Basic Java Language
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 that can change. 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. 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-percision floating poing (32-bit IEEE 754) | Smallest positive non-zero: 14e-45, Largest positive non-zero: 3.4028234e38 |
| double | Double-percision 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 |
Declaring 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;
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
Statements
A statement in Java is generally a line of code that does something. Statements are ended by a semicolon ;. An example of a statement is the declaration of a variable like we saw before. 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.
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 anythiing 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 */
Operators
Equality Operator =
The equality 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;
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.
More comming soon