All syntax java programming

Java Basic Syntax Tutorial

In this Java tutorial we learn what syntax in programming is.

We also discuss documenting our code, naming things, and special reserved keywords.

Lastly, we take a look at how Java defines scope and what it means, as well as curly brace conventions.

What is Syntax

Simply put, syntax is the specific way we write code. It defines a set of rules for developers, and every programming language defines its own syntax.

As an example, consider the following code that we used to create our first app.

You’ll see all kinds of weird words, curly braces, parentheses etc. This makes up the syntax of the document.

We can’t change the code however we want. If we do, the compiler won’t understand it and show us all kinds of scary errors.

The example above doesn’t follow Java syntax rules. The code has to be written in a specific manner, for example the public keyword comes before the class keyword, not the other way around.

Throughout the tutorial course we’ll show what the syntax of a concept looks like, as well as an example of it that can be compiled and run in your IDE.

In this lesson, we’ll discuss some of the basics.

Comments

We’ll start of with comments. Comments allow us to document our code and enhance its readability.

Comments are ignored by the compiler, which means the compiler won’t try to execute any text or code written inside a comment.

In most cases, we shouldn’t need comments, our code should be written cleanly and clearly enough so that it’s obvious as to what the code does.

That said, there are many situations where we should make use of comments.

• When learning programming, or a new language, it’s helpful to comment our understanding of the code.
• When working on large projects as part of a group, we should provide comments so that other developers immediately understand what’s going on.
• When using external libraries it may be useful to include short comments so it’s not neccesary to visit the library’s documentation that often.
• When we’re working with complex code, it may not be immediatly clear what the code is supposed to do. Commenting would make it easier if we need to come back to the code at a later time.
• When we’re debugging/testing. We may want to temporarily remove pieces of code from execution by commenting them out.

Java supports both types of C++ comments, block comments and single line comments.

Block comments consist of an open tag and a close tag. The open tag is a slash, immediately followed by an asterisk. The close tag is an asterisk, immediately followed by a slash.

Anything between the comments, even if it spans multiple lines, is considered a comment by the compiler.

   Line comments are prefixed with two slashes and is only used on a single line.

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Java — Basic Syntax

When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other’s methods. Let us now briefly look into what do class, object, methods, and instance variables mean.

• Object − Objects have states and behaviors. Example: A dog has states — color, name, breed as well as behavior such as wagging their tail, barking, eating. An object is an instance of a class.
• Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type supports.
• Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
• Instance Variables − Each object has its unique set of instance variables. An object’s state is created by the values assigned to these instance variables.

First Java Program

Let us look at a simple code that will print the words Hello World.

Example

public class MyFirstJavaProgram < /* This is my first java program. * This will print 'Hello World' as the output */ public static void main(String []args) < System.out.println("Hello World"); // prints Hello World >>

Let’s look at how to save the file, compile, and run the program. Please follow the subsequent steps −

• Open notepad and add the code as above.
• Save the file as: MyFirstJavaProgram.java.
• Open a command prompt window and go to the directory where you saved the class. Assume it’s C:\.
• Type ‘javac MyFirstJavaProgram.java’ and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
• Now, type ‘ java MyFirstJavaProgram ‘ to run your program.
• You will be able to see ‘ Hello World ‘ printed on the window.

Output

C:\> javac MyFirstJavaProgram.java C:\> java MyFirstJavaProgram Hello World

Basic Syntax

About Java programs, it is very important to keep in mind the following points.

• Case Sensitivity − Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
• Class Names − For all class names the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word’s first letter should be in Upper Case. Example:class MyFirstJavaClass
• Method Names − All method names should start with a Lower Case letter. If several words are used to form the name of the method, then each inner word’s first letter should be in Upper Case. Example:public void myMethodName()
• Program File Name − Name of the program file should exactly match the class name. When saving the file, you should save it using the class name (Remember Java is case sensitive) and append ‘.java’ to the end of the name (if the file name and the class name do not match, your program will not compile). But please make a note that in case you do not have a public class present in the file then file name can be different than class name. It is also not mandatory to have a public class in the file. Example: Assume ‘MyFirstJavaProgram’ is the class name. Then the file should be saved as ‘MyFirstJavaProgram.java’
• public static void main(String args[]) − Java program processing starts from the main() method which is a mandatory part of every Java program.

