CS 4773 Object Oriented Systems

Object Oriented Programming Concepts as applied to Java

Does the language make a difference?

Suppose that in an Eskimo language there was no general word for snow, just words for specific forms of snow.
Does this mean you could not express the concept: It is snowing?

Are there any advantages to a language that forces you to be specific rather than general?
Do these languages make you think in different ways?

Two views

Sapir-Whorf hypothesis:It may be possible for an individual working in one language to imagine thoughts that cannot be expressed in another language, and cannot be understood by those using this other language.

It is possible for both of these to be correct.

Which ever is correct it is certainly true that the language we use influences the way we think.

Object-oriented programming is a programming paradigm: an model or example of how programs should be designed.

OOP Concepts

• Responsibilities
  • Describe behavior in terms of an object in terms of what should be accomplished, rather than the method to accomplish it.
  • This makes the description independent of the implementation
  • Gives more flexibility in design
  • An object can be thought of as an agent responsible for accomplishing a goal.
  • Communication with objects is through messages
  • In Java, messages are sent by calling methods of the object
• Encapsulation
  • The internal structure (implementation) should be hidden from the user (as above).
  • The internal structure should also be protected from the misuser.
  • Parnas's principles: (as quoted in Budd's book)
    • The developer of a software component must provide the intended user with all of the information needed to make effective use of the services provided by the component, and should provide no other information.
    • The developer of a software component must be provided with all the information necessary to carry out the given responsibilities assigned to the component, and should be provided with no other information.
• Classes and Instances
  • An object is an instance of a class, just as 3 is an instance of an integer.
  • A class defines the behavior of the objects in the class.
  • In Java, variables can be either instance variables (one for each instance) or class variables (shared by all instances). Similarly for methods.
• Class Hierarchies and Inheritance
  • In Java, a subclass is produced using extends
  • If A extends B, A is a subclass of B and B is a superclass of A.
  • A inherits the variables and methods of B
  • A can override methods of B
• Uses of Inheritance
  • Subclassing for Specialization (Subtyping)
    • The new class is a specialized form of the parent and satisfies all of the specifications of the parent.
    • Satisfies the Principle of Substitutability:
      If B is a subclass of A, it should be possible to substitute instances of class B for instances of class A in any situation with no observable effect.
    • Example: Window and TextWindow.
      Window operations: move, resize, iconify
  • Subclassing for Specification
    • The parent class may have some operations (methods) that are deferred, or incompletely specified.
    • The subclass must completely specify these.
    • In Java this is done with implements rather than extends
    • In Java, the parent class is called abstract
    • Example: implements Runnable
  • Subclassing for Construction
    • The subclass has new behavior (methods) which use the methods of the parent, but the methods of the parent are not part of the specifications of the subclass.
    • Example: a set may be implemented as a subclass of a list.
      The set does not have the ordering property of the list.
  • Subclassing for Generalization
    • The behavior of the parent is modified to allow a more general behavior.
    • At least one method of the parent must be overridden.
    • Example: A ColoredWindow generalizes a Window which might always have a white background.
    • In theory you should avoid this and have Window as a subclass of ColoredWindow, but this is not always possible.
  • Subclassing for Extension
    • The parent methods are left unchanged.
    • Additional methods are added for increased functionality.
    • Example: A StringSet extends Set to provide string operations.
  • Subclassing for Limitation
    • Some of the operations on the parent are limited.
    • Example: A stack may be a subclass of a double-ended queue.
  • Subclassing for Variance
    • Two or more classes have much in common but one is not logically a subclass of the other.
    • One is logically chosen as the parent and the other as the child.
    • Example: Mouse and Graphics Tablet.
    • Should be avoided with the common parts put in an abstract class.
  • Subclassing for Combination
    • This is usually called multiple inheritance
    • In Java a class can only extend a single class
    • Java supports this using implements
• Replacement
  • Replacement refers to the replacement of a method from the parent in a subclass.
  • When a message is sent to an object, the search for a matching object begins with the examination of the methods associated with the class of the object.
  • If no method is found, the methods associated with the immediate parent class of the object are examined.
  • If no matching method is found, the immediate patent of that class is searched.
  • This continues until a match is found or no parent exists.
  • A method in a class that has the same name as a method in a superclass is said to override the method in the parent class.
  • In Java, the overriding class must have the same parameters types and return value type as the overridden method.
  • In Java it is also possible to override data fields.
• Refinement
  • Refinement refers to supplementing the behavior of the parent class so that the actions described by the code in the child class are combined with the actions described by the parent class.
  • This is often useful during the initialization of the new object.
  • In Java this is accomplished by using super.
  • Sending a message to super indicates that the search for the associated method should begin with the parent class to the current class rather than the current class.
  • You use this in JOTSA when in your applet you execute
    super.init();
• Assignment
  • There are three possible interpretations of assignment
    • Copy semantics: Assignment copies the entire value of the right side, assigning it to the left side.
      The two values are independent and changes to one do not affect the other.
    • Pointer semantics: Assignment changes the reference of the left side to be the right side.
      Also called pointer assignment
      The two variables not only have the same value but also refer to the same object.
    • Copy on Write: Use pointer semantics for assignment but convert the value into a new structure if it is ever modified.
  • Java uses copy semantics for primitive types and pointer semantics for everything else.
  • In Java you can simulate copy semantics by using new or clone.
• Equality
  • There are two possible interpretations of equality
    • identity: Also called pointer equivalence
      The pointers refer to the same objects
    • member equality: Also called structural equivalence
      The members of the objects are the same
  • Java uses identity for non-primitive types.
  • Many classes have an equals method which tests for member equality.
• Polymorphism
  • A polymorphic object is any entity, such as a variable or function argument, that is permitted to hold values of different types during the course of execution.
  • Forms of polymorphism
    • Polymorphic variables can hold values of different types.
      In Java, a variable which is not of primitive type can hold any subclass of the declared type.
    • Overloading refers to 2 or more function bodies associated with the same name.
      Java does not allow user overloading of operators, but does allow Parametric Overloading:
      methods of the same name can have different number and types of arguments.
    • Templates refers to a way of parameterizing a class or function by use of a type.
      Java does not support this.
  • Overriding: Java allows for overriding methods of a parent class.
  • Deferred methods:Java uses abstract classes and implements for this.
  • In Java, the word final can be used to prevent overriding.