how to destroy an object in java

Question

I encountered this question in an interview with following options      How to destroy an object in java      a  System gc      b  Runtime getRuntime gc      c  object delete      d  object finalize      e  Java performs gc by itself  no need to do it manually     The answer should be e   what if e was not there  then   clearly c is not the answer  a and b will do gc for the whole application question requires for one object    I think it is d because finalize   is called just prior to gc but is it necessary that after finalize gc is invoked    or I am wrong   e must be there to answer this question

User · Accepted Answer

Answer E is correct answer  If E is not there  you will soon run out of memory  or  No correct answer   Object should be unreachable to be eligible for GC  JVM will do multiple scans and moving objects from one generation to another generation to determine the eligibility of GC and frees the memory when the objects are not reachable

User · Answer

In java there is no explicit way doing garbage collection  The JVM itself runs some threads in the background checking for the objects that are not having any references which means all the ways through which we access the object are lost  On the other hand an object is also eligible for garbage collection if it runs out of scope that is the program in which we created the object is terminated or ended        Coming to your question the method finalize is same as the destructor in C    The finalize method is actually called just before the moment of clearing the object memory by the JVM  It is up to you to define the finalize method or not in your program  However if the garbage collection of the object is done after the program is terminated then the JVM will not invoke the finalize method which you defined in your program       You might ask what is the use of finalize method      For instance let us consider that you created an object which requires some  stream to external file and you explicitly defined a finalize method to this object which checks wether the stream opened to the file or not and if not it closes the stream  Suppose  after writing several lines of code you lost the reference to the object  Then it is eligible for garbage collection  When the JVM is about to free the space of your object the JVM just checks have you defined the finalize method or not and invokes the method so there is no risk of the opened stream  finalize method make the program risk free and more robust

User · Answer

Set to null  Then there are no references anymore and the object will become eligible for Garbage Collection  GC will automatically remove the object from the heap

User · Answer

Here is the code   public static void main String argso      int big array     new int 100000       Do some computations with big array and get a result   int result   compute big array       We no longer need big array  It will get garbage collected when there    are no more references to it  Since big array is a local variable     it refers to the array until this method returns  But this method    doesn t return  So we ve got to explicitly get rid of the reference    ourselves  so the garbage collector knows it can reclaim the array   big array   null      Loop forever  handling the user s input for     handle input result

User · Answer

To clarify why the other answers can not work    System gc    along with Runtime getRuntime   gc    which does the exact same thing  hints that you want stuff destroyed   Vaguely   The JVM is free to ignore requests to run a GC cycle  if it doesn t see the need for one   Plus  unless you ve nulled out all reachable references to the object  GC won t touch it anyway   So A and B are both disqualified  Runtime getRuntime gc   is bad grammar  getRuntime is a function  not a variable  you need parentheses after it to call it  So B is double-disqualified  Object has no delete method   So C is disqualified  While Object does have a finalize method  it doesn t destroy anything   Only the garbage collector can actually delete an object    And in many cases  they technically don t even bother to do that  they just don t copy it when they do the others  so it gets left behind    All finalize does is give an object a chance to clean up before the JVM discards it   What s more  you should never ever be calling finalize directly    As finalize is protected  the JVM won t let you call it on an arbitrary object anyway    So D is disqualified  Besides all that  object doAnythingAtAllEvenCommitSuicide   requires that running code have a reference to object   That alone makes it  alive  and thus ineligible for garbage collection   So C and D are double-disqualified

User · Answer

Short Answer - E  Answer isE given that the rest are plainly wrong  but     Long Answer - It isn t that simple  it depends      Simple fact is  the garbage collector may never decide to garbage collection every single object that is a viable candidate for collection  not unless memory pressure is extremely high  And then there is the fact that Java is just as susceptible to memory leaks as any other language  they are just harder to cause  and thus harder to find when you do cause them   The following article has many good details on how memory management works and doesn t work and what gets take up by what  How generational Garbage Collectors work and Thanks for the Memory   Understanding How the JVM uses Native Memory on Windows and Linux    If you read the links  I think you will get the idea that memory management in Java isn t as simple as a multiple choice question

[java] how to destroy an object in java?

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