Java – Variable Types

On By Mahesh KariyaIn Uncategorized

​A variable provides us with named storage that our programs can manipulate. Each variable in Java has a specific type, which determines the size and layout of the variable’s memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

You must declare all variables before they can be used. Following is the basic form of a variable declaration −

data type variable [ = value][, variable [ = value] ...] ;

Here data type is one of Java’s datatypes andvariable is the name of the variable. To declare more than one variable of the specified type, you can use a comma-separated list.

Following are valid examples of variable declaration and initialization in Java −

Example

inta,b,c;// Declares three ints, a, b, and c.inta=10,b=10;// Example of initializationbyteB=22;// initializes a byte type variable B.doublepi=3.14159;// declares and assigns a value of PI.chara='a';// the char variable a iis initialized with value 'a'

This chapter will explain various variable types available in Java Language. There are three kinds of variables in Java −

  • Local variables
  • Instance variables
  • Class/Static variables

Local Variables

  • Local variables are declared in methods, constructors, or blocks.

  • 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

Here, age is a local variable. This is defined insidepupAge() method and its scope is limited to only this method.

publicclassTest{publicvoid pupAge(){int age =0;
      age = age +7;System.out.println("Puppy age is : "+ age);}publicstaticvoid main(String args[]){Test test =newTest();
      test.pupAge();}}

This will produce the following result −

Output

Puppy age is: 7

Example

Following example uses age without initializing it, so it would give an error at the time of compilation.

publicclassTest{publicvoid pupAge(){int age;
      age = age +7;System.out.println("Puppy age is : "+ age);}publicstaticvoid main(String args[]){Test test =newTest();
      test.pupAge();}}

This will produce the following error while compiling it −

Output

Test.java:4:variable number might not have been initialized
age = age + 7;
         ^
1 error

Instance Variables

  • 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 value 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 variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object’s 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 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.

  • 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.

  • Instance variables can be accessed directly by calling the variable name inside the class. However, within static methods (when instance variables are given accessibility), they should be called using the fully qualified name.ObjectReference.VariableName.

Example

import java.io.*;publicclassEmployee{// this instance variable is visible for any child class.publicString name;// salary  variable is visible in Employee class only.privatedouble salary;// The name variable is assigned in the constructor.publicEmployee(String empName){
      name = empName;}// The salary variable is assigned a value.publicvoid setSalary(double empSal){
      salary = empSal;}// This method prints the employee details.publicvoid printEmp(){System.out.println("name  : "+ name );System.out.println("salary :"+ salary);}publicstaticvoid main(String args[]){Employee empOne =newEmployee("Ransika");
      empOne.setSalary(1000);
      empOne.printEmp();}}

This will produce the following result −

Output

name  : Ransika
salary :1000.0

Class/Static Variables

  • Class variables also known as static variables 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, regardless 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 the 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 variables can be accessed by calling with the class nameClassName.VariableName.

  • When declaring class variables as public static final, then variable 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.*;publicclassEmployee{// salary  variable is a private static variableprivatestaticdouble salary;// DEPARTMENT is a constantpublicstaticfinalString DEPARTMENT ="Development ";publicstaticvoid main(String args[]){
      salary =1000;System.out.println(DEPARTMENT +"average salary:"+ salary);}}

This will produce the following result −

Output

Development average salary:1000

Note − If the variables are accessed from an outside class, the constant should be accessed as Employee.DEPARTMENT

What is Next?

You already have used access modifiers (public & private) in this chapter. The next chapter will explain Access Modifiers and Non-Access Modifiers in detail.

Java – Modifier Types

Modifiers are keywords that you add to those definitions to change their meanings. Java language has a wide variety of modifiers, including the following −

To use a modifier, you include its keyword in the definition of a class, method, or variable. The modifier precedes the rest of the statement, as in the following example.

Example

publicclass className {// ...}privateboolean myFlag;staticfinaldouble weeks =9.5;protectedstaticfinalint BOXWIDTH =42;publicstaticvoid main(String[] arguments){// body of method}

Access Control Modifiers

Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are −

  • Visible to the package, the default. No modifiers are needed.
  • Visible to the class only (private).
  • Visible to the world (public).
  • Visible to the package and all subclasses (protected).

Non-Access Modifiers

Java provides a number of non-access modifiers to achieve many other functionality.

  • The static modifier for creating class methods and variables.

  • The final modifier for finalizing the implementations of classes, methods, and variables.

  • The abstract modifier for creating abstract classes and methods.

  • The synchronized and volatile modifiers, which are used for threads.

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