Data and Variables

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Chapter 9
Data and Variables

   9.1 Background
   9.2 Literals
   9.3 Variables
   9.4 Assignment Instructions
   9.5 Initialization of Variables
   9.6 Assignment #10: Primitive Types and Wrap Classes

9.1 Background

The task of programs is to process information
Information is presented to programs in raw facts, as data.
To become information and knowledge, the data need to be organized and evaluated.
We are interested here in presentational aspects of data.
Processing aspects of data will be considered later

9.2 Literals

Character-based representations for data values are said to be literals.
The Java language offers literals for 6 primitive data types (int, long, double, float, boolean, char), and for the String class type.

Name	Constants (Literals)
integer numbers
int	0, 10, -12
long	0L, 10L, -12L
short
byte
real numbers
double	0.0, -5.1, 3e-2
float	0.0F, -5.1F, 3e-2F
other types
boolean	false, true
char	'a', '?', '\'' (apostrophe), '\"' (quotation mark), '\n' (newline), '\\' (backslash)
String	"abc"

class literals {
   public static void main (String[] args){
    System.out.println( 123 );
    System.out.println( 12.3e-5 );
    System.out.println( '\'' );
    System.out.println( true );
    System.out.println( "Hello \"World\"!" );
} }

9.3 Variables

Programs use variables for carrying values through computations.

Variables are named memory cells.
Java variables are classified into: fields, parameters, and local variables.
Local variables are defined within bodies of methods and constructors.
When no confusion may arise, the different kinds of variables are referred to just as variables.
Fields, parameters, and local variables are defined with instructions of the form
<optional modifiers> <type> <name>

class Variables{
   static int field;
   void method(int parameter) {
      int variable;
   }
}
In the case of fields and local variables, a single instruction may define more than one variable.
<optional modifiers> <type> <comma-separated list of names>

class example{
int first, second;
}

9.4 Assignment Instructions

Values are assigned to fields, parameters, and local variables through commands of the form
<name> = <value>

class example{
  public static void main(String[] args){
      int a, b;
      a = 10;
      b = a;
      System.out.println( b );
} }
The variables and the values assigned to them should be of compatible types.

9.5 Initialization of Variables

Values may be assigned to fields and local variables in their definition statements.
class example{
  public static void main(String[] args){
      int a = 10, b;
      b = a;
      System.out.println( b );
} }

Exercises

9.6 Assignment #10: Primitive Types and Wrap Classes

Due: Mo, Apr 28, midnight

Write a program that satisfies the following specifications.

The program should define exactly one local variable for each primitive data type, and one local variable of type String.
For each available type of literals: choose a literal, assign the literal to a local variable of the same type, and print out the value of the variable.
For each variable in the program: find a static field of the same type in the Java 2 platform packages, assign the field to the variable, and print out the local variable value.

class example {
  public static void main (String[] args){
    // code fragment for the primitive data type int
    int intVar;
    intVar = 12;
    System.out.print( "integer: " );
    System.out.print( intVar );
    System.out.print( " (literal), " );
    intVar = java.lang.Integer.MAX_VALUE;
    System.out.print( intVar );
    System.out.println( " ( java.lang.Integer.MAX_VALUE ) " );
    // the String and the other primitive data types  were
    // not considered.
} }

Assume ‘lab10’ for the submit program. The program should be stored in a file named ‘lab10.java’, and assume the name ‘lab10’ for the class.

Note: Files that fail to compile, and execute when applicable, will not be examined. They will be awarded a grade of 0 points.

Q&A

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Chapter 9Data and Variables