Basic Java Language: Difference between revisions

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 written in classes. A Class in this context is a section of code that can contain data (variables), and instructions for processing that data (methods). Returning to the Hello World example from the [[Introduction to Java]], the class is defined on the first line, and everything within the curly brackets is within that class.

A class with ''public'' modifier is visible by all other classes. Class with no modifier has package level protection. It means that only other classes within the same package can see it.  

However, in order to get started without being bogged down by choosing one, just declare all of your classes as <code>public</code>. The idea behind protection is that, if only the class itself can make changes to the data stored within itself, the other parts of the program do not need to know where the data is coming from or how it is stored. This is great for two reasons, firstly, it is easy to rewrite the structure of a class without changing the external application, and secondly it allows the class to make assumptions about the data, and do slightly less exception handling. You should know that if your code tried to break the protection of a class, the java compiler will stop and report an error.

===Extends/Implements===

== Statements ==

Comments are written using two forward slashes  

Comments can be written at the end of a line  

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

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== Variables ==

== Operators ==

Operators are the symbols defining a certain operation to be performed.

=== Addition Operator '+' ===

=== Subtraction '-', multiplication '*', division '/', and modulus '%' operators ===

Just like the addition operator, the subtraction, multiplication, division, and modulus operators are used as follows:

int a = 9;

a*2; // evaluates to 18

a%4; // evaluates to 1

Subtraction, multiplication, and division, you have seen before, but the modulus operator is much less commonly used. Actually, it is commonly used, but it known under a different name, the remainder. Nine divided by four gives a remainder of one, so therefore 9%4 (pronounced "9 mod 4") evaluates to 1.

When more than one of the operators are used in the same statement, for example:

int a, b, c, d;

a = a-b/c*d;

the Java language would use the order-of-operations in this case.

==== Plus-equals ====

Example:

int a = 6; // assigns the value 6 to variable a

a += 5; // adds 5 to a, and assigns that value back into a, now a is 11

==== Minus-equals, multiply-equals, divide-equals, and mod-equals ====

Identifiers should also be meaningful, while maintaining practicality. If calculating the sum of a set of numbers, <code>sum<code> is better than <code>cumulativeTotal</code>, but <code>s</code> might be too short.

You can also print out multiple variables on the same line:

Note that this won't work for two literal numeric values:

Now you can do calculations and print the result! For example, say you want to print 6*6*6*6. Here is the code:

public class Multiplying

{

   }

}

== Compiling and running your program ==

If, when running, it reports "invalid class file", the compiler might not have created a file. It should have given an error message at compile time.

''In order for the following to work properly, you must place the following code on the first line of your .java file. Importing packages will be explained later.''

int mine = Integer.parseInt( JOptionPane.showInputDialog( null, "Now I can set an int!" ) );

System.out.println( mine );

And now, of course, you can do whatever you want with that...

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[[Category:Java]]

[[Category:Learning activities]]

[[Category:Basics]]