Comments

There are threee types of comments in Java.

Single line Comment

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

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

Multi-line Comment

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

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

System.out.println("This is the second line comment.");

Documentation Comment. Also called a doc comment

/** documentation */

1	/** documentation */

Source file name

The name of a source file should exactly match the public class name with the extension of .java. The name of the file can be a different name if it does not have any public class.

Case sensitivity

Java is a case-sensitive language, which means that the identifiers AB, Ab, aB, ******and ab are different in Java.

System.out.println("Wrapline"); // valid syntax 
system.out.println("Wrapline"); // invalid syntax because of the first letter of System keyword is always uppercase.

1 2	System.out.println("Wrapline"); // valid syntax system.out.println("Wrapline"); // invalid syntax because of the first letter of System keyword is always uppercase.

Class name

The first letter of the class should be in Uppercase (lowercase is allowed but discouraged).
If several words are used to form the name of the class, each inner word’s first letter should be in Uppercase. Underscores are allowed, but not recommended. Also allowed are numbers and currency symbols, although the latter are also discouraged because they are used for a special purpose (for inner and anonymous classes).

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

class MyJavaProgram // valid syntax

class 1Program // invalid syntax

class My1Program // valid syntax

class $Program // valid syntax, but discouraged

class My$Program // valid syntax, but discouraged (inner class Program inside the class My)

class myJavaProgram // valid syntax, but discouraged

public static void main(String [] args)

The method main() is the main entry point into a Java program; this is where the processing starts. Also allowed is the signature public static void main(String… args).

Method name

All the method names should start with a lowercase letter (uppercase is also allowed but lowercase is recommended).
If several words are used to form the name of the method, then each first letter of the inner word should be in Uppercase. Underscores are allowed, but not recommended. Also allowed are digits and currency symbols.

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

1 2	public void employeeRecords() // valid syntax public void EmployeeRecords() // valid syntax, but discouraged

Identifiers

Identifiers are the names of local variables, instance and class variables, and labels, but also the names for classes, packages, modules and methods. All Unicode characters are valid, not just the ASCII subset.

All identifiers can begin with a letter, a currency symbol or an underscore (_). According to the convention, a letter should be lower case for variables.
The first character of identifiers can be followed by any combination of letters, digits, currency symbols and the underscore. The underscore is not recommended for the names of variables. Constants (static final attributes and enums) should be in all Uppercase letters.
Most importantly identifiers are case-sensitive.
A keyword cannot be used as an identifier since it is a reserved word and has some special meaning.

Legal identifiers: MinNumber, total, ak74, hello_world, $amount, _under_value 
Illegal identifiers: 74ak, -amount

1 2	Legal identifiers: MinNumber, total, ak74, hello_world, $amount, _under_value Illegal identifiers: 74ak, -amount

White spaces

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

Access Modifiers

These modifiers control the scope of class and methods.

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

Keywords

Keywords or Reserved words are the words in a language that are used for some internal process or represent some predefined actions. These words are therefore not allowed to use as variable names or objects.

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