Java Basics

Key Topics

• Comments

• Variables

• Data types

• Operators

• Order of Operations

• Incrementors and Decrementors

• Escape Characters

• Comparative Operators

• Concatenation

Readings

https://books.trinket.io/thinkjava/chapter1.html

Comments

Definition

Comment allows the programmer to document their code. It is not compiled.

Examples

// This is a single line comment

/*
This is 
a multiline
comment
*/

Variables

Definition

Variable stores one item of information.

Naming Variables

• Start with a letter, underscore or a dollar sign

• Can not start with an number

• NO SPACES

• CASE SENSITIVE

  • All different are different variable names: fname, Fname, fName, FNAME, fnamE

• Java as a whole is case sensitive

Declaring A Variable

datatype variableName;
datatype variableName = value;

Examples

// declaring variables
int Age;
String First_Name;
boolean Breathing;

//declaring variables and intializing them with a value
int numberOfStates = 50;
double salary = 50000;

// note that strings get double quotes
// chars get single quotes
String dogName = "Spot";
char middleInital = 'A';

boolean isBreathing = true;

Warning

A variable is usually declared once within a set of braces{}.

Data Types

Data Type	Memory Space	Max Values
Integer Types - whole numbers
byte8	bits - 1 byte	-128 to 128
short	16 bits – 2 bytes	-32,768 to 32,767
int	32 bits – 4 bytes	-2,147,483,648 to 2,147,483,647
long	64 bits – 8 bytes	(+-)9,223,372,036,854,775,808)
Real / Floating Point
float	32 bits - 4 bytes	1.4 E-45 to 3.4 E+38
double	64 bits - 8 bytes	4.9 E -324 to 1.7 E 308
Other
boolean		true / false - Use as flags
char		Individual letters or characters
Acts Like A Variable
String		~ 2 billion characters
Object		Store binaries - “anything”
var		limited use within functions when datatype is not known

Note: for precise decimals i.e. currency – use java.math.BigDecimal – reason – rounding accuracy

Operators

Sign	Operations
+	Addition
-	Subtraction
*	Multiplication
/	Divide
%	Modulus (return the remainder of an integer)

Order Of Operations

• Increment and decrement

• Parentheses ()

• Exponents

• * /

• + -

• Comparisons > < >= <= == !=

• Logical Operations && ||

• Assignment Expressions

Incrementors and Decrementors

• use ++ to add the number 1 to a variable.

• use – to subtract the number one from a variable.

Examples

x++;
age++;

y--;
salary--;

Escape Characters

Definition

Escape Character is a backslash \ followed by the character you want to insert into your for printing or inserting into a string.

Escape Character	Definition
\n	new line
\t	tab
\b	backspace
\r	carriage return
\f	form feed
\	backslash
‘	single quotation mark
\”	double quotation mark
\d	octal
\xd	hexadecimal
\ud	Unicode character

Comparative Operators

Definition

Comparative Operators evaluate is something is true or false.

Operator	Definition
==	Equal to x==y; (NOT x=y)
!=	Not Equal to x != y;
<	Less Than x < y;
>	Greater Than x>y.
<=	Less Than or Equal to x <= y ; (no space between the signs)
>=	Greater Than or Equal to x >=y;

Logical Operators

Definition

Logica Operators connect two or more expression together and evalute to true or false.

Operator	Definition
&&	AND (both sides must be true to evaluate to true)
||	OR (only one side must be true to evaluate to true)

Truth Table

(x < y && z > 10)

expression 1 x<y	operator	expression 2 z >10	outcome
AND
true	&&	true	true
true	&&	false	false
false	&&	true	false
false	&&	false	false
OR
true	||	true	true
true	||	false	true
false	||	true	true
false	||	false	false

Concatenation

Definition

Concatenation is the action of putting strings togeher to form a new string.

Example

String fName = "Bubba";
String lName =  "Smith";
String fullName = fName  + " " + lname;
// Note we concantenate a space " " between the first and last name

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End Of Topic