1. What is difference between JDK,JRE and JVM?

JVM

• JVM is an acronym for Virtual Machine, it is an abstract machine which provides the runtime environment in which bytecode can be executed. It is a specification.
• JVMs are available for many hardware and software platforms (so JVM is platform dependent).

JRE

• JRE stands for Runtime Environment. It is the implementation of JVM.

JDK

• JDK is an acronym for Development Kit. It physically exists. It contains JRE + development tools.

2. How many types of memory areas are allocated by JVM?

Many types:

• Class(Method) Area
• Heap
• Stack
• Program Counter Register
• Native Method Stack

3. What is JIT compiler?

Just-In-Time(JIT) compiler:It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term “compiler” refers to a translator from the instruction set of a virtual machine (JVM) to the instruction set of a specific CPU.

4. What is platform?

A platform is basically the hardware or software environment in which a program runs. There are two types of platforms software-based and hardware-based. provides software-based platform.

5. What is the main difference between platform and other platforms?

The platform differs from most other platforms in the sense that it’s a software-based platform that runs on top of other hardware-based platforms.It has two components:

• Runtime Environment
• API(Application Programming Interface)

We use the class setDaemon(true) to create this thread. We call this method before the start() method, and else we get IllegalThreadStateException.

7. What is classloader?

The classloader is a subsystem of JVM that is used to load classes and interfaces.There are many types of classloaders e.g. Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc.

8. What is the default value of the local variables?

The local variables are not initialized to any default value, neither primitives nor object references.

9.Garbage collector thread is what kind of a thread?

t is a daemon thread. It is a low priority thread running in the background.

10.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 about it.But the JVM will internally load the class only once no matter how many times you import the same class.