Head First Java Chapter 03 Notes

Head First Java Chapter 03

This is the third Chapter learning of Head First Java by Kathy Sierra and Bert Bates,
This markdown file is my notes for the third chapter, not sure if any copyright issues are there.

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

Type          Bit Depth     Value Range 
boolean JVM Specific true/false
char 16 bits 0 to 65535
byte 8 bits -128 to 127
short 16 bits -32768 to 32767
int 32 bits -2147483648 to 2147483647
long 64 bits huge
float 32 bits varies
double 64 bits varies

There are eight primitive types:
boolean char byte short int long float double
you can remember it like,
be carefull 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.

GitHub Code :>

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