Data types in JAVA

Java is a Strongly Typed Language

It is important to state at the outset that Java is a strongly typed language. Indeed, part of
Java’s safety and robustness comes from this fact. Let’s see what this means. First, every
variable has a type, every expression has a type, and every type is strictly defined. Second,
all assignments, whether explicit or via parameter passing in method calls, are checked for
type compatibility. There are no automatic coercions or conversions of conflicting types as
in some languages. The Java compiler checks all expressions and parameters to ensure that
the types are compatible. Any type mismatches are errors that must be corrected before the
compiler will finish compiling the class.

The Primitive Types

Java defines eight primitive types of data: byte, short, int, long, char, float, double, and boolean.
The primitive types are also commonly referred to as simple types, and both terms will be
used in this book. These can be put in four groups:
• Integers This group includes byte, short, int, and long, which are for whole-valued
signed numbers.
• Floating-point numbers This group includes float and double, which represent
numbers with fractional precision.

• Characters This group includes char, which represents symbols in a character set,

like letters and numbers.

• Boolean This group includes boolean, which is a special type for representing

true/false values.

You can use these types as-is, or to construct arrays or your own class types. Thus, they

form the basis for all other types of data that you can create.

The primitive types represent single values—not complex objects. Although Java is

otherwise completely object-oriented, the primitive types are not. They are analogous to

the simple types found in most other non–object-oriented languages. The reason for this is

efficiency. Making the primitive types into objects would have degraded performance too much.

The primitive types are defined to have an explicit range and mathematical behavior.

Languages such as C and C++ allow the size of an integer to vary based upon the dictates

of the execution environment. However, Java is different. Because of Java’s portability

requirement, all data types have a strictly defined range. For example, an int is always 32 bits,

regardless of the particular platform. This allows programs to be written that are guaranteed

to run without porting on any machine architecture. While strictly specifying the size of an

integer may cause a small loss of performance in some environments, it is necessary in order

to achieve portability.

Let’s look at each type of data in turn.

Integers

Java defines four integer types: byte, short, int, and long. All of these are signed, positive

and negative values. Java does not support unsigned, positive-only integers. Many other

computer languages support both signed and unsigned integers. However, Java’s designers

felt that unsigned integers were unnecessary. Specifically, they felt that the concept of unsigned

was used mostly to specify the behavior of the high-order bit, which defines the sign of an integer

value. As you will see in Chapter 4, Java manages the meaning of the high-order bit differently,

by adding a special “unsigned right shift” operator. Thus, the need for an unsigned integer type

was eliminated.

The width of an integer type should not be thought of as the amount of storage it consumes,

but rather as the behavior it defines for variables and expressions of that type. The Java run-time

environment is free to use whatever size it wants, as long as the types behave as you declared

them. The width and ranges of these integer types vary widely, as shown in this table:

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

Let’s look at each type of integer.

byte

The smallest integer type is byte. This is a signed 8-bit type that has a range from –128 to 127.

Variables of type byte are especially useful when you’re working with a stream of data from

a network or file. They are also useful when you’re working with raw binary data that may

not be directly compatible with Java’s other built-in types.

Byte variables are declared by use of the byte keyword. For example, the following

declares two byte variables called b and c:

byte b, c;

short

short is a signed 16-bit type. It has a range from –32,768 to 32,767. It is probably the least-used

Java type. Here are some examples of short variable declarations:

short s;

short t;

int

The most commonly used integer type is int. It is a signed 32-bit type that has a range

from –2,147,483,648 to 2,147,483,647. In addition to other uses, variables of type int are

commonly employed to control loops and to index arrays. Although you might think that

using a byte or short would be more efficient than using an int in situations in which the

larger range of an int is not needed, this may not be the case. The reason is that when byte

and short values are used in an expression they are promoted to int when the expression is

evaluated. (Type promotion is described later in this chapter.) Therefore, int is often the best

choice when an integer is needed.

long

long is a signed 64-bit type and is useful for those occasions where an int type is not large

enough to hold the desired value. The range of a long is quite large. This makes it useful

when big, whole numbers are needed. For example, here is a program that computes the

number of miles that light will travel in a specified number of days.

// Compute distance light travels using long variables.

class Light {

public static void main(String args[]) {

int lightspeed;

long days;

long seconds;

long distance;

// approximate speed of light in miles per second

lightspeed = 186000;

days = 1000; // specify number of days here

seconds = days * 24 * 60 * 60; // convert to seconds

distance = lightspeed * seconds; // compute distance

System.out.print("In " + days);

System.out.print(" days light will travel about ");

System.out.println(distance + " miles.");

}

}

This program generates the following output:

In 1000 days light will travel about 16070400000000 miles.

