To put it differently, what do I gain by using Stream.Builder.add() to add items to the builder and then using Stream.Builder.build(), versus adding the items in a collection or array and creating a Stream from that?
I assume there is a benefit somewhere in some circumstances but it's not obvious to me...
Assuming the machine has enough memory, using Stream.Builder allows to add more than Integer.MAX_VALUE elements to it.
Internally, Stream.Builder uses a SpinedBuffer, which is a non public class.
From SpinedBuffer docs:
An ordered collection of elements. Elements can be added, but not removed.
Goes through a building phase, during which elements can be added, and a
traversal phase, during which elements can be traversed in order but no
further modifications are possible.
One or more arrays are used to store elements. The use of a multiple
arrays has better performance characteristics than a single array used by
ArrayList, as when the capacity of the list needs to be increased
no copying of elements is required. This is usually beneficial in the case
where the results will be traversed a small number of times.
So, it also avoids ArrayList resizing.
It has a very interesting internal data structure that is highly optimized for insertions, without the possibility of removal or random access.
Your array and/or Collection pay an additional price for all other functionality that they support.
Related
I must work with a Collection, and I am not sure about using a List or a Set. This collection must be sorted, but not by the order of insertion but for another one, so each time a new item is added, a Comparator should be executed in order to reorder the Collection. So, for this reason, an ArrayList could be the best option.
Removing objects from that Collection must be possible too, furthermore, I would really appreciate using removeIf method, so a Set would be the best option here.
Getting and iterating over the Collection will be the most repeated scenario, so it must have a good performance in this scenario.
Seeing that, I think that a Set would be a good decision, however, I was thinking about converting the Set into a List when adding items, then, once the list has been resorted, converting it back to a Set. Is it bad performing? What do you think?
Thanks in advance
Unless you have bulk inserts during which you would need no sorting, TreeSet is fine. Simply measure both solutions.
With TreeSet inserting already ordered items, like rereading a set from disk, performs bad in that even a balanced tree, will have a bit too large depth. That however can be remedied.
For better performance you might go for a B-tree (needs 3rd party code) instead of the binary TreeSet. Measure that too, as typically a facet such as deletion with rebalancing might be done suboptimally.
This depends a lot on how you fill and use your collection and performance of which operation is the most important.
Do you fill the collection with items at once? Or add new elements from time to time? Does the performance of adding elements matter? Or only the iteration performance is important?
If performance is critical, it might make sense to implement a few solutions and compare their performance using a benchmark.
I personally don't believe that iteration performance of a TreeSet is that much worse that ArrayLists or LinkedLists or LinkedHashMaps. Especially compared to linked data structures. Iteration on a tree should not be that different in the performance. But I have no data, so this is just a belief here.
Below are two implementation ideas.
First, if you load a lot of data at once and then add new items rather seldom, load the data into an ArrayList and sort it using Collections.sort. If you need to add another item do a binary search (Collections.binarySearch) and insert the element at the corresponding position. Wrap it all in a custom List implementation and you're good to go.
Next, if you fill the collection with the data "in the beginning" and then the collection is hardly modified, you may simply cache the iteration order in an ArrayList. Every time the collection is modified, reset this list and. When iteration is requested and the list is not null, just use it, otherwise first fill it in the order of the sorted set.
Currently, I have a LinkedList which stores a custom Node class. The Nodes are currently removed in order and evaluated, which generally adds more Nodes back into the LinkedList, treating it like a Queue.
But in reality I don't care about maintaining the order of the Nodes because the order they are being added or removed doesn't matter. You can remove the 1st, 54th, or 1032nd Node from the List, it doesn't matter. All that matters is the Nodes are being processed quickly, which means one is removed (at random), mutated, then added back along with several variations of it (once again the order doesn't matter).
Since I haven't been able to find a Java Bag implementation, what is the most efficient way to maintain this type of collection ?
PS Out of laziness I have avoid using arrays because the collection of nodes could theoretically range from 1 Node to 3^64 Nodes in size, though it's more likely to stay under a million.
The Java HashSet or TreeSet types might be good here, since they represent unordered collections of elements that support quick insertion and deletion of elements. That said, you can't possibly hold 364 values in memory, since that's appromately 3.4336838 × 1030, a number vastly bigger than any amount of RAM that I know of can hold.
