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 |