java - Identifiers

 Identifiers are used for class names, method names, and variable names. An identifier may be any descriptive sequence of uppercase and lowercase letters, numbers, or the underscore and dollar-sign characters. They must not begin with a number, lest they be confused with a numeric literal. Again, Java is case-sensitive, so VALUE is a different identifier than Value.

Some examples of valid identifiers are: AvgTemp , count, a4, $test,  this_is_ok

Invalid variable names include: 2count , high-temp , Not/ok

In java there are several points to remember about identifiers. They are as follows:

1.  All identifiers should begin with a letter (A to Z or a to z ), currency character ($) or an underscore (_).

2.  After the first character identifiers can have any combination of characters.

3.  A key word cannot be used as an identifier.

4.  Most importantly identifiers are case sensitive.

5.  Examples of legal identifiers:age, $salary, _value, __1_value.

Variables:  variable represents a memory location which holds data.

In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:

type identifier [ = value][, identifier [= value] ...] ;

The type is one of Java's  datatypes. The identifier is the name of the variable. To declare more than one variable of the specified type, use a comma-separated list.

Here are several examples of variable declarations of various types.

 int a, b, c;         // declares three ints, a, b, and c.

int d = 3, e, f = 5; // declares three more ints, initializing d and f.

byte z = 22;         // initializes z.

double pi = 3.14159; // declares an approximation of pi.

char x = 'x';        // the variable x has the value 'x'.

There are three kinds of variables in Java:

 1.  Local variables     2. Instance variables  3. Class/static variables

Local variables :

1. These are declared in methods, constructors, or blocks.
2.  These are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
3.  Access modifiers cannot be used for local variables.
4.  Local variables are visible only within the declared method, constructor or block.
5.  Local variables are implemented at stack level internally.
6.  There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.

Example 1:  Here height is a local variable. This is defined inside pupHeight() method and its scope is limited to this method only.

public class Test{

   public void pupHeight(){

      int height = 0;

      height = height + 2;

      System.out.println("Puppy height is : " + height);

   }

 public static void main(String args[]){

      Test test = new Test();

      test.pupHeight();

   }

}

output:

Puppy height is: 2

Example 2: Following example uses height without initializing it, so it would give an error at the time of compilation.

public class Test{

   public void pupHeight (){

      int height;

      height = height + 2;

      System.out.println("Puppy height is : " + height);

   }

  

   public static void main(String args[]){

      Test test = new Test();

      test.pupHeight();

   }

}

output:

Test.java:4:variable number might not have been initialized

height = height + 2;

         ^

1 error

Instance variables :

1. These are declared in a class, but outside a method, constructor or any block.
2.  When a space is allocated for an object in the heap a slot for each instance variable value is created.
3.  Instance variables are created when an object is created with the use of the key word 'new' and destroyed when the object is destroyed.
4.  Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object.
5.  Instance variables can be declared in class level before or after use.
6.  Access modifiers can be given for instance variables.
7.  The instance variables are visible for all methods, constructors and block in the class. Normally it is recommended to make these variables private (access level).However visibility for subclasses can be given for these variables with the use of access modifiers.
8.  Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
9.  Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class (when instance variables are given accessibility) the should be called using the fully qualified name.ObjectReference.VariableName.

Example:

import java.io.*;

public class Employee{

 // this instance variable is visible for any child class.

   public String name;

  // salary  variable is visible in Employee class only.

   private double salary;

   // The name variable is assigned in the constructor.

   public Employee (String empName){

   name = empName;

   }

   // The salary variable is assigned a value.

   public void setSalary(double empSal){

      salary = empSal;

   }

  // This method prints the employee details.

   public void printEmp(){

      System.out.println("name  : " + name );

      System.out.println("salary :" + salary);

   }

   public static void main(String args[]){

      Employee empOne = new Employee("rajesh");

      empOne.setSalary(10000);

      empOne.printEmp();

   }

}

This would produce following result:

name  : Rajesh

salary :10000.0

Class/static variables :

1. Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
2.  There would only be one copy of each class variable per class, regardless of how many objects are created from it.
3.  Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
4.  Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
5.  Static variables are created when the program starts and destroyed when the program stops.  
6. Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
7.  Default values are same as instance variables. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.   
8. Static variables can be accessed by calling with the class name . ClassName.VariableName.
9.  When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.

Example:

import java.io.*;

public class Employee{

   // salary  variable is a private static variable

   private static double salary;

   // DEPARTMENT is a constant

   public static final String DEPARTMENT = "software development ";

   public static void main(String args[]){

      salary = 10000;

      System.out.println(DEPARTMENT+"average salary:"+salary);

   }

}

output:

software development average salary:10000

keywords:  There are 49 reserved keywords currently defined in the Java language.

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

Abstract       assert            boolean               break

byte              case                catch                   char

class              const              continue              default

do                  double           else                      enum

extends       final                finally                    float

for                if                       while                 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                                

Separators : In Java, there are a few characters that are used as separators. The most commonly used separator in Java is the semicolon. As you have seen, it is used to terminate statements. The separators are shown in the following table:

Symbol                  Name                                                                    Purpose

( )                     Parentheses                             Used to contain lists of parameters in method definition

and invocation. Also used for defining precedence in expressions, containing  expressions    in control statements, and surrounding cast types.

{ }                    Braces                                     Used to contain the values of automatically initialized

arrays. Also used to define a block of code, for classes, methods,   and local scopes.

[ ]                      Brackets                                Used to declare array types. Also used when dereferencing

 array values.

;                         Semicolon                                 Terminates statements.

,                         Comma                                     Separates consecutive identifiers in a variable declaration.

                                                                          Also used to chain statements together inside a for statement.

.                            Period                                 Used to separate package names from subpackages

and  classes. Also used to separate a variable or method from a     reference variable.