原文：If you are converting from a byte value b to a char and you don’t want signextension, you must use a bit mask to suppress it. This is a common idiom, so nocomment is necessary:char c = (char) (b & 0xff);沒(méi)有g(shù)et到 sign extension的含義！以及為什么這么做就沒(méi)有sign extension