Datatypes And Arrays

            Java is a strongly typed language. All expressions and parameters are checked by the java compiler to ensure that the types are compatible.

             There are 8 simple datatypes defined by Java. They are byte, short, int, long, char, float, double and boolean.

Integers:

             The datatypes byte, short, int and long is grouped  under integers. All of these datatypes are signed, positive and negative values.

Name	Width	Range
long	64	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
int	32	-2,147,483,648  to  2,147,483,647
short	16	-32,768  to  32,767
byte	8	-128  to  127

Floating point types:

              When evaluating expressions that require fractional precision then floating-point numbers are used. There are two types of floating-point types, float and double, which represent single- and double-precision numbers, respectively. Their width and ranges are shown here:

Name	Width in Bits	Approximate Range
double	64	4.9e–324 to 1.8e+308
float	32	1.4e-045 to 3.4e+038

Characters:

                 char is the datatype used to store characters in java. In Java char is a 16-bit type. The range of a char is 0 to 65,536. There are no negative chars. The standard set of characters known as ASCII still ranges from 0 to 127 as always, and the extended 8-bit character set, ISO-Latin-1, ranges from 0 to 255. Since Java is designed to allow applets to be written for worldwide use, it makes sense that it would use Unicode to represent characters.

Booleans:

             The simple datatype used for logical values is boolean. It can have only one of two possible values, true or false. This is the type returned by all relational operators, such as a < b. boolean is also the type required by the conditional expressions that govern the control statements such as if and for.

Arrays:

             An array is a group of like-typed variables that are referred to by a common name. Arrays of any type can be created and may have one or more dimensions. A specific element in an array is accessed by its index. Arrays offer a convenient means of grouping related information.

One-Dimensional Arrays

     A one-dimensional array is, essentially, a list of like-typed variables. To create an array, you first must create an array variable of the desired type. The general form of a one-dimensional array declaration is

type var-name[ ];

Here, type declares the base type of the array. The base type determines the data type of each element that comprises the array. Thus, the base type for the array determines what type of data the array will hold.

            For example, the following declares an array named month_days with the type “array of int”:

int month_days[];

Although this declaration establishes the fact that month_days is an array variable, no array actually exists. In fact, the value of month_days is set to null, which represents an array with no value. To link month_days with an actual, physical array of integers, you must allocate one using new and assign it to month_days. new is a special operator that allocates memory.

  The general form of new as it applies to one-dimensional arrays appears

as follows:

array-var = new type[size];

Here, type specifies the type of data being allocated, size specifies the number of elements in

the array, and array-var is the array variable that is linked to the array. That is, to use new to

allocate an array, you must specify the type and number of elements to allocate. The elements in the array allocated by new will automatically be initialized to zero. This example allocates a 12-element array of integers and links them to month_days.

month_days = new int[12];

After this statement executes, month_days will refer to an array of 12 integers. Further, all elements in the array will be initialized to zero.

Let’s review:

                Obtaining an array is a two-step process. First, you must declare a variable of the desired array type. Second, you must allocate the memory that will hold the array, using new, and assign it to the array variable. Thus, in Java all arrays are dynamically allocated.

     Once you have allocated an array, you can access a specific element in the array by specifying its index within square brackets. All array indexes start at zero.

For example, this statement assigns the value 28 to the second element of month_days.

month_days[1] = 28;

The next line displays the value stored at index 3.

System.out.println(month_days[3]);

Putting together all the pieces, here is a program that creates an array of the number

of days in each month.

// Demonstrate a one-dimensional array. class Array { public static void main(String args[]) { int month_days[]; month_days = new int[12]; month_days[0] = 31; month_days[1] = 28; month_days[2] = 31; month_days[3] = 30; month_days[4] = 31; month_days[5] = 30; month_days[6] = 31; month_days[7] = 31; month_days[8] = 30; month_days[9] = 31; month_days[10] = 30; month_days[11] = 31; System.out.println("April has " + month_days[3] + " days."); } }

When you run this program, it prints the number of days in April. As mentioned, Java array indexes start with zero, so the number of days in April is month_days[3] or 30. It is possible to combine the declaration of the array variable with the allocation of the array itself, as shown here:

int month_days[] = new int[12];

This is the way that you will normally see it done in professionally written Java programs. Arrays can be initialized when they are declared. The process is much the same as that used to initialize the simple types. An array initializer is a list of comma-separated expressions surrounded by curly braces. The commas separate the values of the array elements. The array will automatically be created large enough to hold the number of elements you specify in the array initializer. There is no need to use new.

            For example, to store the number of days in each month, the following code creates an initialized array of integers:

// An improved version of the previous program. class AutoArray  { public static void main(String args[]) { int month_days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; System.out.println("April has " + month_days[3] + " days."); } }

When you run this program, you see the same output as that generated by the previous version. Java strictly checks to make sure you do not accidentally try to store or reference values outside of the range of the array. The Java run-time system will check to be sure that all array indexes are in the correct range. For example, the run-time system will check the value of each index into month_days to make sure that it is between 0 and 11 inclusive. If you try to access elements outside the range of the array (negative numbers or numbers greater than the length of the array), you will cause a run-time error.

Here is one more example that uses a one-dimensional array. It finds the average of a set

of numbers.

// Average an array of values. class Average  { public static void main(String args[])  { double nums[] = {10.1, 11.2, 12.3, 13.4, 14.5}; double result = 0; int i; for(i=0; i<5; i++) result = result + nums[i]; System.out.println("Average is " + result / 5); } }