Making Objects

• Objects
  • The Big Picture (Slides)
  • A class is instantiable if we can create instances of the class.
  • An object is an instance of a class.
  • An object contains
    • Data
    • Methods
  • An object is created with the new operator, and is referred to by an object variable (a reference variable).
    • The new operator specifies the name of the class the instance is to be made from.
    • The data and methods in an object are the instance data and methods in the class.
    • Parameters may be given to the class name in a new statement, which are normally used to give initial values to the data in the object through a constructor method ... more details coming soon.
  • An object's data and methods are accessed using the syntax <object variable>.<data or method name>
  • Example: CurrencyConverter1.java and UseCurrencyConverter1.java
    • Create a project named something appropriate
    • Create the two classes in the src directory
    • Select the class containing the main method to run
    • It is possible to have multiple classes with a main method in one project
  • It is possible to do it all in one file, if a class is very specific to a program
    • Example: CurrencyConverter1OneFile.java
    • Notice the absence of a visibility modifier
    • The class can be used in only that file
    • Seperate files for now ...
  • Many objects can be created from the same class.
    • Example: CurrencyConverter1.java and UseManyCurrencyConverter1.java
    • Notice how there are two classes with a main method in my CurrencyConverter1 project.

• Constructors
  • Imagine what happens without the call to randConverter.setExchangeRate in UseCurrencyConverter1.java
  • A constructor is a special method that is automatically executed when a new instance of the class is created.
    • The purpose of the constructor is to initialize (the data members of) an object to a valid state.
    • The name of a constructor must be the same as the name of the class.
    • Constructors have no return type (not even void)
  • For every class you should define a default constructor that does something (useful)
    • Example: CurrencyConverter2.java and UseCurrencyConverter2.java
    • If no constructor is defined for a class then the Java compiler will include a default constructor that does nothing (useful).
  • Non-default contructors can be added (typically) to provide initial values for variables. (A class can include multiple constructors, by using overloading.)
    • Example: CurrencyConverter3.java and UseCurrencyConverter3.java
  • Constructors may obtain initial information from the user
    • Example: CurrencyConverter4.java and UseCurrencyConverter4.java

• Methods in Objects
  • Objects that IO themselves
    • A well encapsulated class has no public instance variables
    • It is common for an object to have a toString method. println method calls toString when printing an object.
    • It is common for an object to have a method that inputs values for its instance variables.
    • Example: Species1.java and UseSpecies1.java
  • Accessor and Mutators
    • A well encapsulated class has no public instance variables
    • It is therefore necessary to provide
      • Constructors to initialize them
      • Accessor methods to get their values
      • Mutator methods to change their values
    • Output methods are examples of accessors
    • Input methods are examples of mutators
    • Mutators can check for validity of data values
    • Example: Species2.java and UseSpecies2.java
  • Helper Methods
    • Helper methods "help" the public methods complete their tasks.
    • Helper methods that are used only within objects are private.
    • Example: Species3.java and UseSpecies3.java

Exercises

• Do the static or non-static parts of a class define what's in objects that are instances of the class?
• What operator is used to create an object?
• What happens if you don't supply a default constructor method?

Exam Style Questions

• Write a class suitable for storing information about the governor of a state. It must store the name, the fraction of the vote attained, and whether or he (s)he is honest (not a constant :-). Provide no methods except a default constructor.
• When is a constructor method of a class automatically called?
• In the class MyBigFatGreekWedding, what is the name of the constructor method? Does it have a return type?
• What is the benefit of having a toString method in a class?
• What are mutator and accessor methods?