Question

a.java has import java.io.*; I dragged all the java to link to the project, but some ended up with red icon, while a.java with yellow. Those with red don’t have imports, and act like there are no imports, but that’s why I put a.java there. Why doesn’t it work?

Answer 1

Just because A.java has import java.io.*; it doesn’t mean that B.java has access to java.io.*;. B.java will have to import what it wants to use explicitly, even if they are in the same package.
This is specified in §6.3 and §7.5 of the JLS.

From section 6.3:

An import declaration makes types or members available by their simple names only within the compilation unit that actually contains the import declaration. The scope of the type(s) or member(s) introduced by an import declaration specifically does not include the PackageName of a package declaration, other import declarations in the current compilation unit, or other compilation units in the same package.