25. Are the imports checked for validity at compile time? Example: will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, can not resolve symbol
symbol : class ABCD
location: package io
26. Does importing a package imports the subpackages as well? Example: Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
27. What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
Example: String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
28. What is the default value of an object reference declared as an instance variable?
The default value will be null unless we define it explicitly.
29. Can a top level class be private or protected?
No. A top level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.
If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.
30. What type of parameter passing does Java support?
In Java the arguments are always passed by value.