Learn Java

What are they

A variable refers to a place in memory for a value (data) used in an application.

  • By using the variable's name the value it refers to can be changed.

Imagine a variable named counter that keeps track of an event. Each time the event happens counter's value increments or decrements. As counter's value changes so does the value held in memory.

So, if you need somewhere to hold and remember a value, you hold it in a variable.

Where are they

Variables are found in two different places -

  • inside a class
  • inside a method

To understand the difference between these two, you should know the following:

Variable Scope

Scope (or visibility) depends on where you've declared the variable -

  • If declared inside a method, they're only visible to that method
  • If declared as private at the top of a Class (outside any method) they can be seen by any method in that Class
  • If declared as public at the top of a Class (outside any method) they can be seen by all class's methods within that package

Variable Lifespan

Life Span (or Extent) depends on where you've declared the variable -

  • If declared inside a method, the variable lives in memory as long as the method is running
  • If declared at the top of the Class private or public, it lives as long as an object of the class exists.
  • If declared as static at the top of the Class, it lives as long as the program is running.

What to call each

Depending on where they are declared variables are referred to as -

  • Local - if declared inside a method
  • Instance - if declared inside a class (outside any methods) using the keyword private or public
  • Static - if declared inside a class (outside any methods) using the keyword static
  • Variables declared at class level are often called fields

Naming Variables

You choose the names of your variables. There are some naming conventions that you should ALWAYS follow.

  • Use a name that someone besides you will readily understand
  • Use lower case letters
  • If you choose two or more words join them - no spaces allowed
  • When joining words the second and further words should start with a capital. This is called camelCase

Naming variables

  • A good name - dateOfBirth
  • A bad name - dob
  • A good name - uniqueID
  • A bad name - uid
  • Although the maximum recommended length is about 32 characters, be reasonable - you shouldn't have any names that long.

Data Types

  • In Java all variables must have a data type.
  • It's a classification for a value used in a program.
  • The data type assigned determines how the value can be used.
  • You decide what data type to assign.

Here are the primitive ones-

Examples...

Primitive data

Primitive data types:

  • short   16-bit whole number
  • int      32-bit whole number
  • long    64-bit whole number
  • float   32-bit floating point
  • double   64-bit floating point
  • byte    from -128 to 127
  • boolean   either true or false
  • char   holds a single character

Other data types...

More data types

  • boolean   either true or false
  • char   it holds a single character

Note 1 - These are all referred to as primitive data types as opposed to being objects. That basically means they are simple data.

Note 2 - String (which holds multiple characters together) is an object.

Object Types

An object is different from a variable. It's more complex and has properties and methods. But like a variable, to use an object you must first declare it.

The data type of an object is it's name. So a String object has a data type of String and the word 'String' is used in the declaration.

String FirstName = new String();

For more on object data types see the Objects section.

Declaration

Declaration is setting the characteristics of a variable. A declaration includes -

  • It's scope (if declared at class level)
  • Data Type - this gives the data it's meaning
  • Name - a word that describes the value's use
  • An initial value - this is generally optional but sometimes you'll need to provide one. It's called initialisation.

See the examples page

Variable types

boolean data type

boolean choice = false;

int

int amount;

byte

byte b = 89;

char

char c = 'f';

Notice something?

Each statement ends with a semi-colon ;

More...

Variable types

Integer

int counter = 0;

A decimal - double

double interestRate;

Another decimal - short

short angleDegree;

Another decimal - float

float temperature = 32.5;

Boolean - true or false

boolean connected = false;

A byte

byte arrayCount = 4;

A long number

long bigNumber;

Question 2

  • An application is written to calculate someone's age by subtracting the current year from their birth year. What data type would you choose when declaring the variable that would hold the age value?
  • String
  • boolean
  • int
  • number

Question 4

  • An application uses a boolean variable to keep track of whether an item is signed out or not. All items start as not being signed out and the variable is named signedOut.
    Which of these statements is the correct way to initialise the variable?
  • boolean signedOut = false;
  • boolean signedOut == false;

End of Quiz

Well done.