Clearly, the result could not have been held in an int variable.

Floating-Point Types

Floating-point numbers, also known as real numbers, are used when evaluating expressions

that require fractional precision. For example, calculations such as square root, ortranscendentals

such as sine and cosine, result in a value whose precision requires a floating-point type. Java

implements the standard (IEEE–754) set of floating-point types and operators. There are two

kinds of floating-point types, float and double, which represent single- and double-precision

numbers, respectively. Their width and ranges are shown here:

double 64 4.9e–324 to 1.8e+308

float 32 1.4e–045 to 3.4e+038

Each of these floating-point types is examined next.

float

The type float specifies a single-precision value that uses 32 bits of storage. Single precision is

faster on some processors and takes half as much space as double precision, but will become

imprecise when the values are either very large or very small. Variables of type float are useful

when you need a fractional component, but don’t require a large degree of precision. For

example, float can be useful when representing dollars and cents.

Here are some example float variable declarations:

float hightemp, lowtemp;

double

Double precision, as denoted by the double keyword, uses 64 bits to store a value. Double

precision is actually faster than single precision on some modern processors that have been

optimized for high-speed mathematical calculations. All transcendental math functions, such

as sin( ), cos( ), and sqrt( ), return double values. When you need to maintain accuracy over

many iterative calculations, or are manipulating large-valued numbers, double is the best

choice.

Here is a short program that uses double variables to compute the area of a circle:

// Compute the area of a circle.

class Area {

public static void main(String args[]) {

double pi, r, a;

r = 10.8; // radius of circle

pi = 3.1416; // pi, approximately

a = pi * r * r; // compute area

System.out.println("Area of circle is " + a);

}

}

Characters

In Java, the data type used to store characters is char. However, C/C++ programmers beware:

char in Java is not the same as char in C or C++. In C/C++, char is 8 bits wide. This is not the

case in Java. Instead, Java uses Unicode to represent characters. Unicode defines a fully

international character set that can represent all of the characters found in all human

languages. It is a unification of dozens of character sets, such as Latin, Greek, Arabic, Cyrillic,

Hebrew, Katakana, Hangul, and many more. For this purpose, it requires 16 bits. Thus, 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 programs to be written for worldwide use, it makes sense that it would use Unicode to

represent characters. Of course, the use of Unicode is somewhat inefficient for languages such

as English, German, Spanish, or French, whose characters can easily be contained within 8 bits.

But such is the price that must be paid for global portability.

NOTE More information about Unicode can be found at http://www.unicode.org.

Here is a program that demonstrates char variables:

// Demonstrate char data type.

class CharDemo {

public static void main(String args[]) {

char ch1, ch2;

ch1 = 88; // code for X

ch2 = 'Y';

System.out.print("ch1 and ch2: ");

System.out.println(ch1 + " " + ch2);

}

}

This program displays the following output:

ch1 and ch2: X Y

Notice that ch1 is assigned the value 88, which is the ASCII (and Unicode) value that

corresponds to the letter X. As mentioned, the ASCII character set occupies the first 127

values in the Unicode character set. For this reason, all the “old tricks” that you may have

used with characters in other languages will work in Java, too.

Although char is designed to hold Unicode characters, it can also be thought of as an

integer type on which you can perform arithmetic operations. For example, you can add

two characters together, or increment the value of a character variable. Consider the

following program:

// char variables behave like integers.

class CharDemo2 {

public static void main(String args[]) {

char ch1;

ch1 = 'X';

System.out.println("ch1 contains " + ch1);

ch1++; // increment ch1

System.out.println("ch1 is now " + ch1);

}

}

The output generated by this program is shown here:

ch1 contains X

ch1 is now Y

In the program, ch1 is first given the value X. Next, ch1 is incremented. This results in ch1

containing Y, the next character in the ASCII (and Unicode) sequence.

Booleans

Java has a primitive type, called boolean, for logical values. It can have only one of two

possible values, true or false. This is the type returned by all relational operators, as in the

case of a < b. boolean is also the type required by the conditional expressions that govern

the control statements such as if and for.

Here is a program that demonstrates the boolean type:

// Demonstrate boolean values.

class BoolTest {

public static void main(String args[]) {

boolean b;

b = false;

System.out.println("b is " + b);

b = true;

System.out.println("b is " + b);

// a boolean value can control the if statement

if(b) System.out.println("This is executed.");

b = false;

if(b) System.out.println("This is not executed.");

// outcome of a relational operator is a boolean value

System.out.println("10 > 9 is " + (10 > 9));

}

}

The output generated by this program is shown here:

b is false

b is true

This is executed.

