Transcript Numeric literals and named constants

Numeric literals and named constants
Numeric literals
• Numeric literal:
• A numeric literal is a constant value that appears
in a Java program.
Example:
3.14159265358979
1776
The data type of literals
• Important fact:
• Every numerical literal (constant) has a data type
The data type of literals (cont.)
• Type assignment rules for constants in Java:
• A integer literal (integer constant) has the type int
Example:
• 12345 has the type int
• A decimal literal (decimal constant) has the type
double
Example:
•123.45 has the type double
Special tags that change the data type of a
constant
• long tag:
• The tag L or l after an integer literal will change the
data type of the literal to long
Example:
• 12345L has the type long
Special tags that change the data type of a
constant (cont.)
• The float tag:
•The tag F or f after a decimal literal will change the data
type of the literal to float
Example:
• 123.45f has the type float
Exercise: can you tell what's wrong with
these statements ?
• Figure out what cause the error in each of the statements in
the following Java program:
public class Exercise3
{
public static void main(String[] args)
{
int a;
float b;
a = 1L;
// What is wrong ?
b = 1.0; // What is wrong ?
}
}
Exercise: can you tell what's wrong with
these statements ? (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
Exercise3.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Exercise3.java
• Can't run the program, look at the reported errors !
Exercise: can you tell what's wrong with
these statements ? (cont.)
• Here is the compile errors:
Exercise3.java:10: possible loss of precision
found : long
required: int
a = 1L; // What is wrong ?
^
Exercise3.java:12: possible loss of precision
found : double
required: float
b = 1.0; // What is wrong ?
^
2 errors
Assigning integer constants to byte and
short variables
• Strictly speaking, you cannot assign an integer literal
(which has the data type int) to a byte typed or short typed
variable.
Example: strictly speaking, the following assignment
statements are not allowed:
Assigning integer constants to byte and
short variables (cont.)
public class Demo1
{
public static void main(String[] args)
{
byte a;
short b;
a = 1;
// Assigns an int value (1) to a byte variable (a)
b = 1;
}
}
// Assigns an int value (1) to a short variable (b)
Assigning integer constants to byte and
short variables (cont.)
• Strictly speaking, you need to use casting operators:
public class Demo1
{
public static void main(String[] args)
{
byte a;
short b;
a = (byte) 1; // Converts an int value (1) to a byte value
b = (short) 1; // Converts an int value (1) to a short value
}
}
The designers of the Java language deem this to be
inconvenient and introduced a convenience conversion rule
into the Java language
Assigning integer constants to byte and
short variables (cont.)
• Convenience auto-conversion rule:
• If an integer literal (constant) is assigned to a byte
typed or short typed variable, the literal (constant) is
automatically converted into the appropriate type if the
constant is within the range of the data type of the
variable.
Assigning integer constants to byte and
short variables (cont.)
• Example:
• The range of the byte type is −128 ... 127
• Therefore, the following assignment statements are
permitted (through the convenience conversion rule):
byte a;
a = 1;
a = 127;
a = -128;
Assigning integer constants to byte and
short variables (cont.)
• However, the following assignment statements are
illegal (out of range !):
byte a;
a = 128;
a = -129;
Assigning integer constants to byte and
short variables (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
Byte.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Byte.java
• Can't run - look at the compiler errors.
Named literals (constants)
• Named constants in Mathematics:
•π=
3.14159265358979323846264338327950288419716939
937510582097...
• e = 2.7182818284590452353602875.... (base of the
natural log)
Named literals (constants) (cont.)
• We can give a name to any constant in Java
Later on in the program, we can use the name in place of
the actual constant
•
Syntax to define a named constant:
final datatype CONSTANT_NAME =
value ;
Named literals (constants) (cont.)
• Explanation:
• The keyword final introduces the definition of a
named constant
• datatype is the data type of the named constant (Yes,
a named constant also has a data type)
• CONSTANT_NAME is the name of the constant
CONSTANT_NAME is an identifier
• value is the value of the named constant
Named literals (constants) (cont.)
• Example: area of a circle using a named constant myPi
public class AreaOfCircle2
{
public static void main(String[] args)
{
double r;
// variable containing the radius
double area; // variable containing the area
final double myPi = 3.14159265358979;
r = 4;
// Give the radius
area = myPi * r * r; // Compute the area of the circle
System.out.print("The radius = ");
System.out.println(r);
System.out.print("The area = ");
System.out.println(area);
}
}
Named literals (constants) (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
AreaOfCircle2.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac AreaOfCircle2.java
• To run:
java AreaOfCircle2