Question

write a short reflection about kinds of variables and their uses.​

Answer 1

Answer:

Type variables and reflection

The class Class whose objects represent information about Java classes is now a generic class that takes a type parameter. Thus, the type String is represented by an object of type Class<String>, type ETicket is represented by an object of type Class <ETicket> etc.

Unfortunately, type variables cannot be used in all contextss where a type name is expected. As we have seen, type variables can be used to specify the types of arguments and return values to functions and also to specify the types of variables within a function or a class. However in expressions such as

if (s instanceof Shape){ ... }

we cannot replace Shape by a type variable and write

if (s instanceof T){ ... } // T a type variable

This means that the following version of the function classequal (see page 83 in the older notes) would not work:

public static <S,T> boolean classequal(S o1, T o2){

return (o1 instanceof T) && (o2 instance of S); // Illegal!

}

However, we can fruitfully exploit the fact that Class now has a type parameter. Suppose we want to write a reflective function that takes as input a class and creates a new object of that type. In the normal setup, we would have written:

public Object createinstance(Class c){

return c.newInstance();

}

The return type of this function is Object because we cannot infer any information about the nature of the class c which is received as the argument.

In the revised framework, we if c is the Class object corresponding to a class T, then it is actually of type Class<T>. Thus, we can rewrite this function as follows:

public T createinstance(Class<T> c){

return c.newInstance();

}

Now, notice that we are able to extract the underlying type T from c and use it to specify a more precise return value, thus avoiding the need to use a cast when calling this function.

One important point about Java's parameterized classes is that they are more sophisticated than macros or templates. It is not correct to think of a definition like

public class LinkedList<T>{...}

as a skeleton that is instantiated into a ``real'' Java class each time it is actually used with a concrete value of T. There is truly only one class LinkedList<T> and all specific instantiations of LinkedList<T> have the same underlying object. In other words, if we declare

LinkedList<String> stringlist = new LinkedList<String>();

LinkedList<Integer> intlist = new LinkedList<Integer>();

then

classequal(stringlist,intlist)

returns true.

This seems to have some unfortunate implications. For instance, suppose we write:

LinkedList<String> sl = new LinkedList<String>();

LinkedList<String> newsl = createinstance(sl.getClass());

Since all instantiations LinkedList<T> are of the same type, the value returned by sl.getClass() is of the ``generic'' type LinkedList<T> and the Java compiler is unable to reconcile this with the specific type LinkedList<String> defined for newsl. (Try this for yourself and see.)

Explanation:

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Answer 2

There are six types of variables and they have a variety of usages within statistics and the calculation of data.

• A variable is defined as a characteristic that can be measured and that can be assumed to have different values. From this universal definition, we can look at the various types of variables such as; categorical, nominal, ordinal, numeric, continuous, and discrete.
• Categorical variables are variables that cannot be quantified. They are also called a qualitative variable. Examples of this include; gender, race, ethnicity, place of birth, etc. These variables will have some impact on your data depending on your aim but they will also not be calculated or measured.
• Nominal variables describe variables without any natural order. They can consist of names, labels, or categories. Examples include; sex, type of transport, etc.
• Ordinal variables are variables whose values are said to be defined by an order relation between two different categories. For example, if we had a scale from very bad to excellent we will be able to know that the variable of good is better than bad and less than very good. Therefore, one variable has a relation with another one.
• Numeric variables otherwise known as quantitative variables are variables that can be measured in numbers, for example, height, weight, age, etc. They are also divided into discrete and continuous variables.
• Continuous variables are variables that are assumed to be infinite in number and have a real value within a given interval. For example, if a person's weight cannot be negative therefore it is theoretically finite in the sense that it cannot go backward.
• Discrete variables are only finite numbers of real values within a given interval. For example, if someone score on a test with the highest mark 10. They can only get up to 10 however, they will still have finite possibilities with the help of decimal points.

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