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Java in a Nutshell

Previous Chapter 2
How Java Differs from C

2.6 Primitive Data Types

Java adds byte and boolean primitive types to the standard set of C types. In addition, it strictly defines the size and signedness of its types. In C, an int may be 16, 32, or 64 bits, and a char may act signed or unsigned depending on the platform. Not so in Java. In C, an uninitialized local variable usually has garbage as its value. In Java, all variables have guaranteed default values, though the compiler may warn you in places where you rely, accidentally or not, on these default values. Table 2.2 lists Java's primitive data types. The subsections below provide details about these types.

Table 2.2: Java Primitive Data Types
        Min Value
Type Contains Default Size Max Value
boolean true or false false 1 bit N.A.
char Unicode character \u0000 16 bits \u0000
byte signed integer 0 8 bits -128
short signed integer 0 16 bits -32768
int signed integer 0 32 bits -2147483648
long signed integer 0 64 bits -9223372036854775808
float IEEE 754 0.0 32 bits +/-3.40282347E+38
  floating-point     +/-1.40239846E-45
double IEEE 754 0.0 64 bits +/-1.79769313486231570E+308
  floating-point     +/-4.94065645841246544E-324

The boolean Type

boolean values are not integers, may not be treated as integers, and may never be cast to or from any other type. To perform C-style conversions between a boolean value b and an int i, use the following code:

b = (i != 0);   // integer-to-boolean: non-0 -> true; 0 -> false; 
i = (b)?1:0;    // boolean-to-integer: true -> 1; false -> 0;

The char Type

char values represent characters. Character literals may appear in a Java program between single quotes. For example:

char c = 'A';

All of the standard C character escapes, as well as Unicode escapes, are also supported in character literals. For example:

char newline = '\n', apostrophe = '\", delete = '\377', aleph='\u05D0';

Values of type char do not have a sign. If a char is cast to a byte or a short, a negative value may result.

The char type in Java holds a two-byte Unicode character. While this may seem intimidating to those not familiar with Unicode and the techniques of program internationalization, it is in fact totally transparent. Java does not provide a way to compute the size of a variable, nor does it allow any sort of pointer arithmetic. What this means is that if you are only using ASCII or Latin-1 characters, there is no way to distinguish a Java char from a C char.

Integral Types

The integral types in Java are byte, short, char, int, and long. Literals for these types are written just as they are in C. All integral types, other than char, are signed. There is no unsigned keyword as there is in C. It is not legal to write long int or short int as it is in C. A long constant may be distinguished from other integral constants by appending the character l or L to it.

Integer division by zero or modulo zero causes an ArithmeticException to be thrown. [3]

[3] Exceptions signal errors in Java. Exception handling is described later in this chapter.

Floating-Point Types

The floating-point types in Java are float and double. Literals for these types are written just as they are in C. Literals may be specified to be of type float by appending an f or F to the value; they may be specified to be of type double by appending a d or D.

float and double types have special values that may be the result of certain floating-point operations: positive infinity, negative infinity, negative zero and not-a-number. The java.lang.Float and java.lang.Double classes define some of these values as constants: POSITIVE_INFINITY, NEGATIVE_INFINITY, and NaN.

NaN is unordered--comparing it to any other number, including itself, yields false. Use Float.isNaN() or Double.isNaN() to test for NaN. Negative zero compares equal to regular zero (positive zero), but the two zeros may be distinguished by division: one divided by negative zero yields negative infinity; one divided by positive zero yields positive infinity.

Floating-point arithmetic never causes exceptions, even in the case of division by zero.

String Literals

Strings in Java are not a primitive type, but are instances of the String class. However, because they are so commonly used, string literals may appear between quotes in Java programs, just as they do in C. When the compiler encounters such a string literal, it automatically creates the necessary String object.

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