Variables store values. They are the most basic entity used to store data such as text, numbers, etc. in a program.
These are case sensitive,i.e, small letter and capital letter must be the same as when declared.
In Java, variables are strongly typed, which means you have to define the type for each variable whenever you declare it. Otherwise, the compiler will throw an error at compile time. Therefore, each variable has an associated ‘data-type‘ of one of the following:
|Primitive Type||int, short, char, long, boolean, byte, float, double|
|Wrapper Type||Integer, Short, Char, Long, Boolean, Byte, Float, Double|
|Reference Type||String, StringBuilder, Calendar, ArrayList, etc.|
You may have noticed that the Wrapper Type consists of types spelled exactly like the Primitive Type, except for the capitalized alphabet in the beginning (like the Reference Type). This is because the Wrapper Types are actually a part of the more general Reference Types but closely linked with their primitive counterparts via autoboxing and unboxing. For now, you just need to know that such a ‘Wrapper Type’ exists.
Typically, you can declare (i.e., create) variables as per the following syntax:
// Primitive Data Type int i; // Reference Data Type Float myFloat;
You can assign a value to the variable either simultaneously when you are declaring it (which is called initialisation), or anywhere in the code after you have declared it. The symbol = is used for the same.
// Initialise the variable of Primitive Data Type 'int' to store the value 10 int i = 10; double amount = 10.0; boolean isOpen = false; char c = 'a'; // Note the single quotes String s = "Hello World"; // String is created with double quotes //Variables can also be declared in one statement, and assigned values later. int j; j = 10; // initiates an Float object with value 1.0 // variable myFloat now points to the object Float myFloat = new Float(1.0); //Bytes are one of the data types in Java and can be //represented with this code int byteValue = 0B101; byte anotherByte = (byte)0b00100001;
Variable Data Types
As evident from the above example, variables of Primitive type behave slightly differently from variables of Reference (& Wrapper) type – while Primitive variables store the actual value, Reference variables refer to an ‘object’ containing the actual value.
Java Programming language defines mainly three kind of variables.
- Local Variable – It is declared inside the method and can only be used within the method itself.
- Static variable – Any variable which has the keyword static is called a static variable.
It cannot be created as a local variable. Once created, it can be used among all the instances of the class.
- Instance Variable – It is declared inside a class but not in a specific method body.
You can find out more in the sections linked below.
Rules for naming Variable
- All variable names must begin with a letter of the alphabet, an underscore, or ( _ ), or a dollar sign ($). The convention is to always use a letter of the alphabet. The dollar sign and the underscore are discouraged.
- After the first initial letter, variable names may also contain letters and the digits 0 to 9. No spaces or special characters are allowed.
- The name can be of any length, but don’t get carried away. Remember that you will have to type this name.
- Uppercase characters are distinct from lowercase characters. Using ALL uppercase letters are primarily used to identify constant variables. Remember that variable names are case-sensitive.
- You cannot use a Java keyword (reserved word) for a variable name.