FQA ON GARBAGE COLLECTION IN JAVA


INTERVIEW QUESTIONS ON GARBAGE COLLECTION IN JAVA
  1.  What is structure of Java Heap 
When a Java program started Java Virtual Machine gets some memory from Operating System. Java Virtual Machine or JVM uses this memory for all its need and part of this memory is call java heap memory. Heap in Java generally located at bottom of address space and move upwards. whenever we create an object using new operator or by any another means the object is allocated memory from Heap and When object dies or garbage collected, memory goes back to Heap space in Java.

  1. When does an Object becomes eligible for Garbage collection in Java ?
Answer : An object becomes eligible for garbage collection when there is no live reference for that object or it can not be reached by any live thread. Cyclic reference doesn’t count as live reference and if two objects are pointing to each other and there is no live reference for any of them, than both are eligible for GC. Also Garbage collection thread is a daemon threadwhich will run by JVM based upon GC algorithm and when runs it collects all objects which are eligible for GC.


  1. What is finalize method in Java ? When does Garbage collector calls finalize method in Java ?
Answer : Finalize method in Java also called finalizer is a method defined in java.lang.Object and called by Garbage collector before collecting any object which is eligible for GC. Finalize() method provides last chance to object to do cleanup and free any remaining resource, to learn more about finalizers, read What is finalize method in Java.
  1. If Object A has reference to Object B and Object B refer to Object A, apart from that there is no live reference to either object A or B, Does they are eligible to Garbage collection ?
This Garbage collection interview questions is related question 5 “When object become eligible for Garbage collection”. An object becomes eligible for Garbage collection if there is no live reference for it. It can not be accessible from any Thread and cyclic dependency doesn’t prevent Object from being Garbage collected. Which means in this case both Object A and Object B are eligible of Garbage collection.

  1. Can we force Garbage collector to run at any time ?
Answer : No, you can not force Garbage collection in Java. Though you can request it by calling Sytem.gc() or its cousin Runtime.getRuntime().gc(). It’s not guaranteed that GC will run immediately as result of calling these method.
6.       Which part of the memory is involved in Garbage Collection? Stack or Heap?
Ans) Heap
7.      What is responsiblity of Garbage Collector?
Ans) Garbage collector frees the memory occupied by the unreachable objects during the java program by deleting these unreachable objects.
It ensures that the available memory will be used efficiently, but does not guarantee that there will be sufficient memory for the program to run.
8.     Is garbage collector a daemon thread?
Ans) Yes GC is a daemon thread. A daemon thread runs behind the application. It is started by JVM. The thread stops when all non-daemon threads stop.
9.      How is Garbage Collection managed?
Ans)The JVM controls the Garbage Collector; it decides when to run the Garbage Collector. JVM runs the Garbage Collector when it realizes that the memory is running low. The behavior of GC can be tuned by passing parameters to JVM. One can request the Garbage Collection to happen from within the java program but there is no guarantee that this request will be taken care of by jvm.
  1.  When does an object become eligible for garbage collection?
Ans) An object becomes eligible for Garbage Collection when no live thread can access it.
11.    What is the purpose of overriding finalize() method?
Ans) The finalize() method should be overridden for an object to include the clean up code or to dispose of the system resources that should to be done before the object is garbage collected.
12.  How many times does the garbage collector calls the finalize() method for an object?
Ans) Only once.
13.  What happens if an uncaught exception is thrown from during the execution of the finalize() method of an object?
Ans) The exception will be ignored and the garbage collection (finalization) of that object terminates.
14.   What are different ways to call garbage collector?
Ans) Garbage collection can be invoked using System.gc() or Runtime.getRuntime().gc().
15.    How to enable/disable call of finalize() method of exit of the application
Ans) Runtime.getRuntime().runFinalizersOnExit(boolean value) . Passing the boolean value will either disable or enable the finalize() call.

16. What is the purpose of garbage collection in Java, and when is it used ?

The purpose of garbage collection is to identify and discard those objects that are no longer needed by the application, in order for the resources to be reclaimed and reused.

17. What does System.gc() and Runtime.gc() methods do ?

These methods can be used as a hint to the JVM, in order to start a garbage collection. However, this it is up to the Java Virtual Machine (JVM) to start the garbage collection immediately or later in time.

18.When is the finalize() called ? What is the purpose of finalization ?

The finalize method is called by the garbage collector, just before releasing the object’s memory. It is normally advised to release resources held by the object inside the finalize method.

19. If an object reference is set to null, will the Garbage Collector immediately free the memory held by that object ?

No, the object will be available for garbage collection in the next cycle of the garbage collector.
20. What Is The Algorithm Jvm Internally Uses For Destroying Objects?
"mark and swap" is the algorithm JVM internally uses.









 

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