What are the big advantages from JMS over Webservices or vice versa?
(Are webservices bloated? Is JMS overall better for providing interfaces?)
javajmsweb services
What are the big advantages from JMS over Webservices or vice versa?
(Are webservices bloated? Is JMS overall better for providing interfaces?)
Immutable means that once the constructor for an object has completed execution that instance can't be altered.
This is useful as it means you can pass references to the object around, without worrying that someone else is going to change its contents. Especially when dealing with concurrency, there are no locking issues with objects that never change
e.g.
class Foo
{
private final String myvar;
public Foo(final String initialValue)
{
this.myvar = initialValue;
}
public String getValue()
{
return this.myvar;
}
}
Foo
doesn't have to worry that the caller to getValue()
might change the text in the string.
If you imagine a similar class to Foo
, but with a StringBuilder
rather than a String
as a member, you can see that a caller to getValue()
would be able to alter the StringBuilder
attribute of a Foo
instance.
Also beware of the different kinds of immutability you might find: Eric Lippert wrote a blog article about this. Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state (and therefore can't be shared safely between threads).
Summary ArrayList
with ArrayDeque
are preferable in many more use-cases than LinkedList
. If you're not sure — just start with ArrayList
.
TLDR, in ArrayList accessing an element takes constant time [O(1)] and adding an element takes O(n) time [worst case]. In LinkedList adding an element takes O(n) time and accessing also takes O(n) time but LinkedList uses more memory than ArrayList.
LinkedList
and ArrayList
are two different implementations of the List interface. LinkedList
implements it with a doubly-linked list. ArrayList
implements it with a dynamically re-sizing array.
As with standard linked list and array operations, the various methods will have different algorithmic runtimes.
For LinkedList<E>
get(int index)
is O(n) (with n/4 steps on average), but O(1) when index = 0
or index = list.size() - 1
(in this case, you can also use getFirst()
and getLast()
). One of the main benefits of LinkedList<E>
add(int index, E element)
is O(n) (with n/4 steps on average), but O(1) when index = 0
or index = list.size() - 1
(in this case, you can also use addFirst()
and addLast()
/add()
). One of the main benefits of LinkedList<E>
remove(int index)
is O(n) (with n/4 steps on average), but O(1) when index = 0
or index = list.size() - 1
(in this case, you can also use removeFirst()
and removeLast()
). One of the main benefits of LinkedList<E>
Iterator.remove()
is O(1). One of the main benefits of LinkedList<E>
ListIterator.add(E element)
is O(1). One of the main benefits of LinkedList<E>
Note: Many of the operations need n/4 steps on average, constant number of steps in the best case (e.g. index = 0), and n/2 steps in worst case (middle of list)
For ArrayList<E>
get(int index)
is O(1). Main benefit of ArrayList<E>
add(E element)
is O(1) amortized, but O(n) worst-case since the array must be resized and copiedadd(int index, E element)
is O(n) (with n/2 steps on average)remove(int index)
is O(n) (with n/2 steps on average)Iterator.remove()
is O(n) (with n/2 steps on average)ListIterator.add(E element)
is O(n) (with n/2 steps on average)Note: Many of the operations need n/2 steps on average, constant number of steps in the best case (end of list), n steps in the worst case (start of list)
LinkedList<E>
allows for constant-time insertions or removals using iterators, but only sequential access of elements. In other words, you can walk the list forwards or backwards, but finding a position in the list takes time proportional to the size of the list. Javadoc says "operations that index into the list will traverse the list from the beginning or the end, whichever is closer", so those methods are O(n) (n/4 steps) on average, though O(1) for index = 0
.
ArrayList<E>
, on the other hand, allow fast random read access, so you can grab any element in constant time. But adding or removing from anywhere but the end requires shifting all the latter elements over, either to make an opening or fill the gap. Also, if you add more elements than the capacity of the underlying array, a new array (1.5 times the size) is allocated, and the old array is copied to the new one, so adding to an ArrayList
is O(n) in the worst case but constant on average.
So depending on the operations you intend to do, you should choose the implementations accordingly. Iterating over either kind of List is practically equally cheap. (Iterating over an ArrayList
is technically faster, but unless you're doing something really performance-sensitive, you shouldn't worry about this -- they're both constants.)
The main benefits of using a LinkedList
arise when you re-use existing iterators to insert and remove elements. These operations can then be done in O(1) by changing the list locally only. In an array list, the remainder of the array needs to be moved (i.e. copied). On the other side, seeking in a LinkedList
means following the links in O(n) (n/2 steps) for worst case, whereas in an ArrayList
the desired position can be computed mathematically and accessed in O(1).
Another benefit of using a LinkedList
arises when you add or remove from the head of the list, since those operations are O(1), while they are O(n) for ArrayList
. Note that ArrayDeque
may be a good alternative to LinkedList
for adding and removing from the head, but it is not a List
.
Also, if you have large lists, keep in mind that memory usage is also different. Each element of a LinkedList
has more overhead since pointers to the next and previous elements are also stored. ArrayLists
don't have this overhead. However, ArrayLists
take up as much memory as is allocated for the capacity, regardless of whether elements have actually been added.
The default initial capacity of an ArrayList
is pretty small (10 from Java 1.4 - 1.8). But since the underlying implementation is an array, the array must be resized if you add a lot of elements. To avoid the high cost of resizing when you know you're going to add a lot of elements, construct the ArrayList
with a higher initial capacity.
If the data structures perspective is used to understand the two structures, a LinkedList is basically a sequential data structure which contains a head Node. The Node is a wrapper for two components : a value of type T [accepted through generics] and another reference to the Node linked to it. So, we can assert it is a recursive data structure (a Node contains another Node which has another Node and so on...). Addition of elements takes linear time in LinkedList as stated above.
An ArrayList, is a growable array. It is just like a regular array. Under the hood, when an element is added at index i, it creates another array with a size which is 1 greater than previous size (So in general, when n elements are to be added to an ArrayList, a new array of size previous size plus n is created). The elements are then copied from previous array to new one and the elements that are to be added are also placed at the specified indices.
Best Answer
EDITED after correction from erickson:
JMS requires that you have a JMS provider, a Java class that implements the MessageListener interface for processing messages, and a client that knows how to connect to the JMS queue. JMS means asynchronous processing - the client sends the message and doesn't necessarily wait for a response. JMS can be used in a point-to-point queue fashion or publish/subscribe.
"Service" is a fluid term. I think of a service as a component that lives out on a network and advertises a contract: "If you send me X I'll perform this task for you and return Y."
Distributed components have been around for a long time. Each one communicated using a different protocol (e.g., COM, Corba, RMI, etc.) and exposed their contract in different ways.
Web services are the latest trend in distributed services. They use HTTP as their protocol and can interoperate with any client that can connect via TCP/IP and make an HTTP request.
You can use SOAP or RPC-XML or REST or "contract first" styles, but the underlying idea of a distributed component using HTTP as its protocol remains.
If you accept all this, web services are usually synchronous calls. They need not be bloated, but you can write bad components in any style or language.
You can start designing any distributed component by designing the request and responses first. Given those, you choose JMS or web services based on what kind of clients you want to have and whether or not the communication is synchronous or asynchronous.