Every compilation unit implicitly imports every public type name declared in the predefined package java.lang, as if the declaration import java.lang.*; appeared at the beginning of each compilation unit immediately after any package statement. As a result, the names of all those types are available as simple names in every compilation unit.
There are no new questions, but there may be new answers.
The java classes such as String, Number, StringBuffer, StringBuilder, Object, etc. are present in package "java.lang". In every java class this java.lang package is by default imported.
Hence all these classes are available in a class.