Java Basic Syntax

Java is an object-oriented programming language, meaning it relies on the concept of objects, which interact through method calls to function together. This article covers fundamental concepts in Java, such as classes, objects, methods, instance variables, syntax, and semantics.

Basic Terminologies in Java

1. Class: A class acts as a blueprint for creating objects. It is a logical template that defines common properties and methods.
   • Example 1: A blueprint of a house represents a class.
   • Example 2: In real life, Alice is an object of the “Human” class.
2. Object: An object is an instance of a class, having its own state and behavior.
   • Example: Dogs, cats, and monkeys are objects of the “Animal” class.
   • Behavior: Running on the road.
3. Method: A method defines the behavior of an object.
   • Example: A fuel gauge shows the amount of fuel left in a car.
4. Instance Variables: These are unique to each object and define its state through the values assigned to them.

   Steps to compile and run a Java program in a console:

   javac LT.java
   java LT

   import java.util.*;
   public class LT {
       public static void main(String[] args) {
           System.out.println("LearnTricking!");
       }
   }

   Output:

   LearnTricking!

Note: If a class is public, the file name must match the class name.

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Java Syntax Overview

Java syntax defines how Java code is written and understood by the Java compiler. Here are the essential elements of Java syntax:

1. Comments in Java

There are three types of comments in Java:

• Single-line Comment: Begins with //.

  // System.out.println("This is a comment.");

• Multi-line Comment: Enclosed between /* and */.

  /*
      System.out.println("This is the first line of the comment.");
      System.out.println("This is the second line of the comment.");
  */

• Documentation Comment: Also known as a doc comment, starts with /** and ends with */.

  /** documentation */

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2. Source File Name

The source file name must match the public class name and end with .java. If the file contains a public class named LT, the file name should be:

LT.java  // valid syntax
lt.java  // invalid syntax

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3. Case Sensitivity

Java is case-sensitive, meaning identifiers like AB, Ab, aB, and ab are different.

System.out.println("LearnTricking"); // valid syntax
system.out.println("LearnTricking"); // invalid syntax due to incorrect capitalization of 'System'

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4. Class Names

• The first letter of a class name should be uppercase (though lowercase is allowed, it's discouraged).
• For class names with multiple words, each inner word’s first letter should be uppercase.

class MyJavaProgram  // valid syntax
class 1Program       // invalid syntax
class My1Program     // valid syntax
class $Program       // valid but discouraged
class My$Program     // valid but discouraged (indicates inner class Program inside My)
class myJavaProgram  // valid but discouraged

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5. public static void main(String[] args)

The main() method is the entry point of a Java program. An alternative signature is public static void main(String... args).

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6. Method Names

• Method names should start with a lowercase letter (though uppercase is allowed, it's discouraged).
• For multi-word method names, each inner word's first letter should be uppercase.

public void employeeRecords() // valid syntax
public void EmployeeRecords() // valid but discouraged

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7. Identifiers in Java

Identifiers are names for variables, classes, methods, etc., and must follow these rules:

• Can start with a letter, currency symbol, or underscore (_). Conventionally, variables should start with a lowercase letter.
• Can contain letters, digits, currency symbols, and underscores after the first character. Constants should be all uppercase.
• Case-sensitive.
• Keywords cannot be used as identifiers.

Legal identifiers: MinNumber, total, ak74, hello_world, $amount, _under_value

Illegal identifiers: 74ak, -amount

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8. White Spaces in Java

Blank lines, which may contain only white spaces or comments, are ignored by the Java compiler.

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9. Access Modifiers

Modifiers control the scope of classes and methods.

• Access Modifiers: default, public, protected, private
• Non-access Modifiers: final, abstract, static, transient, synchronized, volatile, native

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10. Understanding Access Modifiers

Access Modifier	Within Class	Within Package	Outside Package by Subclass Only	Outside Package
Private	Yes	No	No	No
Default	Yes	Yes	No	No
Protected	Yes	Yes	Yes	No
Public	Yes	Yes	Yes	Yes

11. Java Keywords

Keywords are reserved words in Java used for specific purposes and cannot be used as identifiers.

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