EDIT: Based on the described use case (support efficient insertion and removal of random elements), you might want to adopt the approach described in this older question for building a data structure that does just that. Intuitively, you would use an ArrayList, then remove elements by swapping them to the end of the ArrayList and removing them. This gives O(1) insertion and O(1) removal with extremely low overhead.
Hope this helps!
I'm returning large numbers of collections from a DTO object and was wondering if anyone could point me in right direction. Any type of collection will do, but I don't know which one is best suited for the task of returning a large number of objects.
I know this can change based on threading and the like, but I'm at least looking for general guidance and benchmarks. Also, I'm required to stay within standard Java collections (no third-party libraries).
As irreputable says: If you need a simple collection, than ArrayList should perform good because it is based on an Array which is fast by definition using the System functions.
If you set the initial capacity to a higher value (don't know what you call a large number), than it will be even faster because it reduces the amount of incremental reallocation.
Any other collection has some kind of an overhead like looking for hashcodes or beeing synchronized.
An ArrayList initialized at the correct size (if you know how many DTOs you'll be adding, or an upper bound) is the simplest and smallest Collection you'll find. By setting its size at initialization, it won't need to resize its internal array, an operation which produces garbage. It's better that directly using an array, which is really low level, and which you'd need to manage manually if it needs resizing (that's what the ArrayList does for you).
To create a pre-sized ArrayList, use the ArrayList(int capacity) constructor.
I'm a student and fairly new to Java. I was looking over the different speeds achieved by the two collections in Java, Linked List, and ArrayList. I know that an ArrayList is much much faster at looking up and placing in values into its indexes. My question is:
how can one make a linked list faster, if at all possible?
Thanks for any help.
zmahir
When talking about speed, perhaps you mean complexity. Insertion and retrieval operations for ArrayList (and arrays) are O(1), while for LinkedList they are O(n). And this cannot be changed - it is 'by definition'.
O(n) means that in order to insert an object at a given position, or retrieve it, you must traverse, in the worst case, all (n) the items in the list. Hence n operations. For ArrayList this is only one operation.
You probably can't. You don't know the size (well, ok you can), nor the location of each element. To find element 100 in a linked list, you need to start with item 1, find it's link to item 2, etc. until you find 100. This makes inserting into this list a tedious job.
There are many alternatives depending on your exact goals. You can use b-trees or similar methods to split the large linked list into smaller ones. Or use hashlists if you want to quickly find items. Or use simple arrays. But if you want a list that performs like an ArrayList, why not use an ArrayList?
You can split off regions which are linked to the main linked list, so this gives you entry points directly inside the list so you don't have to walk up to them. See the subList method here: http://download.oracle.com/javase/1.4.2/docs/api/java/util/AbstractList.html. This is useful if you have a number of 'sentences' made out of words, say. You can use a separate linked list to iterate over the sentences, which are sublists of the main linked list.
You can also use a ListIterator when adding, removing, or accessing elements. This helps greatly with increasing the speed of sequential access. See the listIterator method for this, and the class: http://download.oracle.com/javase/1.4.2/docs/api/java/util/ListIterator.html.
Speed of a linked list could be improved by using skip lists: http://igoro.com/archive/skip-lists-are-fascinating/
a linked list uses pointers to walk through the items, so for example if you asked for the 5th item, the runtime will start from the first item and walks through each pointer until it reaches the 5th item.
there is really not much you can do about it. a linked list may not be a good choice if you need fast acces to items. although there are some optimizations for it such as creating a circular linked list or a double linked list where you can walk back and forth the list but this really depends on the business logic and the application requirements.
my advise is to avoid linked lists if it does not match your needs and changing to a different data structure might be the best approach.
As a general rule, data structures are designed to do certain things well. LinkedLists are designed to be faster than ArrayLists at inserting elements and removing elements and about the same as ArrayLists at iterating across the list in order. When you change the way a LinkedList works, you make it no longer a true LinkedList, so there's not really any way to modify them to be faster at something and still be a LinkedList.
You'll need to examine the way you're using this particular collection and decide whether a LinkedList is really the best data structure for your purposes. If you share with us how you're using it, and why you need it to be faster, then we can advise you on which data structure you ought to consider using.
