<<Java Language Specification>>列出了java关键字、保留关键字，
并举例说明true，false和null都不是java的keywords，而是literals ，尽管他们看起来很像是keywords。

3.9 Keywords

The following character sequences, formed from ASCII letters, are reserved for use as keywords and cannot be used as identifiers (§3.8):

						Keyword: one of
	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 not currently used. This may allow a Java compiler to produce better error messages if these C++ keywords incorrectly appear in programs.

While true and false might appear to be keywords, they are technically Boolean literals (§3.10.3). Similarly, while null might appear to be a keyword, it is technically the null literal (§3.10.7).

3.10 Literals

A literal is the source code representation of a value of a primitive type (§4.2), the String type (§4.3.3), or the null type (§4.1):

						Literal:
	IntegerLiteral
	FloatingPointLiteral
	BooleanLiteral
	CharacterLiteral
	StringLiteral
	NullLiteral