This lesson discusses the basics of the Java language, including variables, primitives, operators, statements and Java writing conventions.

Touch on Classes

Everything, in Java, is done inside classes. Essentially, in a brief talk, classes are bodies of statements, methods (you will learn these later), and variables. There is a method called main that all classes have. This is the start point of your program - where everything must begin. To create a class, the syntax (or basic outline in code) is:

[Protection] class [Name] [Extend/Implements...]

Right now, worry about the first 3. Protection will be discussed later. "class" is required to create a class. "Name" can be any valid identifier (see Valid Identifiers...). The name will be used to create objects (more later...). Here is what you type to create a class, as for now, and all you must worry about:

public class NAME

NAME, again,is the identifier.

Classes also have a body - where you actually put the things. To create the body you have a { and }. For example:

public class NAME
{

}

This, essentially creates a class. All the code goes between { and }.

Protection

Java allows you to create a "protection value". This means you can set who (or what - your program, other people's programs, etc.) can access the variable, method, or class that you set to a protection. You have already seen where this goes in a class, and will learn where in variables and methods. Right now, just call everything public. Here is a chart (which you might not understand right now).

Name	Desc.	Example + Meaning
private	Variable only accessible from class it is in	private int myInt;//myInt can only be edited and viewed from the class it is in
	(No modifier) - only accessible from class and subclass	int myInt;//A subclass can access it, as the class it is in
protected	Accessible from package, subclass, and class	protected int myInt;//Class, subclass, and package classes can access it
public	Fully accessible	public int myInt;//Any class can access it

Statements

All calculations are done in statements. Every statements end in a semicolon: ;. Statements can set a variable with the '=' operator. This sets the variable you wish to set to another variable or a value. Values can only be used on primitive types. As you saw in the HelloWorld program, there was on statement: System.out.println("Hello, World"). This is one type of statement called calling a method. More will be discussed later. Finally, you can declare a variable. If a class has no statements, it would do nothing.

Variables

Variables Variables are used to store data into your computer memory, that can change. All you need to worry about is that you can create a variable and change its value. There are two main categories of variables: reference variables and primitive variables. Reference variables are place holders for objects (More on objects in Lesson 4, Java Objects and Classes.) Primitive variables are given a value of a primitive, a basic value. Variables are all given a type when they are declared, such as "int" or "String".

Declaring Variables

To declare a variable, use this syntax:

[protector] [classname] [identifier]

This is one type of statement. The class name is the type of variable you want. Currently, you will only use basic primitive variables, but you will learn about objects soon. Here is an example of declaring a variable:

public int myInt

myInt has protection of public. It is of class int, or Integer (see Primitive Types). int is a class in Java. When you say that, you are basically making a primitive object.

Primitive Data Types

Primitives are the simplest type of data. Primitives consist of numbers, letters and strings of letters (more than one letter in one variable). The following table shows all the types of primitives.

Primitive	Description	Values
byte	A brief, 8-bit integer	Whole numbers -128 through 127
short	A short, 16-bit integer	Whole numbers -32,768 through 32,767
int	A 32 bit integer	Whole numbers -2,147,483,648 through 2,147,483,647
long	A long, 64 bit integer	Whole numbers -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807
float	Single-precision floating point (32-bit IEEE 754)	Smallest positive non-zero: 14e^-45, Largest positive non-zero: 3.4028234e³⁸
double	Double-precision floating point (64-bit IEEE 754)	Smallest positive non-zero: 4.9e^-324, Largest positive non-zero: 1.797693157e³⁰⁸
char	A single character	All Unicode characters
boolean	A Boolean value	true or false
String	A group of characters	Any formation of characters

Operators

Assignment Operator '='

The assignment operator sets the variable on its left equal to the value on its right. This code creates the variable a and sets it equal to 5

int a;
a = 5;

Fairly simple, right? Here is where it can get a little more tricky.

int a;
int b;
b = 6;
a = b; //a equals 6

At first glance, you might think that the last statement is setting a equal to the letter "b", but instead it is setting a equal to the value of b, which is 6. Essentially, a is now 6. You can also set a variable and declare it on the same line:

int a = 6;

Addition Operator +

The addition operator is different from the = operator. It does not set anything. You would need the = operator to do any setting.

If you want to you want to use the addition operator to add 6 to the value of a, it would be used like this:

a = a + 6;

Or you can use a shortcut in order to reach the same result as the above.

a += 6;

There are shortcuts to all of the operators, which will be shown in the following sections.

Subtraction -, Division / and Multiplication * Operators

Just like the addition operator, the Subtraction, Division, and Multiplication operators are used as follows:

int a;

a = a-1;
a = a/1;
a = a*1;

The result of "a" in each example would be simply

1st a - "a" minus 1
2nd a - "a" divided by 1
3rd a - "a" multiplied by 1

When all of the operators are used at once, for example:

int a, b, c, d;

a = a-b/c*d;

The Java Programming Language would use the order of operations in this example.

Here is a shortcut for subtracting a value from int myInt:

myInt -= 4;

Valid Identifiers

There are several rules for the naming of your variables. Identifiers can be named using all the letters from A through Z (capital or lowercase, you choose), numbers 0-9 and underscore _ and the dollar sign $. The first character of the identifier cannot be a number, but it can be a letter, _ or $.

You can't create two variables with the same identifier, all identifiers must be unique. You also can't use an identifier that is a Java keyword. Here is a list of all the Java Keywords. Flashcards for Java keywords provides some assistance in memorizing the concepts behind specific keywords.

Compiling and Running your program

To compile your program, go to the directory you saved it. Note that if your class was named NaMe then your file must be named NaMe.java. Anyways, type: javac NaMe.java where NaMe is your class name. To run, type java NaMe.

Note that if the compiler gives errors with your code, you must fix them.

If, when running, it says invalid class file, perhaps the compiler did not create a file. It might have given error messages.

Exercises

A. Create a program that creates a String with the value "ABC". Then print the value using System.out.println(StringIdentifierHere). You could also do System.out.prinln("ABC"), but we are focusing on creating variables.

B. Do the above with a long set to 10000. Note that you have to say System.out.println(LongIdentifier + "").

You are ready to move on! Java Decision Structures
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