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1.4 Variables & ScopeHomepage « Java6 Certification « 1.4 Variables & Scope
In this lesson we examine the declaration, initialization and usage of primitives, arrays, enums, and objects as static, instance and local variables and their respective scopes.
Lets take a look at the points outlined at the Oracle Website for this part of the certification.
·Section 1: Declarations, Initialization and Scoping
oDevelop code that declares, initializes, and uses primitives, arrays, enums, and objects as static, instance, and local variables. Also, use legal identifiers for variable names.
Variables in Java Top
Java comes with two types of variables to choose from, primitives and reference, both of which can be declared as static variables, instance variables, method parameters and local variables. Java is a strongly typed language and as such cares about the type of the variables we declare. So what does this statement actually mean? Well whenever you declare a variable in Java, you have to give it a type, or the compiler complains when you come to compile your code. You also have to name the variable and there are rules on naming.
Naming Rules and Keywords
These rules apply to all variables, as well as classes and methods (collectively known as identifiers)
·Must start with a letter, the underscore symbol (_) or the dollar sign ($). A name can’t start with a number or any other symbol.
·You can use numbers after the first letter, you just can’t start with a number.
·You cannot use keywords or names recognized by the compiler. See the table below for the words you can’t use.
Click a link in the table below to show lesson usage for any keyword you’re interested in:
Java6 Keywords
abstract
assert
boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
enum
extends
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
Although not a rule as such it’s also important to know that identifiers are case-sensitive.
Variable Scope
In Java we can declare variables anywhere within a block of code. A block starts with its opening curly brace and ends with its closing curly brace. A block and where it resides defines the type of the variable and the scope of the enclosed data. Therefore each time you start a new block of code you start a new scope. Scope determines the visibility and lifetime of the variables within it. Java comes with three kinds of scope and we name variables according to the scope they reside in as detailed in the table below.
Variable
Scope
Lifetime
static
Static variables apply to the class as a whole and are declared within the class but outside a method.
Exists for as long as the class it belongs to is loaded in the JVM.
See the Static Members lesson for more information.
instance
Instance variables apply to an instance of the class and are declared within the class but outside a method.
Exists for as long as the instance of the class it belongs to.
See the Instance Variables & Scope lesson for more information.
local
Local variables apply to the method they appear in.
Exists until the method it is declared in finishes executing.
See the Method Scope lesson for more information.
Access Modifiers
The table below shows the four types of access available in Java for members (variables, inner classes and methods).
Access modifier
Description
public
A member may be declared with the public access modifier, and if it is the member is accessible to all other classes everywhere, assuming the class it resides in is accessible.
protected
A member may be declared with the protected access modifier, and if so, is only accessible within its own package and also by a subclass of its class in other packages..
See the Packages lesson for more information on packaging.
See the Inheritance Basics lesson for more information on subclassing.
no modifier
package-private / (the default)
If a member has no explicit access modifier it is only accessible within its own package.
private
A member may be declared with the private access modifier, and if it is the member is only accessible within its own class.
Non-access Modifiers
The table below shows the types of non-access modifiers available for use with variables.
static variable and instance variable modifiers
local variable modifiers
final
transient
volatile
final
Primitive Variables Top
The table below lists the eight primitive types available in Java6:
Primitive Types
Type
Description
Bit Width
Range
boolean and char
boolean
true/false values
JVM specific
true or false
char
Character
16
0 to 65535 – (‘u0000’ to ‘uffff’ Unicode)
signed numeric integers
byte
8-bit integer
8
-128 to 127
short
Short integer
16
-32,768 to 32,767
int
Integer
32
-2,147,483,648 to 2,147,483,647
long
Long integer
64
-9,233,372,036,854,775,808 to 9,233,372,036,854,775,807
signed floating point
float
Single-precision float
32
varies
double
Double-precision float
64
varies
Reference Variables Top
Reference variables are concerned with objects and how we access them. The reference variable doesn’t hold the object itself but some bits that act as a pointer to an address in memory where the object exists. How the JVM holds this bits isn’t important for us as programmers, just the fact that this memory address pointer is used by the JVM to access a unique object. There are three steps involved in object creation:
So lets go through the three step process above:
1.Declaration – Here. we declare a reference variable named moggy of type Cat and the JVM allocates space for it.
2.Creation – Tells the JVM to allocate space on The Heap for a new Cat object.
3.Assignment – Assign the new Cat object to the reference variable moggy.
Arrays Top
An array in Java is an object that contains a collection of values which can be a collection of primitive data types or a collection of reference variable types.
·An array of primitives data types is a collection of values that constitute the primitive values themselves.
·An array of reference variable types is actually a collection of pointer values which point to the memory address where each object is stored on The Heap.
Whichever variable type the array holds, primitive or reference, the array itself is still an object. In Java you can define a one dimensional array which is an object that refers to a collection of values repeated one or more times, or multi dimensional arrays which are a collection of array objects (arrays of arrays). Multi dimensional arrays may contain the same number of elements in each row or column and are known as regular arrays or an uneven number of elements in each row or column which are known as irregular arrays.
Array Creation
Array creation is a three step process as outlined below and can be achieved using separate statements or combined into a single statement.
1.Array declaration
2.Array allocation
3.Array element initialization
Array Notes
As you can see from the table above there is a lot of ways to create arrays in Java. There are several points about arrays shown in the table above that we will highlight again here before we go into some code examples:
·Array indexes are zero-based.
·When declaring an array the square brackets can appear after the type, after the array name or in the case of multi dimensional arrays a combination of both.
·After allocation of an array, each element of the array is initialized with the default for the array type:
oobject – null
oboolean – false
ochar – /u0000
ointeger types (byte, short, int and long) – 0
ofloating-point types (float, double) – 0.0
·For multiple statement array creation the new keyword is mandatory.
·For single statement array creation the size of the array is calculated by the number of values that are assigned to the array and should not be specfied.
Enums Top
In their simplest form enumerations are just a list of constants that define a new data type. Before Java6 the only way to do this would have been using static final variable to define Java constants. The difference between using enumerations and static final variables to define Java constants, is that with static final variable we can’t guarantee that another piece of code won’t set an invalid value, instead of using one of our static final variables. With enumerations, objects of the enumerated type can only hold values defined in the list of constants such as months of the year and days of the week for example. Lets look at some code to see this in action:
/*
Enumeration of soups
*/
enum Soup {
TOMATO, CHICKEN, PRAWN
}
We create an enumeration using the enum keyword. The identifers TOMATO, CHICKEN and PRAWN are known as enumeration constants and are implicitly declared as public, static members of Soup. When we declare enumeration constants it’s not obligatory to use all uppercase letters, but it lets others see this is a constant and so we use this convention here.
Related Java6 Tutorials
Beginning Java6 – Primitive Variables
Beginning Java6 – Method Scope
Objects & Classes – Arrays
Objects & Classes – Reference Variables
Objects & Classes – Enumerations
Objects & Classes – Static Members
Objects & Classes – Instance Variables & Scope
OO Concepts – Interfaces
<< 1.3 Abstract Classes 1.5 Overriding & Overloading >>
Java6 Tutor Homepage Top
All the Java6 Certification Declarations, Initialization and Scoping lessons are listed below. Click a link to go to that lesson.
1: Declarations
1.1 Classes, Packages & Imports
1.2 Interfaces
1.3 Abstract Classes
1.4 Variables & Scope
Variables in Java
Primitive Variables
Reference Variables
Arrays
Enums
1.5 Overriding & Overloading
1.6 Constructors / Nested Classes
