Implementing a Time Abstract Data Type with a Class

• The Time class definition has 3 int instance variables: hour, minute, and second.
• Member access modifiers:
  • public: When a member (data or method) is public, it is accessible wherever the program has access to an object of the class (e.g., the Time class).

    For example, if we have the declaration

    Time bomb;

    then anywhere within the scope of the object bomb, we can access any of bomb's public members, as defined in the Time class.

  • private: When a member (data or method) is private, it is accessible only to methods of the class.
• The default access modifier is public.
• Typically:
  • a class's data members (i.e., its instance variables) are declared private
  • a class's methods are declared public, at least those methods used by client methods (i.e. methods of other classes).

    The strategy is to insulate users of the class from its implementation details. For example, we could implement the Time class with only 1 instance variable,

    private int seconds; // 0 <= seconds <= 60 * 60 * 24

    without changing what the methods deliver to clients.

    We can change our implementation in many ways without having to change client code.

    A method should be declared private when the intention is for it to be invoked only by the class's methods (i.e., it is a utility method).

• The method with the same name as the class is called a constructor method.

  It initializes the instance variables of objects in the class.

  A class usually has several constructors, each with a unique parameter list (i.e., the constructor typically is an overloaded method).

  Constructors cannot return a value.

• Once a class has been defined, it can be used as a type in declarations.

  Java consequently is referred to as an extensible language.

  Time HappyHour, squashSchedule[], renAndStimpy, PeteAndPete;

• The applet TimeTest declares a Time object named t.
• t is instantiated in the init method (i.e., t does not refer to a Time object until a new operator is executed).
  1. Declarations convey your intent to use a particular identifier to refer to a particular kind of object or primitive data type.
  2. The new operator allocates memory for the object.
  3. The constructor gives the object an initial value.

  The constructor for Time invokes the setTime method to initialize the instance variables to 0.

• The instance variables of a Time object can never be invalid:
  • When the object is constructed it is given valid instance variable values;
  • When setTime is invoked, it ensures that the instance variables are given valid values.

  Designing classes this way eliminates a major source of errors, making object-oriented programming less error-prone than, say, ordinary C programming.

• A class's instance variables can be initialized:
  • Where they are declared in the class body;
  • By the class's constructor[s];
  • By the class's "set" methods (methods used to set the value of the instance variables).
• The toMilitaryString and toString methods have no parameters.

  They have access to all instance variables implicitly.

  This feature keeps method argument lists short, and less likely to have type errors at compile-time.