Core Java - Interview Questions and Answers for 'Jvm' | Search Interview Question

Core Java - Interview Questions and Answers for 'Jvm' - 47 question(s) found - Order By Rating

Q1. What are different methods to get the permanent gen space of running application?

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Q2. Difference between JVM, JRE and JDK ?

Core Java

Ans. JDK(Java Development kit) = Development Kit comprising of JVM , library and development tools for developers

JRE (Java Run time Environment) - Comprise of JVM and set of libraries

JVM(Java Virtual Machine) = Interpreter which reads the .class file line by line.

When we install JDK, JRE also get installed so we can write,compile and excute our code. Used by developer. Without JDK we can only execute the program using JRE.

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Q3. Why is JVM called Virtual machine ?

Core Java

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Q4. In which memory segment - heap or stack, the following stored by JVM

1. static members
2. objects
3. object references
4. local or method variables

Core Java

Ans. static members are stored in method area of heap

objects are stored on heap

object references are stored on stack

local or method variables are stored on stack

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Q5. Is JDK required on each machine where a Java application needs to be run ?

Core Java

Ans. No, to run application, machine needs just a JRE (Java Runtime Environment)

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Q6. Can we compile and execute a Java class without main method ?

Core Java

Ans. No without main method can not be executed it will throw an error illegal start of type. Main method is the entry point of application.

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Q7. What is JVM ?

Core Java

Ans. JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is createdThe JVM performs following operation:Loads codeVerifies codeExecutes codeProvides runtime environment

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Q8. What is an execution engine within JVM ?

Core Java

Ans. Execution engine contains interpreter and JIT compiler, which covert byte code into machine code.

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Q9. What is Shutdown hook ?

Core Java

Ans. Shutdown hook is a thread that is invoked before the JVM is shut down. we can use it perform resource cleaning task.

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Q10. What is the best Memory setting for JVM ?

Core Java

Ans. In Java JVM memory settings is done by use the arguments -Xms -Xmx. Use M or G after the numbers for indicating Megs and Gigs of bytes respectively. -Xms indicates the minimum and -Xmx the maximum.

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Q11. How can we find the memory usage of JVM from Java code?

Core Java

Ans. Using Runtime object.

Runtime runtime = Runtime.getRuntime();

runtime.totalMemory();
runtime.freeMemory();

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Q12. How can one determine if JVM is 32-bit or 64-bit from Java Program ?

Core Java

Ans. There is a Java system property "sun.arch.data.model" that can tell if JVM is 32 bit of 64 bit

System.getProperty("sun.arch.data.model") can be used to get that property.

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Q13. What are the disadvantages of running multiple apps in single JVM ?

Core Java

Ans. 1. Issue with Jar and ClassPath Conflicts

2. Killing JVM will terminate all applications

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Q14. Can I import same package/class twice? Will the JVM load the package twice at runtime?

Core Java

Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

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Q15. Different types of memory used by JVM ?

Core Java

Ans. Class , Heap , Stack , Register , Native Method Stack.

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Q16. What is a class loader ? What are the different class loaders used by JVM ?

Core Java

Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

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Q17. Explain java.lang.OutOfMemoryError ?

Core Java

Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.

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Q18. Is JVM, a compiler or interpretor ?

Ans. Its an interpretor.

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Q19. Difference between loadClass and Class.forName ?

Ans. loadClass only loads the class but doesn't initialize the object whereas Class.forName initialize the object after loading it.

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Q20. Which kind of memory is used for storing object member variables and function local variables ?

Ans. Local variables are stored in stack whereas object variables are stored in heap.

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Q21. Why do member variables have default values whereas local variables don't have any default value ?

Core Java

Ans. member variable are loaded into heap, so they are initialized with default values when an instance of a class is created. In case of local variables, they are stored in stack until they are being used.

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Q22. Why Java don't use pointers ?

Ans. Pointers are vulnerable and slight carelessness in their use may result in memory problems and hence Java intrinsically manage their use.

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Q23. Do we need to import java.lang package ?

Core Java

Ans. No, It is loaded by default by the JVM.

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Q24. What are various types of Class loaders used by JVM ?

Core Java

Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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Q25. How are classes loaded by JVM ?

Core Java

Ans. Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.

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Q26. Difference between static vs. dynamic class loading?

Core Java

Ans. static loading - Classes are statically loaded with Java new operator.

dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.

Class.forName (Test className);

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Q27. What are strong, soft, weak and phantom references in Java ?

Core Java

Ans. Garbage Collector wont remove a strong reference.

A soft reference will only get removed if memory is low.

A weak reference will get removed on the next garbage collection cycle.

A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected.

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Q28. Name few tools for probing Java Memory Leaks ?

Ans. JProbe, OptimizeIt

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Q29. Which memory areas does instance and static variables use ?

Core Java

Ans. instance variables and objects are stored on heap and the references are stored on stack whereas static variables are stored in the method area of heap.

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Q30. What is PermGen or Permanent Generation ?

Core Java

Ans. The memory pool containing all the reflective data of the java virtual machine itself, such as class and method objects. With Java VMs that use class data sharing, this generation is divided into read-only and read-write areas. The Permanent generation contains metadata required by the JVM to describe the classes and methods used in the application. The permanent generation is populated by the JVM at runtime based on classes in use by the application. In addition, Java SE library classes and methods may be stored here.

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