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

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Q31. What is Byte Code ? Why Java's intermediary Code is called Byte Code ?

Ans. Bytecode is a highly optimized set of instructions designed to be executed by the Java run-time system. Its called Byte Code because each instruction is of 1-2 bytes.

Sample instructions in Byte Code -

1: istore_1
2: iload_1
3: sipush 1000
6: if_icmpge 44
9: iconst_2
10: istore_2

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Q32. Difference between PATH and CLASSPATH ?

Operating System

Ans. PATH is the variable that holds the directories for the OS to look for executables. CLASSPATH is the variable that holds the directories for JVM to look for .class files ( Byte Code ).

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Q33. Should we create system software ( e.g Operating system ) in Java ?

Core Java

Ans. No, Java runs on a virtual machine called JVM and hence doesn't embed well with the underlying hardware. Though we can create a platform independent system software but that would be really slow and that's what we would never need.

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Q34. What is OutOfMemoryError in Java?

Core Java

Ans. OutOfMemoryError in Java is a subclass of VirtualMachineError and is thrown by JVM when it runs out of heap memory.

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Q35. Which is of the following is NOT TRUE for JVM ?

a. JVM reads Byte Code and generates Machine Code.
b. JVM is a virtual Machine that acts as a intermediary between Java Application and Host Operating System.
c. JVM reads Source Code and generates Byte Code.
d. JVM acts as a translator that translates different Machine code ( on the basis of Host Machine ) for a common Byte Code.

Ans. JVM reads Source Code and generates Byte Code.

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Q36. 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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Q37. 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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Q38. 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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Q39. 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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Q40. 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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Q41. 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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Q42. 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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Q43. 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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Q44. 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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Q45. 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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Q46. Why is JVM called Virtual machine ?

Core Java

^{This question is still unanswered. Can you please provide an answer.}

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Q47. What are different methods to get the permanent gen space of running application?

Core Java

^{This question was recently asked at 'Apcolite'.This question is still unanswered. Can you please provide an answer.}

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