Java interview questions on packages

Which package is always imported by default?
No. It is by default loaded internally by the JVM. The java.lang package is always imported by default.

Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains anything about it. And the JVM will internally load the class only once no matter how many times you import the same class.

Does importing a package imports the sub packages as well? E.g. Does importing com.bob.* also import com.bob.code.*?
No you will have to import the sub packages explicitly. Importing com.bob.* will import classes in the package bob only. It will not import any class in any of its sub package’s.

Explain the usage of Java packages.
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
For example: The Java API is grouped into libraries of related classes and interfaces; these libraries are known as package.

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.BOB 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, cannot resolve symbol.