Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.
Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.
java.lang package is imported by default even without a package declaration.
Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.
The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.
static final int PI = 2.14; is an example for constant.