The two previous posters' comments are good. Allow me to point to an earlier post of mine that discusses the structure of Hughes' poem "Harlem" and to add that there are (depending on how you define the term) probably many more figures of speech in the poem, other than simile and metaphor; among these are alliteration and anaphora.
The poem is a series of similes that appeal to different senses (taste, touch, smell, visual, sound). Hughes expertly manipulates the reader via these metaphors. He posits a question in the first line and then offers six tentative answers in the form of five similes and one metaphor (the last line). However, because of the repetition, the reader could turn that metaphor into a simile on their own (Or does it explode . . . like a bomb?).
Each of these questions refers to, of course, racial equality. The first line of the poem is an allusion to Martin Luther King\'s \"I Have a Dream\" speech. Together these questions, phrases as similes, gives up a greater idea of the depth and seriousness of the problem of racial equality.
1 problem about it alluding to king his speach was after this was made