Basic Java Language

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

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".

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

Declaring and Setting Variables

In a nutshell, declaring a variable has this syntax:

 [variable type] [identifier];

This works for both reference variables and primitive variables. [variable type] is the type of variable, like int or char. [identifier] is the name for the variable you wish to create, for example, password or number_of_fish. If you wanted to create a int variable called age, you would type this:

int age;

Notice the semicolon at the end of the statement. All statements in Java must end in this, otherwise you will get a compiler error.

You can set the values of variables while declaring them, like this:

int age = 100;

You can declare multiple variables on one line, if you wish:

 boolean a, b;

Also, you can change variables after you already give them a value.

[v you want to change] = [v you want it to be];

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.

Statements

A statement in Java is a chunk of code that does something specific completely. Statements are ended by a semicolon ;. An example of a statement is the declaration of a variable like we saw before.

private int static numberOfObjectsInstantiated;

The above declares a single memory location per class which may be incremented each time an object of that class is instantiated by calling its constructor method.

Another example you will recall from the "Hello World" program we made in Lesson 1.

System.out.println("Hello World");

This line of code prints Hello World onto the console.

A third example

if (ourShip.speed<10) then ourShip.mainThrustPulse(2);

The above checks a ship object's speed and if it is less than ten then it commands the ship object to fire two pulses from its main propulsion rockets. It is a single specific application task completed at the semi-colon when it moves to the next chunk of code.

Comments

Comments are notes that the developer can insert into their code. This allows the developer to keep track of their code and helps when developers are collaborating on a project. It is good to write a summary of the following code at the beginning of your methods, classes, etc. The Java compiler completely ignores comments, so you can say anything in them

Comments are written using two forward slashes

//This is a comment

Comments can be written at the end of a line

System.out.println("I love comments"); //This is also a comment

Comments can also be written on multiple lines by starting with /* and ending with*/.

/* Comment 1
Comment 2
Comment 3 */

Sun recommends the above form be reserved for information intended for automatic JavaDoc documentation.

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.

Addition Operator +

Unlike the = operator, a statement that simply uses the addition operator and two other values on either side does not do anything significant. For example, neither of the following three lines of code do anything.

5 + 6;
a + 9;
a + b;

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;

You would then use the order or operations, otherwise known as PEMDAS, to find the result of "a"; which would be:

a = a - (b/c)d
or
         b
a = a - ---
       (c*d)

(either way the result is the same)

Or if you prefer actual numbers, it would look like so:

int a = 1;
int b = 2;
int c = 2;
int d = 1;

a = 1-2/2*1;

thus:

a = 1 - 1
or 
a = 0;

Public, Private - What are they?

These statments are used to protect data. For example, "private int myInt" will make myInt only accessible from the class it is in. Here is a complete description:

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

Make a Class

Since you do not know classes and objects yet, you can put everything like this:

public class NAME
{
   public static void main(String args[])
   {
      //CODE HERE
   }
}

You will also later learn about methods. Note that this file must be saved into NAME.java. You can change NAME to any name you want (that is valid). This way, you can test all of your programs. In the command line, type:

javac NAME.java
java NAME

Note that NAME is still the program's name!

Convenient Statements

System.out.println("TEXT HERE"); -- Prints text onto command console (include quotes)

JOptionPane.showMessageDialog(null, "TEXT HERE"); -- Opens a message box with the text in the quotes

String message = JOptionPane.showInputDialog(null, "TEXT HERE"); -- See above; also allows user to input something. In this case, the input goes to String message

Finalizing the Basics

Let this information sink through before continuing to the next chapter. Take the time to test out some things you learned (see Make a Class). Then you'll really understand some of it.

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You are ready to move on! Java Decision Structures
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