (CSCI) Department
College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops
Java Basics

Fall 2004

Types
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

There are two different categories of types in Java
- primitive types
  - from the AP CS Java Subset
  - The primitive types int, double, and boolean are part of the AP Java subset.
    int numberOfTrys = 0; double moneyEarned = 38.95; boolean moreToCome = true;
  - The other primitive types short, long, byte, char, and float are not in the subset.
- abstract data types
  - classes define new types - BankAccount.java
  - they can then be used to create objects
  - they can also be used to declare an object variable that will hold a reference to an object - BankAccountTest.java
  - Java does not have pointers
Comparing primitive data types and objects
- primitive types hold values
- assignment of primitive types causes the value of one variable to be copied to the other
  double balance1 = 1000;
  double balance2 = balance1;
  a subsequent change in balance2 does not affect balance1
  balance2 = balance2 + 500;
- object variables hold references to object
- assignment of object variables causes the reference stored in one variable to be copied to the other
  BankAccount account1 = new BankAccount(1000);
  BankAccount account2 = account1;
  Change through account2 is also visible through account1
  account2.deposit(500);
- Object variables hold references, not the object itself

Exercise - set up, debug, and modify project Bank in Visual Studio .NET

start VS .NET
create another new project
- choose File>>Add Project>>New Project
- click on Visual J# Projects
- scroll down and click on Empty Project
- give the project the name Bank
- click OK
bring in an existing file
- right click and save BankAccount.java in the jett\Bank directory
- right click and save BankAccountTest.java too
- choose File>>Add Existing Item and add both of these files
compile the Bank project
- click on the Bank project
- choose Build>>Build Bank
execute the Bank project
- right click on the Bank project
- choose Set As StartUp Project
- choose Debug>>Start without Debugging
use the debugger
- right click on project Bank
- choose Debug>>Step into new instance
- on the menu bar at the top, choose Debug>>Windows>>Locals
- to execute one statement at a time, choose Debug>>Step into
modify BankAccountTest.java to create other BankAccount objects
- call methods on those objects
- try copying an object reference with assignment
play with the primitive data types
- create some local variables
- assign a variety of values to them
- watch them change with the debugger