Notes

Primitive data types
holds only one piece of data at a time
the variable holds the actual value
int num1 = 17;
example: int and double

class
holds more than one piece of data at a time
can hold data of different types
has built in methods (tools)
users can create their own classes
the variable holds the memory location of the actual data -refers to it
string word1 = "hello";
example strings

quick example
string a = "what";
string b = a;

string a holds the what in a location
b refers to the variable address.
Both b and a point to the same location (the string what)

what if there is no reference?

for example
string word;
null- special value that means "no reference"
string alpha = "the final frontier";
alpha = null; <---- No place to hold the variable "the final frontier"
It is garbage collected. Java goes through and cleans up any unattached values in memory.

Example:
String alpha = "the final frontier";
String beta = alpha;
alpha = null; <---- the reference for the final frontier is erased for string alpha but not for beta

We don't use == because it tests identity. THE == tests the value stored directly in a and b.

String is an example of a class data type because it has methods and tools to work on

how strings work

the letters in strings are stored as individual characters. Each is given an address.
the addresses start at zero. You can find the length by word.length()

concatenation: add strings
word3 = word1 + word2
the + symbol is overloaded
this means it has multiple uses depending on the situation

tools for working on strings
.equals() tests for equality
this method is built into string
it tests if the contents are equal

.charAt()
tells us hwat letter is at a spot in the String
the fist spot is 0
string w = "variable"
system.out.println(w.charAt(2));
prints r

.indexOf(c)
put in a character, tells were it is

string w = "variable";
system.out.println(w.indexOf(('i'));
prints 3

.subString(start, stop)
pulls of a section of a string from start to stop -1)
string w = "variable";
System.out.println(w.substring(2,5));
prints ria

.subString(start)
pulls of a section of string from start to the end of the array
string w = "variable";
system.out.println(w.subString(4));
prints able

.toUpperCase()
.toLowerCase()

.CompareTo(other)
compares two strings alphabetically

String s = "ardvark";
System.out.print(s.compareTo("zebra");
prints -25

object
in java, an object is a variable of a class type
it can hold lots of data and has methods
it is different



arrays are objects
an array is an object that holds more than one thing in memory
all items in the array are the same data


to create:

int[] list = new int[7];
this creates an array called list of 7 integers
use the new command - this is what stores the reference to where the data is in memory

new
this is a java command
it creates a new reference: this is when java stores the memory address in the variable
we used this with Scanner

element: the box of array. each box holds a value. all values are the same data type

index: the address of the elements. Starts at 0 and ends at (length -1)
initialize: stores known values in an array

The end is always length -1

The number inside the square brackets tell us the address.

initializer lists
automatically sets the values in an array.

int[] list = {14, -10, 1, 0, 9,}