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

JETT Workshops Object-Oriented Programming Fall 2004

(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann)

Objects and Object Variables

• 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.
        
        
  • user defined 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
  • from the AP CS Java Subset
    • Students are expected to understand the difference between object equality (equals) and identity (==).
    • The implementation of the equals methods is not in the subset.
      
      
• 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 balance

    balance2 = balance2 + 500;

  
  
• object variables hold references to objects
  • 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);

    
    
• A null reference
  • from the AP CS Java Subset
    • The null reference is part of the AP Java subset.
      
      
  • null is a Java keyword that means "no reference"
  • it can be used to test an object reference

    BankAccount newAccount; ... if (newAccount == null) System.out.println("Oops!");

  
  
  

• from the AP CS Java Subset

  • The use of this is restricted to passing the implicit parameter in its entirety to another method (e.g., obj.method(this)) and to descriptions such as "the implicit parameter this".
  • Using this.var or this.method(args) is not in the subset.
  • In particular, students are not required to know the idiom "this.var = var", where var is both the name of an instance variable and a parameter variable.
  • Calling other constructors from a constructor with the this(args) notation is not in the subset.
    
    

• Explicit and Implicit Parameters

  public void withdraw(double amount) {  double newBalance = balance - amount; balance = newBalance; }

  
  
• balance is the balance belonging to the object to the left of the dot when the method is called

  momsSavings.withdraw(500)

  
  
  • momsSavings is the implicit parameter
    • it is passed implicitly to the withdraw method
    • the balance that is affected in the withdraw method is the one belonging to the object called momsSavings
    • Java has a reserved word that acts as a variable name for the implicit parameter inside the method - this
      

      public void withdraw(double amount) {  double newBalance = this.balance - amount; this.balance = newBalance; }

      
      
  • 500 is an explicit parameter
    • it is passed explicitly to the withdraw method
    • it is copied into the parameter variable called amount
  • there can be several explicit parameters
  • there is only one implicit parameter