Java Identifiers

All Java components require names. Names used for classes, variables, and methods are called identifiers.

In Java, there are several points to remember about identifiers. They are as follows −

• All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
• After the first character, identifiers can have any combination of characters.
• A key word cannot be used as an identifier.
• Most importantly, identifiers are case sensitive.
• Examples of legal identifiers: age, $salary, _value, __1_value.
• Examples of illegal identifiers: 123abc, -salary.

Java Modifiers

Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers −

• Access Modifiers − default, public , protected, private
• Non-access Modifiers − final, abstract, strictfp

We will be looking into more details about modifiers in the next section.

Java Variables

Following are the types of variables in Java −

• Local Variables
• Class Variables (Static Variables)
• Instance Variables (Non-static Variables)

Java Arrays

Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct, and initialize in the upcoming chapters.

Java Enums

Enums were introduced in Java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.

With the use of enums it is possible to reduce the number of bugs in your code.

For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium, and large. This would make sure that it would not allow anyone to order any size other than small, medium, or large.

Example

class FreshJuice < enum FreshJuiceSize< SMALL, MEDIUM, LARGE >FreshJuiceSize size; > public class FreshJuiceTest < public static void main(String args[]) < FreshJuice juice = new FreshJuice(); juice.size = FreshJuice.FreshJuiceSize.MEDIUM ; System.out.println("Size: " + juice.size); >>

The above example will produce the following result −

Output

Note − Enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

abstract	assert	boolean	break
byte	case	catch	char
class	const	continue	default
do	double	else	enum
extends	final	finally	float
for	goto	if	implements
import	instanceof	int	interface
long	native	new	package
private	protected	public	return
short	static	strictfp	super
switch	synchronized	this	throw
throws	transient	try	void
volatile	while

Comments in Java

Java supports single-line and multi-line comments very similar to C and C++. All characters available inside any comment are ignored by Java compiler.

Example

public class MyFirstJavaProgram < /* This is my first java program. * This will print 'Hello World' as the output * This is an example of multi-line comments. */ public static void main(String []args) < // This is an example of single line comment /* This is also an example of single line comment. */ System.out.println("Hello World"); >>

Output

Using Blank Lines

A line containing only white space, possibly with a comment, is known as a blank line, and Java totally ignores it.

Inheritance

In Java, classes can be derived from classes. Basically, if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.

This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario, the existing class is called the superclass and the derived class is called the subclass.

Interfaces

In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.

An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.

What is Next?

The next section explains about Objects and classes in Java programming. At the end of the session, you will be able to get a clear picture as to what are objects and what are classes in Java.

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Java Syntax

In the previous chapter, we created a Java file called Main.java , and we used the following code to print «Hello World» to the screen:

Example explained

Every line of code that runs in Java must be inside a class . In our example, we named the class Main. A class should always start with an uppercase first letter.

Note: Java is case-sensitive: «MyClass» and «myclass» has different meaning.

The name of the java file must match the class name. When saving the file, save it using the class name and add «.java» to the end of the filename. To run the example above on your computer, make sure that Java is properly installed: Go to the Get Started Chapter for how to install Java. The output should be:

The main Method

The main() method is required and you will see it in every Java program:

public static void main(String[] args) 

Any code inside the main() method will be executed. Don’t worry about the keywords before and after main. You will get to know them bit by bit while reading this tutorial.

For now, just remember that every Java program has a class name which must match the filename, and that every program must contain the main() method.

System.out.println()

Inside the main() method, we can use the println() method to print a line of text to the screen:

public static void main(String[] args) < System.out.println("Hello World"); > 

Note: The curly braces <> marks the beginning and the end of a block of code.

System is a built-in Java class that contains useful members, such as out , which is short for «output». The println() method, short for «print line», is used to print a value to the screen (or a file).

Don’t worry too much about System , out and println() . Just know that you need them together to print stuff to the screen.

You should also note that each code statement must end with a semicolon ( ; ).

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