necessary vs incumbent

necessary and incumbent both are nouns.

Word	Noun	Adjective	Verb	Adverb
necessary	Yes	Yes	No	No
incumbent	Yes	Yes	No	No

necessary (noun)	incumbent (noun)
anything indispensable	the official who holds an office

necessary (adjective)	incumbent (adjective)
absolutely essential	currently holding an office
unavoidably determined by prior circumstances	necessary (for someone) as a duty or responsibility; morally binding
	lying or leaning on something else