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Do Java arrays have a maximum size?
(10 answers)
Closed 6 years ago.
How many integers can an array in java and python have? And if I want to enter more integers to an array and I reached that limit how to handle this?
Java arrays can have 2^31-1 element in theory, although some JVM cannot allocate an object larger than 2^31-8 elements due to OS limitations. (See the ArrayList code for an example of this limit in practice)
If you have more elements, you can either
have an array of arrays.
use a memory mapped file to store the data. This has the advantage of potentially greater than main memory and is persisted.
Related
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Theoretical limit for number of keys (objects) that can be stored in a HashMap?
(4 answers)
Closed 5 years ago.
I was reading about HashMap. HashCodereturns int value. What if i have Huge Huge HashMap, which needs to store more objects than int range. Consider that for every object HashCode() method will returns unique value. In this case what will happen
Is any exception thrown ? Or
It will behave randomly?
You mean storing more than 2 billion entries? Java collections or maps can't do this, their size is always an int value.
There are 3rd party libraries for huge maps.
Are you sure you can store these many objects in memory anyway? One object takes at least 24 bytes (you will be out of the range of Compressed OOPS), so you will be using beyond 100 gigabytes of RAM, and that is with very small objects stored in the HashMap.
PS: I don't understand what you mean with "hashCode returning a unique value". Hash codes don't have to be unique. For a 2+ billion entry hash map, a 32 bit hash code is a bit weak, but still theoretically possible.
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How can I increase the JVM memory? [duplicate]
(4 answers)
Closed 6 years ago.
I am currently working my way through a project, and came across an issue. The program I am writing requires big numbers, bigger than int can hold. So, I used Math.BigInteger. After adding many BigIntegers together, I got a OutofMemory error. I am assuming that means the data type is not large enough. Is there a larger data type, and if not, how can I resolve this issue?
Is there a variable type bigger than BigInteger?
No there isn't. Biginteger uses byte arrays behind the scenes and just allocates more space for it whenever it needs it. It's size limit is the available memory.
You will probably hit the limit when you have used half of memory. At that point if it needs to grow the number it has it must first allocate a space bigger than the space it is using and run out.
The error occurs because JVM is running out of heap space. You can try increasing the heap size using
java -Xmx256m MyClass.java
Xmx- Max Heap Size
It would be better if you pasted your code here, but I'll try to give an answer with what you've posted so far.
Java's BigIntegers are made as large as necessary to house the result of whatever operation you're using. BigIntegers are just like Integers without the overflow exceptions. From oracle docs:
Semantics of arithmetic operations exactly mimic those of Java's integer arithmetic operators, as defined in The Java Language Specification.
Most likely you're not allocating enough memory to your java process. Not sure how you're executing your java processes, but you're looking for these arguments to be provided:
java -Xmx1024M -Xms1024M
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Finding out the minimum difference between elements in an array
(9 answers)
Closed 7 years ago.
Well I have to find the smallest difference between any two elements of a large array. By large i mean really large like 10 million or more elements array. Although this is not a practical scenario i like to know the fastest possible ones. Getting an element, comparing with the rest of the elements , sorting the difference and getting the least difference is really unimaginable. Is there some efficient way to do this task? I would be happy.
How fast this can be done depends entirely on your notion of 'difference' - for example, if you just have an array of numbers and want the smallest difference between two numbers (that is, you have a total ordering available for your elements) then you can just sort and get the smallest difference between neighbors. If there is no total ordering, you'll have to minimize your checks by clustering your elements somehow (e.g. creating a spatial tree for an array of 3D points).
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Why start an ArrayList with an initial capacity?
(11 answers)
Closed 8 years ago.
Is it considered a best pratice to initialize Collections ( eg: Arraylist ) with initial capacity ?
From my observation through docjar, it looked like initialization reduced need for 'expansion' of initial array and also reduce the size, of internal datastructure due to resizing.
As the comment mentions, if you know the number of elements that will be eventually needed, then it will gain you some performance since you do not have to re-allocate the backing array when the number of elements increases to exceed the current capacity.
However, there is a tradeoff because if you can't estimate it well (within an order of magnitude), then you may waste memory by allocating too much at the beginning.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
How many data a list can hold at the maximum
What is the maximum number of elements a list can hold?
Assuming you mean implementations of the java.util.List interface, methods like get() and size() use int, so the upper theoretical boundary would be Integer.MAX_VALUE entries. You might run out of memory before you reach this limit though!
The index type in Java arrays is int too, so you're definitely limited to Integer.MAX_VALUE entries for regular arrays.
If you're talking about an ArrayList, they're indexed using integers (which are always signed in Java), so they can theoretically hold 2^31 elements (not 2^32). At that point you're probably going to have memory issues anyway.
LinkedList is also an implementation of List which stores the elements as a Linked List. So theoretically its size is equivalent to the amount of memory you can allocate.