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

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Q11. Different types of memory used by JVM ?

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

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Q12. 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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Frequently asked to fresh graduates.

Q13. Is JVM, a compiler or interpretor ?

Ans. Its an interpretor.

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Q14. 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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Q15. 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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Q16. 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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Q17. 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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Q18. 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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Q19. What is metaspace ?

Core Java

Ans. The Permanent Generation (PermGen) space has completely been removed and is kind of replaced by a new space called Metaspace. The consequences of the PermGen removal is that obviously the PermSize and MaxPermSize JVM arguments are ignored and you will never get a java.lang.OutOfMemoryError: PermGen error.

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Q20. How to find if JVM is 32 or 64 bit from Java program. ?

Core Java

Ans. You can find JVM - 32 bit or 64 bit by using System.getProperty() from Java program.

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Q21. How substring() method of String class create memory leaks?

Core Java

Ans. substring method would build a new String object keeping a reference to the whole char array, to avoid copying it. Hence you can inadvertently keep a reference to a very big character array with just a one character string.

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Q22. 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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Q23. 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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Q24. Do we need to import java.lang package ?

Core Java

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

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Q25. Name few tools for probing Java Memory Leaks ?

Ans. JProbe, OptimizeIt

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Q26. Why is Java considered Portable Language ?

Core Java

Ans. Java is a portable-language because without any modification we can use Java byte-code in any platform(which supports Java). So this byte-code is portable and we can use in any other major platforms.

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Q27. What are the platforms supported by Java Programming Language?

Core Java

Ans. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc

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Q28. What environment variables do I need to set on my machine in order to be able to run Java programs?

Ans. CLASSPATH and PATH are the two variables.

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Q29. What is Java bytecode ?

Ans. Java bytecode is the instruction set of the Java virtual machine. Each bytecode is composed by one, or two bytes that represent the instruction, along with zero or more bytes for passing parameters.

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Q30. Is JVM a overhead ?

Ans. Yes and No. JVM is an extra layer that translates Byte Code into Machine Code. So Comparing to languages like C, Java provides an additional layer of translating the Source Code.

C++ Compiler - Source Code --> Machine Code
Java Compiler - Source Code --> Byte Code , JVM - Byte Code --> Machine Code

Though it looks like an overhead but this additional translation allows Java to run Apps on all platforms as JVM provides the translation to the Machine code as per the underlying Operating System.

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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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