★ syntax

below is the basic setup of a java class (the main method/code).

https://lh7-rt.googleusercontent.com/docsz/AD_4nXeB-_TVoXISapGOR5ZFyjJAOcmT1t8x_5UlZkphSwn_iJBsCNIT5Tj0BLrlZZQNtd9GzCzgoYpTstwy9a2gCDG6etABJ5Bp526m809z07Atmm1bvY8bimeBDnWhI0Jgtr29SWaHHcLG8_iaUX2ssd733uI?key=HwOS_EcXozl-R-_TXHh4Hg

• a usual program starts with “public class” followed by the name of the class.
• note the curly brackets { } — they come in pairs (body of the class in this case)
• also note the capitals! (CASE MATTERS)
• semicolons matter too!!!
• spacing doesn’t matter, done for readability and organization

printing

System.out.print(" "); //no line break
System.out.println(" "); // with a line break

commenting

• // this is a single line comment

• /* this can comment

  out several lines */

AP Exam Tip: Use comments in your FRQ to make it clear to the grader what you’re doing!

types of errors

• Syntax errors are typos, missing { }, missing ;
• Run time errors are code that crashes. Think num/0
• Logic errors are code that works but doesn’t produce the desired result

helpful commands

• ctrl / — highlighted code gets commented out
• ctrl [ — highlighted code tabs
  • ctrl ] — tabs back

★ variables & data types

variables

Primitive Types: int, double (decimals), char (’a’), boolean (true/false)

Object (aka Reference) Types: String (”hello”)

String concatenation operator ( + )

//Example of using it: 
System.out.println(”This is my name: “ + name);

Example of how to declare and initialize a String variable:

String name = "Josie";

Example of how to declare and initialize an int variable:

int shoeSize = 9;

Declaring a variable: int shoeSize; | String name;

Initializing a variable: shoeSize = 9; | name = “Josie”;

★ expressions & assignment statements

Assignment statements initialize or change the value stored in a variable using the assignment operator (=)

int score = 0;
int points = 5;
score = points*10;

• Arithmetic expressions can be of type int or double

  • An arithmetic expression that uses at least one double value will evaluate to a double value

• Math Operators: (done in PEMDAS order)

  • addition (+), subtraction (-)
  • multiplication (*)
  • division (/) — dividing by 0 will result in an ArithmeticException error
  • mod (%) — remainder from division

• Equality Operators:

  • (==) checks if the value on the right is equal to the value on the left (DOESN’T WORK FOR STRINGS! you need to use .equals() )
  • (!=) checks if the values are not equal

Increment operator

x++;
/*This is equal to:
x = x + 1
x += 1
*/

Decrement Operator

x--;
/*This is equal to:
x = x - 1
x -= 1
*/

★ casting & ranges of values

Type casting is used to convert value from one type to another

• Casting operators include (int) and (double) and can be used to convert a data type into another
• Casting a double value to an int causes the digits to the right of the decimal point to be truncated (cut off and thrown away)
• In expressions involving doubles, the doubles will cause the ints to be automatically converted into doubles

double d = (double) 5/2; //2.5

★ compound assignment operators

Compound assignment operators are shortcuts that combine an operation with assignment. So, instead of writing x = x + 5, it is possible to write x +=5, which achieves the same thing.

Common Operators

| --- | --- | --- |

★ application program interfaces (API) and libraries

An API is a collection of prewritten classes and methods you can use instead of writing everything from scratch.

//Examples
Math.sqrt(25);
Math.random