Java Beginner's Guide[4] - Data Types, Variables and Modifiers

Hi Everyone! In this tutorial, we will learn Data types, Variables and Modifiers in Java.

Variables are nothing but reserved memory locations to store values/data. This means that when you create a variable you reserve some space in the memory. So let's learn about data types first.

Data Types

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.

There are two data types available in Java. They are,

Primitive Data Types
Reference/Object Data Types

Primitive Data Types

There are eight primitive datatypes supported by Java. Primitive datatypes are predefined by the language and named by a keyword. There are eight primitive data types. They are,

byte
short
int
long
float
double
boolean
char

You can find more about the primitive data types from here.

Reference Datatypes

Reference variables are created using defined constructors of the classes. They are used to access objects. These variables are declared to be of a specific type that cannot be changed.

Class objects and various type of array variables come under reference datatype.

Example:

Person person = new Person("teacher");

Variables

Variable declaration and initialization in Java

Examples:

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'; // the char variable a iis initialized with value 'a'

Types of the variables

Now we're going to learn about the various variable types available in Java Language. There are three kinds of variables in Java. They are,

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:

public void add(){

int a= 1, b=4, c=0;

c = a + b;

System.out.println("Answer is : " + c);

}

Instance variables

Instance variables are declared in a class, but outside a method, constructor or any block.
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 sub-classes 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. Example "John.age"

Example for using instance variables.

Run & See the Output of this program.

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 name.
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.

Ok! We learnt about variables, So let's move on to modifiers.

Modifiers

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

Java Access Modifiers
Non Access Modifiers

To use a modifier, you include its keyword in the definition of a class, method, or variable.

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 keyword is needed.

A variable or method declared without any access control modifier is available to any other class in the same package. The fields in an interface are implicitly public static final and the methods in an interface are by default public.

Examples:

String age= "11"; // default variable

boolean passExam() { //default method
return true;
}

Visible to the class only (private).

Methods, variables, and constructors that are declared private can only be accessed within the declared class itself.

Private access modifier is the most restrictive access level. Class and interfaces cannot be private.

Variables that are declared private can be accessed outside the class, if public getter methods are present in the class.

Visible to the world (public).

A class, method, constructor, interface, etc. declared public can be accessed from any other class. Therefore, fields, methods, blocks declared inside a public class can be accessed from any class belonging to the Java Universe.

However, if the public class we are trying to access is in a different package, then the public class still needs to be imported. Because of class inheritance, all public methods and variables of a class are inherited by its sub-classes.

Visible to the package and all subclasses (protected).

Variables, methods, and constructors, which are declared protected in a super-class can be accessed only by the sub-classes in other package or any class within the package of the protected members' class.

The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.

Protected access gives the subclass a chance to use the helper method or variable, while preventing a non-related class from trying to use it. Non-Access Modifiers

Non - Access Modifiers

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

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.

For further knowledge check this tutorial.

So, Let's meet again with the next tutorial. Till then Good Luck!.

References:
https://www.tutorialspoint.com/java

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Thank you!

Kalpani Ranasinghe

email: kalpanibhagya.kb@gmail.com