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Numeric literals and named constants

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• Numeric literals
  • Numeric literal:

    A numeric literal is a constant value that appears in a Java program.

    Example:

    3.14159265358979 1776

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• The data type of literals
  • Important fact:

    Every numerical literal (constant) has a data type

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  • 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

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• 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

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  • 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

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• 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 ? } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    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 !

    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

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• 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:

    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) } }

    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

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  • 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.

    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; However, the following assignment statements are illegal (out of range !): byte a; a = 128; a = -129;

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    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.

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• Named literals (constants)
  • Named constants in Mathematics:

    π = 3.14159265358979323846264338327950288419716939937510582097... e = 2.7182818284590452353602875.... (base of the natural log)

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  • 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

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  • Syntax to define a named constant:

    final datatype CONSTANT_NAME = value ;

    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

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  • 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); } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac AreaOfCircle2.java To run:          java AreaOfCircle2