These terms are all colorful colloquialisms used by the main character, Holden Caufield, in his narrative. "Madman" means crazy, like the "stuff that happened to me around last Christmas" (Chapter 1). To "get a bang" out of something is similar to get a "kick out of it", and both refer to things or events that really amuse someone, like Mr. Spencer's Navajo blanket and the old teacher's joke about his own health, which really struck him as significant and funny, or "knocked him out" (Chapter 2). A "hot shot" is someone who is or acts like he is exceptionally good at something, like Stradlater says Holden is in Mr. Hartzell's English class (Chapter 4); it can also refer to someone who thinks he is better than everybody else. A "racket" is a commotion, or a lot of noise.