Lots of people smarter than you or I have looked at the implementation of the Java collection classes. If there were an optimization to be made, they would have found it and already made it.
Since the collection classes are pretty much as optimized as they can be, our primary task should be to choose the correct one.
When choosing your collection type, don't forget about things like HashSet. If order doesn't matter, and you don't need to put duplicates in the collection, then HashSet may be appropriate.
I'm a student and fairly new to Java. ... how can one make a linked list faster, if at all possible?
The standard Java collection type (indeed all data structures implemented in any language!) represent compromises on various "measures" such as:
The amount of memory needed to represent the data structure.
The time taken to perform various operations; e.g. for a "list" the operations of interest are insertion, removal, indexing, contains, iteration and so on.
How easy or hard it is to integrate / reuse the collection type; see below.
So for instance:
ArrayList offers lower memory overheads, fast indexing (O(1)), but slow contains, random insertion and removal (O(N)).
LinkedList has higher memory overheads, slow indexing and contains (O(N)), but faster removal (O(1)) under certain circumstances.
The various performance measures are typically determines by the maths of the various data structures. For example, if you have a chain of nodes, the only way to get the ith node is to step through them from the beginning. This involves following i pointers.
Sometimes you can modify the data structures to improve one aspect of the performance. But this typically comes at the cost of some other aspect of the performance. (For example, you could add a separate index to make indexing of a linked list faster. But the cost of maintaining the index on insertion / deletion would mean that you'd probably be better of using an ArrayList.)
In some cases the integration / reuse requirements have significant impact on performance.
For example, it is theoretically possible to optimize a linked list's space usage by adding a next field to the list element type, combining the element and node objects and saving 16 or so bytes per list entry. However, this would make the list type less general (the member/element class would need to implement a specific interface), and has the restriction that an element can belong to at most one list at any time. These restrictions are so limiting that this approach is rarely used in Java.
For a second example, consider the problem of inserting at a given position in a linked list. For the LinkedList class, this is normally an O(N) operation, because you have to step through the list to find the position. In theory, if an application could find and remember a position, it should be able to perform the insertion at that position in O(1). Unfortunately, neither the List APIs provides no way to "remember" a position.
While neither of these examples is a fundamental roadblock to a developer "doing his own thing", they illustrate that using general data structure APIs and general implementations of those APIs has performance implications, and therefore represents a trade-off between performance and ease-of-use.
I'm a bit surprised by the answers here. There are big difference between the theoretical performance of LinkedLists and ArrayLists compared to the actual performance of the Java implementations.
What makes the Java LinkedList slower than a theoretical LinkedList is that it does a lot more than just the operations. For example it checks for concurrent modifications and other safeties.
If you know your use case, you can write a your own simple implementation of a LinkedList and it will be much faster.
I need a class that implements Iterable, and does not need to be safe for concurrent usage. Of the various options, such as LinkedList, HashSet, ArrayList etc, which is the lightest-weight?
To clarify the use-case, I need to be able to add a number of objects to the Iterable (typically 3 or 4), and then something else needs to iterate over it.
ArrayList. From the Javadoc
The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.
That entirely depends on what you mean by "lightest weight". What operations do you need to do, and how often? Do you know the final size beforehand? Are you trying to save execution time or memory?
I would agree that zkarthik that ArrayList is very often a good choice... but it will behave very badly if you want to create a large collection and then repeatedly remove the first element, for example. There's a good reason for there being so many different collections: they have different performance characteristics for different situations.
They all have very different features and behavior, so you should base your choice on how you will use them. For example, for random access and high locality, use an ArrayList; if you need fast unordered insertion and querying, use a HashSet.
If by 'lightweight', you mean 'best performance' then the question is almost impossible to answer without understanding how the collection will be used. All you've told us so for is that it doesn't need to support concurrent usage, but in order to have any hope of answering the question we'd need to know things like
How many objects will be stored in the collection (on average)
What is the relative frequency of read and write access
Is random-access required
Is ordered access required
A number of people have suggested ArrayList may be best. However, I seem to recall reading (possibly in Effective Java 2nd edition), that for certain patterns of usage, Queue performs better than List, because it does not incurr the penalty of random access. In other words, you can add/remove items from a List in any order, but you can only add/remove items in a queue in a specific order (i.e. add to the tail, and remove from the head).