Java Variable Types

Java Variable Types

  • A Variable assigned with a data type.
  • Variable is a name of memory location.
  • Three types variables in Java : - local, instance and static.

Variable

Variable is name of reserved area allocated in memory. In other words, it is a name of memory location. It is a combination of "vary + able" that means its value can be changed.

Declaring (Creating) Variables

To create a variable, you must specify the type and assign it a value:

Syntax

Data_type variable_name = value;
String name = "hpkingdom.com";

Declare Many Variables

To declare more than one variable of the same type, use a comma-separated list:

Example

int a = 5, b = 6, c = 50; System.out.println(a + b + c);

Declare Some Example Variables

int a, b, c;   // Declares three ints, a, b, and c. int a = 110, b = 120;   // Example of initialization byte B =12;   // initializes a byte type variable B. double pi = 3.14159;   // declares and assigns a value of PI. char a = 'h';   // the char variable a iis initialized with value 'h'.

Types of Variables

There are three types of variables in Java:
  • local variable
  • instance variable
  • static variable
Java Variable Types  
class Hp { int i=55;  //instance variable static int s=150;  //static variable void method() { int l=50;  //local variable } }  //end of class

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.
 

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.
 

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 name ClassName.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.
 
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