Java contains 53 reserved words out of which 50 are keywords out of 50 only 48 keywords are used and two keywords const and goto are not in use and 3 are literals values true, false and null.

These reserved words can not be used as identifiers.

Java Keywords:   keyword is a reserved word that has a predefined meaning in the language, therefore, we cannot use keywords as names as a  class, methods, variable name or as any other identifier. Keywords of java are:

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

const and goto are not in use
strictfp added in jdk 1.2
enum added in jdk 1.5

Reference

1. Java Language Keywords
2. List of Java keywords