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

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 Q1. What are different ways to create String Object? Explain.Core Java
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Ans. When we create a String using double quotes, JVM looks in the String pool to find if any other String is stored with same value. If found, it just returns the reference to that String object else it creates a new String object with given value and stores it in the String pool.

When we use new operator, JVM creates the String object but dont store it into the String Pool. We can use intern() method to store the String object into String pool or return the reference if there is already a String with equal value present in the pool.

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 Q2. Difference between JVM, JRE and JDK ?Core Java2017-10-31 08:36:22

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. Explain java.lang.OutOfMemoryError ?Core Java
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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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 Q4. What are various types of Class loaders used by JVM ?Core Java
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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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 Q5. What are strong, soft, weak and phantom references in Java ?Core Java
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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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 Q6. Can I import same package/class twice? Will the JVM load the package twice at runtime?Core Java
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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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 Q7. Different types of memory used by JVM ?Core Java
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Ans. Class , Heap , Stack , Register , Native Method Stack.

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 Q8. What is a class loader ? What are the different class loaders used by JVM ?Core Java
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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.
 Q9. Is JVM, a compiler or interpretor ?
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Ans. Its an interpretor.

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 Q10. Why do member variables have default values whereas local variables don't have any default value ?
Core Java
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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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 Q11. How are classes loaded by JVM ?Core Java
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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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 Q12. Difference between static vs. dynamic class loading?Core Java
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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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 Q13. Which memory areas does instance and static variables use ?Core Java
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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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 Q14. What is PermGen or Permanent Generation ?Core Java
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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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 Q15. What is metaspace ?Core Java
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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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 Q16. Can we call the garbage collector explicitly ?Core Java
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Ans. Yes, We can call garbage collector directly but it doesn't guarantees that the gc will start executing immediately. This gc( ) method appears in both Runtime and System classes of java.lang package.

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 Q17. How to find if JVM is 32 or 64 bit from Java program. ?
Core Java
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Ans. You can find JVM - 32 bit or 64 bit by using System.getProperty() from Java program.

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 Q18. How Java provide high Performance ?Core Java
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Ans. Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode into instructions that can be sent directly to the processor.

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 Q19. How substring() method of String class create memory leaks?Core Java
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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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 Q20. Difference between loadClass and Class.forName ?
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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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 Q21. Which kind of memory is used for storing object member variables and function local variables ?
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Ans. Local variables are stored in stack whereas object variables are stored in heap.

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 Q22. Why Java don't use pointers ?
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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
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Ans. No, It is loaded by default by the JVM.

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 Q24. Name few tools for probing Java Memory Leaks ?
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Ans. JProbe, OptimizeIt

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 Q25. Why is Java considered Portable Language ?Core Java
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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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 Q26. What are the platforms supported by Java Programming Language?Core Java
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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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 Q27. What environment variables do I need to set on my machine in order to be able to run Java programs?
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Ans. CLASSPATH and PATH are the two variables.

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 Q28. Why Java provides default constructor ?Core Java
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Ans. At the beginning of an object's life, the Java virtual machine (JVM) allocates memory on the heap to accommodate the object's instance variables. When that memory is first allocated, however, the data it contains is unpredictable. If the memory were used as is, the behavior of the object would also be unpredictable. To guard against such a scenario, Java makes certain that memory is initialized, at least to predictable default values before it is used by any code.

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 Q29. What is Java bytecode ?
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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 ?
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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 ?
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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
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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
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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
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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.
Anonymous
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 Java2016-06-13 15:40:56

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 Java2016-11-17 11:33:00

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 Java2016-11-17 11:36:33

Ans. Using Runtime object.

Runtime runtime = Runtime.getRuntime();

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

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Frequently asked in Alibaba (Based on 2 feedback)
 Q39. What is the best Memory setting for JVM ?Core Java2016-12-13 10:18:56

 This question was recently asked at 'Alibaba'.This question is still unanswered. Can you please provide an answer.


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 Q40. What is Shutdown hook ?Core Java2017-01-12 14:22:44

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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