Variables and Subroutines

• A way to refer to an unknown value
• Variable is a name for storage space of a value
• Variables are typed → only accept values of certain type
• Variables must be declared before being used
• Variables can change their value through an assignment
• At runtime, variables are stored in memory

int count; // declaration
count = 10; // assignment

Values

Primitive types

name	bytes	bits	range
byte	1	8	-127 to +127
short	2	16	-32 768 to +32 767.
int	4	32	-2 147 483 648 to +2 147 483 647
long	8	64	-9 223 372 036 854 775 808 to +9 223 372 036 854 775 807
float	4	32	up to ${10}^{38}$
double	8	64	up to ${10}^{308}$
char	2	16	a single character
boolean	0	1	true or false

Expressions

int duration = 3215; // declaration + assignment
int sec = duration % 60; // modulo
int min = duration / 60; // integer division
min = min - 60 * hr; // re-assignment
boolean isOK = (duration == sec + 60 * (min + 60*hr)); // boolean

String done = "done!"; // String assignment

Booleans

true and false are values of type boolean

• Operators
  • v1 && v2 (and)
  • v1 || v2 (or)
  • !v (not)
• Conditional evaluation of && and || (lazy evaluation)
• With & and | both sides are evaluated

Subroutines

Used to reduce repetition.

• Subroutines are created to improve structure and to allow for reusability of code.
• Execution flow moves to subroutine and may return a result.

Example

• The Method or subroutine Math.sqrt(x) can be used to calculate the square root of x.
• You don't need to know how Math.sqrt(x) is implemented. (black box)

Strings

String is a pre-defined Java class to represent pieces of text.

• Example: String advice = "Seize the day!";

String functions:

• int s1.length(): returns length of s1
• boolean s1.equals(s2): returns true if s1 and s2 are the same String
• char s1.charAt(n): returns char value in position n (first position 0!)
• Concatenate Strings: "Hello " + "World" same as "Hello World".

Print guide

System.out.print(text) // prints text to standard output
System.out.println(text) // prints text followed by line feed
System.out.printf(format, args) // prints args according to format

System.out.printf("Formatted string: %8d %.3f %c %s", x , y, c, s)
/* prints an int left padded with 8 spaces, a floating point number with 3 
digits after the comma, a character and a string

Ex: Formatted string:       10 3.210 T test
*/

Constants

• Declared with final
• Purpose: Understandability and maintainability
• use caps by convention.

public static final int FAVORITE_NUMBER = 3.14;

TIP

Use enum to define a list of constants.

enum ChessPiece { ROOK, KNIGHT, BISHOP }

Summary

• Java is a strongly typed language.

  • a variable can only hold a particular type of data.
  • the compiler will give a syntax error if this rule is violated.

• Variables refer to positions in memory.

• Strings are special java pre-defined objects (String class)

• Subroutines or methods, provide reusability

Sources:

• Eck Chapter1