Primitive Data

Last modified Sunday, 07-Jul-2024 16:06:39 UTC.

Data Values
- Classes, objects, and methods can contain data values (see The Big Picture)
  - A class data value (indicated by the static modifier) is used to maintain information shared by all instances or aggregate information about the instances.
  - An instance data value is used to maintain information specific to individual instances.
  - Method data values (including parameters) are local to the method
  - Example: MetricConversion.java
- Two types of data values (Slides)
  - Primitive variables contain raw data
  - Reference variables point at objects
- Declarations
  - When a variable declaration is made, memory space is allocated to store the values of the variables.
  - A variable has three properties:
    - A memory location to store the value
    - The type of value stored
    - The name used to refer to the memory location.
  - The syntax for declarations is
```
<modifiers> <type> <names>; 
```
  - Example: MetricConversion.java
  - Variables have (in principle) garbage in them when declared.
    - Java is kind ... it will zeroize some types of variables, but you should never trust a prankster.
    - Java is kind ... it won't let you use a variable that has not been assigned a value.
  - Geoff's rule ... declare all variables at the start of the class or method.
  - Example: PizzaCost.java
- Assignment
  - Values are assigned into variables using assignment statements.
  - The syntax for assignments is
```
<name> = <value>; 
```
  - Example: MetricConversion.java
- Initialization assignment
  - Variables may initialized by assignment in their declaration.
  - Example: NumericVariables.java
  - For final variables, this is their only assignment
  - Example: MetricConversion.java
- Class constants
  - Class constants (indicated by the static and final modifiers) contain values that are global to the class, or even the world.
  - Class constants for the class should be private
  - Class constants for the world should be public
  - Example: MetricConversion.java
Primitive numeric data values
- Example: NumericVariables.java
- Example: NumericOperators.java
- Example: NumericPrecedence.java
- Example: NumericCasting.java
- Example: FinalVariables.java
- Math class
  - See SUN's documentation
  - Example: MathFunctions.java
Primitive Boolean data values
- The boolean values (after George Boole) are true and false
- Example: Boolean.java
- Boolean values are used to control the flow of execution in selection and repetition statements (coming to a class near you soon)
- Boolean values can be created with relational operators (Slides)
- Example: MakeBooleans.java
- Just like mathematical operators, Boolean operators combine Boolean values.
- Example: CombineBooleans.java
- Example: ShortCircuit.java
- Boolean values are used to form conditional expressions, whose value is determined by a Boolean expression.
- Example: Conditional.java

Primitive character data values (Slides)

Example: ASCIITranslate.java
Characters can be compared
Example: CharOrder.java

Special characters

Special characters are written as escape sequences, which represent single characters by using a \ prefix

Name	Escape
Tab	`\t`
Newline	`\n`
Backspace	`\b`
Single quote	`\'`
Backslash	`\\`
Bell	`(char)7`

Example: EscapeCharacters.java

Character class
- See SUN's documentation
- Example: CharGames.java
- Example: CharGamesCompressed.java

Some rules for safe programming with data values
- Declare constant data as final
- Always declare public static data as final (to avoid warts).
- Declare static variables private in preference to public
- Always declare non-static variables private to provide encapsulation.
- The public, private, and static modifiers are not used for local variables

Exercises

What numeric data type would be most appropriate to store:
- An amount of money
- The number part of a UM student number
- The day number in a year
What is the data type of each of the following:
- 234.567
- 0L
- (short) 234
- 2e2f
Why is the result of the following 27?
```
5+2*11 
```
?
How could the previous expression be modified to make the result 77?
What will give you warts?

Exam Style Questions

Name the six primitive numeric data types in Java.
What is the data type of each of these expressions:
- 2 * 2.0f * 2.0
- (long) -2 * 2.0
- (byte) 2 * (byte) 2
- 2 * 2L

What is the output from this program

[an error occurred while processing this directive]

What are the escape characters for:
- Tab
- Backspace
- Backslash
- Newline
How are characters represented in the memory of a computer?
What is "short circuit evaluation" of Boolean expressions?

What is the output from the following:

height = 10;
System.out.println("The cat " + (height > 10 ? "flew" : "sat") + " on the\b big\nmat");