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Do I need to import java.lang package any time? (JVM)
A: We need not, as it is by default loaded internally by the JVM.

Can I import same package/class twice? Will the JVM load the package twice at runtime? (JVM)
A: One can write code to import the same PACKAGE or same CLASS multiple times. Neither compiler nor JVM complains about it. And the JVM will internally load the class ONLY ONCE no matter how many times you import the same class.

* What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? (JVM)
The JVM SPECIFICATION is the BLUE-PRINT for the JVM generated and owned by Sun.
The JVM IMPLEMENTATION is the actual implementation of the spec by a VENDOR.
And the JVM RUNTIME is the actual running INSTANCE of a JVM implementation.

* What is a compilation unit? (JVM)
A compilation unit is a Java SOURCE code file (ClassName.java).

* Difference between JRE And JVM and JDK. (JVM)
The JVM is used to execute class file.
The "JRE" is the Java Runtime Environment. JRE is an implementation of the Java Virtual Machine which actually executes Java programs. Java Runtime Environment is used to execute through browser.
The "JDK" is the Java Development Kit. JDK is bundle of software that is use to develop Java based software. Typically, each JDK contains one (or more) JRE's along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers, development libraries, etc.

* What gives java it's "write once and run anywhere" nature? (JVM)
Java is compiled to be a BYTE CODE which is the intermediate language between SOURCE code and MACHINE code. This byte code is not platform specific and hence can be fed to any platform. After being fed to the JVM, which is specific to a particular operating system, the platform specific machine code is generated thus making java platform independent.

What environment variables do I need to set on my machine in order to be able to run Java programs? (environment)
A: CLASSPATH and PATH are the two variables.

* When does the compiler supply a DEFAULT constructor for a class?
If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided. If the class is the origin and does not have any super classes then the default constructor has an empty body. Otherwise, the default constructor takes no parameters and simply invokes the superclass constructor with no arguments.

* What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.


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