10 > 9 is true

There are three interesting things to notice about this program. First, as you can see, when

a boolean value is output by println( ), “true” or “false” is displayed. Second, the value of a

boolean variable is sufficient, by itself, to control the if statement. There is no need to write

an if statement like this:

if(b == true) ...

Third, the outcome of a relational operator, such as <, is a boolean value. This is why the

expression 10 > 9 displays the value “true.” Further, the extra set of parentheses around 10 > 9

is necessary because the + operator has a higher precedence than the >.

A Closer Look at Literals

Literals were mentioned briefly in Chapter 2. Now that the built-in types have been formally

described, let’s take a closer look at them.

Integer Literals

Integers are probably the most commonly used type in the typical program. Any whole

number value is an integer literal. Examples are 1, 2, 3, and 42. These are all decimal values,

meaning they are describing a base 10 number. There are two other bases which can be used

in integer literals, octal (base eight) and hexadecimal (base 16). Octal values are denoted in Java

by a leading zero. Normal decimal numbers cannot have a leading zero. Thus, the seemingly

valid value 09 will produce an error from the compiler, since 9 is outside of octal’s 0 to 7 range.

A more common base for numbers used by programmers is hexadecimal, which matches

cleanly with modulo 8 word sizes, such as 8, 16, 32, and 64 bits. You signify a hexadecimal

constant with a leading zero-x, (0x or 0X). The range of a hexadecimal digit is 0 to 15, so A

through F (or a through f ) are substituted for 10 through 15.

Integer literals create an int value, which in Java is a 32-bit integer value. Since Java is

strongly typed, you might be wondering how it is possible to assign an integer literal to one

of Java’s other integer types, such as byte or long, without causing a type mismatch error.

Fortunately, such situations are easily handled. When a literal value is assigned to a byte or

short variable, no error is generated if the literal value is within the range of the target type.

An integer literal can always be assigned to a long variable. However, to specify a long

literal, you will need to explicitly tell the compiler that the literal value is of type long. You

do this by appending an upper- or lowercase L to the literal. For example, 0x7ffffffffffffffL

or 9223372036854775807L is the largest long. An integer can also be assigned to a char as

long as it is within range.

Floating-Point Literals

Floating-point numbers represent decimal values with a fractional component. They can be

expressed in either standard or scientific notation. Standard notation consists of a whole number

component followed by a decimal point followed by a fractional component. For example, 2.0,

3.14159, and 0.6667 represent valid standard-notation floating-point numbers. Scientific notation

uses a standard-notation, floating-point number plus a suffix that specifies a power of 10 by

which the number is to be multiplied. The exponent is indicated by an E or e followed by a

decimal number, which can be positive or negative. Examples include 6.022E23, 314159E–05,

and 2e+100.

Floating-point literals in Java default to double precision. To specify a float literal, you

must append an F or f to the constant. You can also explicitly specify a double literal by

appending a D or d. Doing so is, of course, redundant. The default double type consumes 64

bits of storage, while the less-accurate float type requires only 32 bits.

Boolean Literals

Boolean literals are simple. There are only two logical values that a boolean value can have,

true and false. The values of true and false do not convert into any numerical representation.

The true literal in Java does not equal 1, nor does the false literal equal 0. In Java, they can only

be assigned to variables declared as boolean, or used in expressions with Boolean operators.

Character Literals

Characters in Java are indices into the Unicode character set. They are 16-bit values that can

be converted into integers and manipulated with the integer operators, such as the addition

and subtraction operators. A literal character is represented inside a pair of single quotes. All

of the visible ASCII characters can be directly entered inside the quotes, such as ‘a’, ‘z’, and ‘@’.

For characters that are impossible to enter directly, there are several escape sequences that allow

you to enter the character you need, such as ‘\’’ for the single-quote character itself and ‘\n’for

the newline character. There is also a mechanism for directly entering the value of a character in

octal or hexadecimal. For octal notation, use the backslash followed by the three-digit

number. For example, ‘\141’ is the letter ‘a’. For hexadecimal, you enter a backslash-u (\u), then

exactly four hexadecimal digits. For example, ‘\u0061’is the ISO-Latin-1 ‘a’ because the top byte

is zero. ‘\ua432’is a Japanese Katakana character. Table 3-1 shows the character escape sequences.

String Literals

String literals in Java are specified like they are in most other languages—by enclosing

a sequence of characters between a pair of double quotes. Examples of string literals are

“Hello World”

“two\nlines”

“\”This is in quotes\”“

The escape sequences and octal/hexadecimal notations that were defined for character

literals work the same way inside of string literals. One important thing to note about Java

strings is that they must begin and end on the same line. There is no line-continuation escape

sequence as there is in some other languages.