Token and keywords

Token

The smallest individual unit in a program is known as a token. In fact, every unit that makes a sentence is a token.

Java has the following token:

• Keywords
• Identifiers
• Literals
• Punctuators
• Operators

Keywords

Keywords are the words that convey a special meaning to the language compiler. These are reserved for special purposes and must not be used as normal identifier names.

The following character sequences, formed from ASCII letters, are reserved for use as keywords and cannot be used as identifiers:

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

The keywords const and goto are reserved, even though they are currently not in use. This may allow a Java compiler to produce better error messages if these Java Keywords incorrectly appear in programs.

While true and false might appear to be keywords, they are technically boolean literals. Similarly, while null might appear to be a keyword, it is technically the null literal. Thus true, false and null are not keywords but reserved words.

Note: also read about the Simple Hello World Program

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