Java Variables

Java Variable Types
Java Variable Declaration
Java Variable Assignment
Java Variable Reading
Java Variable Naming Conventions
Java Local-Variable Type Inference

Jakob Jenkov
Last update: 2018-10-03

A Java variable is a piece of memory that can contain a data value. A variable thus has a data type. Data types are covered in more detail in the text on Java data types.

Variables are typically used to store information which your Java program needs to do its job. This can be any kind of information ranging from texts, codes (e.g. country codes, currency codes etc.) to numbers, temporary results of multi step calculations etc.

In the code example below, the main() method contains the declaration of a single integer variable named number. The value of the integer variable is first set to 10, and then 20 is added to the variable afterwards.

public class MyClass {

    public static void main(String[] args) {

        int number = 10;

        number = number + 20;
    }

}

Java Variable Types

In Java there are four types of variables:

Non-static fields
Static fields
Local variables
Parameters

A non-static field is a variable that belongs to an object. Objects keep their internal state in non-static fields. Non-static fields are also called instance variables, because they belong to instances (objects) of a class. Non-static fields are covered in more detail in the text on Java fields.

A static field is a variable that belongs to a class. A static field has the same value for all objects that access it. Static fields are also called class variables. Static fields are also covered in more detail in the text on Java fields.

A local variable is a variable declared inside a method. A local variable is only accessible inside the method that declared it. Local variables are covered in more detail in the text on Java methods.

A parameter is a variable that is passed to a method when the method is called. Parameters are also only accessible inside the method that declares them, although a value is assigned to them when the method is called. Parameters are also covered in more detail in the text on Java methods.

Java Variable Declaration

Exactly how a variable is declared depends on what type of variable it is (non-static, static, local, parameter). However, there are certain similarities that

In Java you declare a variable like this:

type name ;

Instead of the word type, you write the data type of the variable. Similarly, instead of the word name you write the name you want the variable to have.

Here is an example declaring a variable named myVariable of type int.

int myVariable;

Here are examples of how to declare variables of all the primitive data types in Java:

byte    myByte;
short   myShort;
char    myChar;
int     myInt;
long    myLong;
float   myFloat;
double  myDouble;

Here are examples of how to declare variables of the object types in Java:

Byte       myByte;
Short      myShort;
Character  myChar;
Integer    myInt;
Long       myLong;
Float      myFloat;
Double     myDouble;
String     myString;

Notice the uppercase first letter of the object types.

When a variable points to an object the variable is called a "reference" to an object. I will get back to the difference between primitive variable values and object references in a later text.

The rules and conventions for choosing variable names are covered later in this text.

Java Variable Assignment

Assigning a value to a variable in Java follows this pattern:

variableName = value ;

Here are three concrete examples which assign values to three different variables with different data types

myByte   = 127;

myFloat  = 199.99;

myString = "This is a text";

The first line assigns the byte value 127 to the byte variable named myByte. The second line assigns the floating point value 199.99 to the floating point variable named myFloat. The third line assigns the String value (text) this is a text to the String variable named myString.

You can also assign a value to a variable already when it is declared. Here is how that is done:

byte   myByte   = 127;

float  myFloat  = 199.99;

String myString = "string value";

Java Variable Reading

You can read the value of a Java variable by writing its name anywhere a variable or constant variable can be used in the code. For instance, as the right side of a variable assignment, as parameter to a method call, or inside a arithmetic expression. For instance:


float myFloat1 = 199.99;

float myFloat2 = myFloat1;           // right hand side value in assignment

float myFloat3 = myFloat2 + 123.45;  // as part of arithmetic expression


System.out.println(myFloat3);        // as parameter in method call.

Java Variable Naming Conventions

There are a few rules and conventions related to the naming of variables.

The rules are:

Java variable names are case sensitive. The variable name money is not the same as Money or MONEY.
Java variable names must start with a letter, or the $ or _ character.
After the first character in a Java variable name, the name can also contain numbers (in addition to letters, the $, and the _ character).
Variable names cannot be equal to reserved key words in Java. For instance, the words int or for are reserved words in Java. Therefore you cannot name your variables int or for.

Here are a few valid Java variable name examples:

myvar
myVar
MYVAR    
_myVar
$myVar
myVar1
myVar_1

There are also a few Java variable naming conventions. These conventions are not necessary to follow. The compiler to not enforce them. However, many Java developers are used to these naming conventions. Therefore it will be easier for them to read your Java code if you follow them too, and easier for you to read the code of other Java developers if you are used to these naming conventions. The conventions are:

Variable names are written in lowercase. For instance, variable or apple.
If variable names consist of multiple words, each word after the first word has its first letter written in uppercase. For instance, variableName or bigApple.
Even though it is allowed, you do not normally start a Java variable name with $ or _ .
Static final fields (constants) are named in all uppercase, typically using an _ to separate the words in the name. For instance EXCHANGE_RATE or COEFFICIENT.

Java Local-Variable Type Inference

From Java 10 it is possible to have the Java compiler infer the type of a local variable by looking at what actual type that is assigned to the variable when the variable is declared. This enhancement is restricted to local variables, indexes in for-each loops and local variables declared in for-loops.

To see how the Java local-variable type inference works, here is first an example of a pre Java 10 String variable declaration:

String myVar = "A string!";

From Java 10 it is no longer necessary to specify the type of the variable when declared, if the type can be inferred from the value assigned to the variable. Here is an example of declaring a variable in Java 10 using local-variable type inference:

var myVar = "A string!";

Notice the var keyword used in front of the variable name, instead of the type String. The compiler can see from the value assigned that the type of the variable should be String, so you don't have to write it explicitly.

Here are a few additional Java local-variable type inference examples:

var list = new ArrayList();

var myNum = new Integer(123);

var myClassObj = new MyClass();

Next: Java Data Types

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	Jakob Jenkov