3 Primitives and references

Variables must always be declared with a name and a type. Variables come in two flavors: primitive and reference.

Variables can be used in many places:

as object state: instance variables
variables declared within a method: local variables
values sent to a method by the calling code: arguments
values sent back to the caller of the method: return types

Naming¶

You can name a class, method, or variable according the following rules (the real rules are sightly more flexible, but these will keep you safe:

It must start with a letter, underscore (_), or dollar sign ($). You can’t start a name with a number.
After the first character, you can use numbers as well. Just don’t start it with a number.
It can be anything you like, subject to those two rules, just so long as it isn’t one of Java’s reserved words.

Primitive Variable¶

A primitive variable value is the bits representing the value.

Primitives hold fundamental values.
Each primitive variable has a fixed number of bits.

Reference variable¶

A reference variable value is the bits representing a way to access an object on the heap.
The only way you can access an object is through a reference variable.
It doesn’t hold the object itself, but it holds something like a pointer. Or an address.
A reference variable is like a remote control. Using the dot operator(.) on a reference variable is like pressing a button on the remote control to access a method or instance variables.
A reference variable has a value of null when it is not referencing any object.

The three steps of object declaration, creation and assignment.

(e.g. Dog myDog = new Dog());

declare a reference variable: tell the JVM to allocate space for a reference variable, and names that variable myDog
create an object: tells the JCM to allocate space for a new Dog object on the heap.
Assigns the object to the reference variable.

three steps of object declaration, creation, and assignment

The size of reference variables: All references for a given JVM will be the same size regardless of the objects they reference, but each JVM might have a different way of representing reference, so references on one JVM may be smaller or larger than references on another JVM.

Array¶

An array is like a tray of cups:

Declare an int array variable. An array variable is a remote control to an array object. int[] nums;
Create a new int array with a length of 7, and assign it to the previously-declared int[] variable nums. nums = new int[7];
Give each element in the array an int value. nums[0] = 6;

an_array_is_like_a_tray_of_cups

An array is always an object, even if the array is declared to hold primitives.
Every element in an array is just a variable.

If you want to initialize an array, try using Array Initializer:

int[] data = {10,20,30,40,50,60,71,80,90,91};

// or

int[] data;
data = new int[] {10,20,30,40,50,60,71,80,90,91};