|Java Language Tutorial|
|What is Java?|
|Java Core Concepts|
|Java Data Types|
|Java Access Modifiers|
|Java Abstract Classes|
|Java Interfaces vs.
|Java Main Program|
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.
Here is a list of the topics covered in this text:
In Java there are four types of variables:
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.
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
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.
In Java you assign a value to a variable like this:
myByte = 127; myFloat = 199.99; myString = "string value";
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";
You can read the value of a Java variable by writing its name where 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.
There are a few rules and conventions related to the naming of variables.
The rules are:
moneyis not the same as
forare reserved words in Java. Therefore you cannot name your variables
There are also a few variable naming conventions. Conventions are not necessary to follow. The compiler to not enforce them. But it may still be a good idea to follow them. The conventions are: