Basic Java Language: Difference between revisions

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

Everything in Java is specified inside classes. Classes are bodies of statements, methods, and variables. A statement can be thought of as the smallest standalone element of an imperative programming language (or a language that explicitly specify an algorithm to achieve a goal, opposed to declarative programming, which explicitly specifies a goal and leaves the implementation of that goal to algorithms within the support software). A program is formed by a sequence of one or more statements (e.g. if x, then y). A statement will have internal components (e.g. expressions). A method usually consists of a sequence of statements to perform an action, a set of input parameters to customize those actions, and possibly an output value (called return value) of some kind. 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:

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This, essentially creates a class. All the code goes between the {, called the opening brace, and the }, called the closing brace.

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Java allows you to abstract the internal data of a class from other code that uses that class. The idea here is to create a "black box" effect, where classes that use your class have no knowledge of the internal workings of the class. This simplifies changing the class if done consistently, as other classes don't depend on anything within the class.

The protected and default levels deal with [[Java/Inheritance|inheritance]] and [[Java Deployment|packages]] respectively; these concepts are dealt with in later lessons. For now, declare all classes public (private classes are only useful — or even possible — when nested, and nested classes definitely comes later). Methods should be public if they're meant to be called by other classes. Declare a method private if it should only be called by its own class.

All operations are done in statements. Every statement ends 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 one statement: <code>System.out.println("Hello, World")</code>. 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 are used to store information in a computer's memory. Like its name implies, variables are values that can be changed. 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".

To declare a variable, use this syntax:

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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.

Primitives are the simplest type of data. The following table shows all the types of primitives.

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<code>double myDouble;</code>

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

<pre>int a;

Now, why would you want to set a variable? For further use, ie, to access at a later time. For example, you can do calculations with other operators...

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

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

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int a;

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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 $. Identifiers can be no longer than 32 characters.

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.

This section is about printing to the console using <code>System.out.println();</code>.

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:

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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  

   

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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.''

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

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| Printing variables