CSC/ECE 517 Fall 2007/wiki2 6 mxz

Assignment 2 - Topic 6

Type v.s. Class Survey the differences on type vs. class in object-oriented languages. Often, the distinction is that class pertains to an object, whereas type pertains to a variable. Consider the distinction between the two terms in several different programming languages. Cover the differences between type and class, from type-theoretic definitions to practical aspects.

Type and Class

Types limit the values that a variable can hold or that an expression can produce, limit the operations supported on those values, and determine the meaning of the operations.[1] For example, a string type variable in c++ holds a string of characters with the operation length.

        string str;
	str="I am string type";
	cout<<str.length();

A class is the blueprint from which individual objects are created. It may be a combination of a type, a constructor and optionally a subtyping relation.[2] In following example, we define a class MyString inherited from String:

        class MyString < String
          def initialize
            ...
          end
          def method
            ...
          end
        end
        str = MyString.new

Type v.s. Class - Theoretic Differences

Generally speaking, type differs from class in the following ways:

The primary difference between type and class is that class pertains to an object, whereas type pertains to a variable.[8]

Type is generally a compile-time concept which limits the values of a variable. The class of an object, though, exists and is important at runtime, and in the behavior of casting operations which provide the models to create the objects.

Classes have their own class methods and class variables which don't invoke any of its instances. For instance, we may have a Car Class which creates different kinds of cars. In this class we may have a variable counting the number of objects created. However, a type does not have such variables holding information of its instances.

In OOP languages, classes are types, while most types are not classes. In pure object languages, by opposition (e.g. Smalltalk, Scala, perhaps Python), every type maps to a class.

Type, Class and Interface in Java

In the Java programming language, every variable and every expression has a type that can be determined at compile time. The type may be a primitive type or a reference type. Reference types include class types and interface types. Reference types are introduced by type declarations, which include class declarations and interface declarations. We often use the term type to refer to either a class or an interface[3]

Every object belongs to some particular class: the class that was mentioned in the creation expression that produced the object, the class whose Class object was used to invoke a reflective method to produce the object, or the String class for objects implicitly created by the string concatenation operator "+". This class is called the class of the object. An object is said to be an instance of its class and of all superclasses of its class.

Sometimes a variable or expression is said to have a "run-time type". This refers to the class of the object referred to by the value of the variable or expression at run time, assuming that the value is not null.

The compile time type of a variable is always declared, and the compile time type of an expression can be deduced at compile time. The compile time type limits the possible values that the variable can hold or the expression can produce at run time. If a run-time value is a reference that is not null, it refers to an object or array that has a class, and that class will necessarily be compatible with the compile-time type.

Type v.s. Class in Ruby

We say that objects are instances of classes and variables are instances of types. While this generally holds things are a bit different in Ruby. As Ruby is a purely object oriented programming language every variable is an object. Also there is no distinction between primitive types and object types. Thats why in Ruby every type maps to a class. As with several other object-oriented languages the Object class is the root of all types. In Ruby, you are able to superclass built-in types like strings and lists.

In Ruby an objects type is not determined by which class it belongs to but by what it can do. That is why we can use duck typing Ruby. In Ruby an object is of a certain type if it behaves as that type. Class objects in Ruby are not explicitly declared to have a certain type. In Ruby the type of a variable depends upon what was last assigned to it. A variable can hold many different types during the lifetime of a script.

Java uses inheritance as the mechanism for defining the type of an object. An object "is a" X if it implements X. While Ruby also supports inheritance, establishing "is a" relationships is not its main purpose.

Type v.s. Class in Phyton

Phyton is an important example. In earlier versions of Phyton derived types were not created from built in types but just from user defined classes. After Phyton 2.2 this has changed and "type/class unification" is accomplished. That is like Ruby, every type started mapping on a single class. Python intended to remove most of the differences between built-in types and user-defined classes. Perhaps the most obvious one is the restriction against using built-in types (such as the type of lists and dictionaries) as a base class in a class statement.IN the old version class and type used to not be the same thing. All instances of old-style classes are of type 'instance'. New-style classes elevate Python-level classes to types equal to the built-in ones, which is why the word "type" is now sufficient for both type and class.

Even though Phyton has made changes there are still differences with Ruby which is also dynamically typed language. Emulating container types in Phyton is harder because there are different methods you have to use for "Emulating callable Objects", "Emulating numeric types", "Emulating sequences" etc.. In Ruby, you just derive from the respective base classes (or extend them directly). So, Ruby's type system is much simpler and cleaner and allows a smaller and simpler language core.

Types v.s. Class in C++

C++ like Java has different kinds of types. Types can be built in, user defined or derived from preexisting classes. Built in types are types that are already defined in the language like int or String. User defined types are types created from user written classes. Lastly derived types are types created form extending the other two types of classes. "derived class" in C++ is "subclass" in Java. When a class extends a base class, to derive a new class with all the base's variables and methods, plus some of its own, we call the new class the subclass, and the base class the new class's "superclass".

In C++ like Java every variable has a type at compile time and the variable needs to be type casted if the type of the variable needs to be change.

Practical Differences

An example in Java

In the following code,

The variable p of class Test has type Point and is initially assigned a reference to a new instance of class Point.

The variable cp similarly has as its type ColoredPoint, and is initially assigned a reference to a new instance of class ColoredPoint.

The assignment of the value of cp to the variable p causes p to hold a reference to a ColoredPoint object. This is permitted because ColoredPoint is a subclass of Point, so the class ColoredPoint is assignment compatible with the type Point.

The variable c has as its type the interface type Colorable, so it can hold a reference to any object whose class implements Colorable; specifically, it can hold a reference to a ColoredPoint.

public interface Colorable {
	void setColor(byte r, byte g, byte b);
}
class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {
	byte r, g, b;
	public void setColor(byte rv, byte gv, byte bv) {
		r = rv; g = gv; b = bv;
	}
}
class Test {
	public static void main(String[] args) {
		Point p = new Point();
		ColoredPoint cp = new ColoredPoint();
		p = cp;
		Colorable c = cp;
	}
}

Note that even though a variable or expression whose type is a compile-time interface type can reference any object whose class implements that interface, there are no instances of interfaces. An expression such as "new Colorable()" is not valid because it is not possible to create an instance of an interface, only of a class. [3]

Syntax Differences

Here is how we define a class in java. Defining classes in different O-O languages are not so different:

class Car
{
       int numOfWheels = 4;

       void changeWheelNum(int newValue) 
       {
            numOfWheels = newValue;
       }
}

Its even easier for Ruby:

class Car
       numOfWheels = 4

       def changeWheelNum(newValue) 
       
            numOfWheels = newValue
       end
end

The class Car that we defined in the above code is from now on a Type. But a user cannot define a built in type which does not stick to a class. Instances of built in types can be created as below:

String str = "The Type is defined"

While in Java you have to define the type every time you create a new variable, in Ruby this is not needed as stated above. In Ruby we don't need type String be written. This is because in Ruby types are defined as how they behave.

str = 'No Type is defined'

When you want to change the type of an object in Java you need to type cast it, in Ruby this is not needed. We saw how instances of built in types could be called for Java and Ruby. To call instances of class derived classes we need to use the 'new' keyword. Her it is how we do that in Java:

Car mycar = new Car();

It's similar in Ruby we just don't need the 'Car' type definition at the beginning.

Types can also be created from inheritance.

class SportsCar extends Car 
{
     int capacity = 2;
}

Now we have a Car type defined and also a SporsCar that is a subtype of Car. There is a special relationship between the type of a subclass and the type of a superclass, in that a subclass of a class defines a subtype of the superclass’ type.