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There are several methods to create Instance in Java
(Different ways are new operator, cloning, reflection and DE-serialization etc)
So among them which is the fastest of all ?
In such a case fastest means which one of those executes less operations before actually allocating the memory for the object. It is easy to determine that new is the fastest among those because it doesn't bear the overhead created from the others such as clone etc.
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I have a requirement where I read string from huge file and process it in HashMap. I want to throw exception when HashMap size is > 5GB. How do I set the max memory size of HashMap?
You subclass HashMap and override methods to keep track of what is allocated and freed, and throw the exception yourself.
However, be aware that determining how much memory an object occupies is itself a significant task... how deep do you go if the object contains references to other objects?
This is a non-trivial problem of deciding what you mean and what you want to track. There is no one-size-fits-all answer.
And, as suggested by #ModusTollens in a comment, this is likely an XY Problem.
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I understand we can do this with file input and output, but why would we want to do this?
It is simply called persistence.
You nailed it: you want to be able to store information (for example after intensive computations) in a way that survives the lifetime of the current JVM process.
In that sense serialization is a (poor) version of database storage.
But of course, that comment is correct: this does not prevent the creation of objects. It is a mechanism to resurrect previous state into "new" objects.
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What is the difference between Collectors & Collector. I know that one is interface & one is class. I want to know with example what is the actual difference and When to use which one with real time example.
Hi all I already mentioned in my question I know the basic difference and I gone through the documentation also, I just want to know the purpose to introduce this two thing with example and When should use which one?
Collectors is just a class with static methods which create commonly used Collectors.
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I have a list of objects which get created by a factory class and for the time being I want to limit the list size to 10 objects. I also need to maintain the list through out the program as multiple classes will be referencing its values. I don't want to create it as a singleton and im not happy about having a master class maintain it, so is there another way? (Best Practice)
For setting the max size you can refer to this post here Any way to set max size of a collection?
Not sure what you mean by "maintaining" the list though. There is no way (that I know of) to make that list accessible to all classes without having it in a "master class". The only other option is passing it through functions.
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I need a thread-safe Queue and my question is that are there any performance difference between the queue I implement using List and wait/notify (simplest implementation) and classes such ArrayBlockingQueue and etc.?
The question is too vague to be answerable. Here is my advice:
If there is a standard class that does the job, use it in preference to rolling out your own.
Profile your code on realistic inputs to see where the bottlenecks are.
Optimize as appropriate.