Two types of data types available in java:
Primitive Data Types
Reference/Object Data Types
Primitive Data Type:
byte,short,int,long,float,double,boolean,char
Reference data types:
Default value of any reference variable is null.
A reference variable can be used to refer to any object of the declared type or any compatible type.
Example:Animal animal = new Animal(“giraffe”);
Java Literals:
\n newline
\r carriage return
\f formfeed
\b backspace
\s space
\t tab
\” double quote
\’ single quote
\\ backslash
\ddd octal character
\uxxxx hexadecimal UNICODE character
Variable Types:
following are the valid examples of variable declaration and initialization in Java:
int a,b,c; //declares three ints,a,b, and c int a=10,b=10; //example of initialization byte B=22; //initializes a byte type variable B double pi=3.14159; //declares and assigns a value of PI char a = 'a'; a char variable a is initialized with value 'a'
Local Variable:
some rules for local variable:
– Local variables are created when the method,costructor or block
– Local variables are created when the method,constructor or block is entered and the variable will be destroyed once it exits the method,constructor or block
– Access modifiers cannot be used for local variables.
– Local variables are visible only within the declared method, constructor or block.
– Local variables are implemented at stack level internally
– 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:
public class Test {
public void pupAge()
{
int age=0;
age=age+7;
System.out.println("Puppy age is: "+age);
}
public static void main(String []args)
{
Test testu=new Test();
testu.pupAge();
}
}
output:
Puppy age is: 7
and this example without initializing will give error:
public class Test {
public void pupAge()
{
int age;
age=age+7;
System.out.println("Puppy age is: "+age);
}
public static void main(String []args)
{
Test testu=new Test();
testu.pupAge();
}
}
Output:
Exception in thread "main" java.lang.Error: Unresolved compilation problem: The local variable age may not have been initialized at Test.pupAge(Test.java:6) at Test.main(Test.java:14)
Instance Variables:
some rules for this:
-Instance variables are declared in a class,but outside a method,constructor or any block.
-When a space is allocated for an object in the heap, a slot for each instance variable values is created.
-Instance variables are created when an object is created with the use of the keyword ‘new’ and destroyed when the object is destroyed.
– Instance variable hold values that must be referenced by more than one method,constructor or block, or essential parts of an objects state that must be present throughout the class.
-Instance variables can be declared in class level before or after use.
– Access modifiers can be given for instance variables.
– The instance variables are visible for all method,constructors and blcok in the class.Normally it is recommended to make these variables private(access level). However visiblily for sbclasses can be given for these variables with the use of access modifiers.
– Instance variables have default values.For numbers the default value is 0, for booleans it is false and for object reference it is null.
-Values can be assigned during the declaration or within the constructor.
– 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 ) should be called using the fully qualified name. ObjectReference.VariableName.
import java.io.*;
public class EmployeeInstance {
// this instance variable is visible for any child class
public String name;
//salary variable is visible in EmployeeInstance class only
private double salary;
//The name variable is assigned in the constructor
public EmployeeInstance(String empName)
{
name = empName;
}
//The salary variable is assigned a value
public void setuSalary(double empSalary)
{
salary = empSalary;
}
//This method prints the employee details
public void printEmployee()
{
System.out.println("name: "+name);
System.out.println("salary: "+salary);
}
public static void main(String args[])
{
EmployeeInstance empuOne=new EmployeeInstance("Zaki");
empuOne.setuSalary(5000);
empuOne.printEmployee();
}
}
Output:
name: Zaki salary: 5000.0
Class/Static variables:
– a class variables also known as static variables and they are declared with the static keyword in a class, but outside a method, constructor or a block.
– There would only be one copy of each class variable per class,regarless of how many objects are created from it.
– 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.
– 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.
– Static variables are created when the program starts and destroyed when the program stops.
– Visibility is similar to instance variables.However, most static variables are declared public since they must be available for users of the class.
– 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.
– Static variable can be accessed by calling with the class name ClassName.VariableName.
– When declaring class variables as public static final, then variables names 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 EmployeeStatic {
//salary variable is a private static variable
private static double salary;
//DEPARTMENT is a constant
public static final String DEPARTMENT="Development";
public static void main(String args[])
{
salary=10000;
System.out.println(DEPARTMENT+" average salary: "+salary);
}
}
Output:
Development average salary: 10000.0
Note if the variables are access from an outside class the constant should be accessed as EmployeeStatic.DEPARTMENT
