Head First Java Chapter 03 summary

Declaring a variable

• Java cares about type., So for this type safety to work, you must declare the type of your variable.
  Variables are of two type:
  1 Primitive
  2 Object Reference

There is only these two declaration rule.
1 variables must have a type.
2 variables must have a name.

Primitive Type

There are eight primitive types:
Boolean char byte short int long float double
you can remember it like,
be care full bears shouldn’t ingest large furry dog

• You can put a very large value say(long) into a small value(int), without losing information
• But you can put small value in a large variable.

Primitive Naming Convention.

Here is the rules for naming a variables.
1. It must start with a letter, underscore(_), or dollar sign ($). You can’t start a name with a number.
2. After the first character, you can use numbers as well, just don’t start it with a number.
3. It cannot be from the Java reserved words.

Object Reference

• There is actually no such thing as an object variable.
• There’s only an object reference variable.
• An Object reference variable holds bits that represent a way to access an object.
• It doesn’t hold the object itself.
• Object reference are like Remote Control for actual object.
• You cannot do arithmetic on a reference variable.

Array

• 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 3.

nums = new int[3]

• Give each element in the array an int value.

nums[0] = 6;

nums[1] = 5;

nums[2] = 4;

• Arrays are always objects, whether they’re declared to hold primitives or object reference.
• Once you’ve declared an array, you can’t put anything in it except things that are of the declared type.

Bullet Point

• Variables come in two flavors: primitive and reference
• Variable must always be declared with a name and a type.
• A primitive variable value is the bits representing the value (5, ‘a’, true, etc)
• A reference variable value is the bits representing a way to get to an object on the heap.
• 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 a access a method or instance variable.
• A reference variable has a value of null when it is not referencing any object.
• An array is always an object, even if the array is declared to hold primitives.