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understanding java's synchronized collections

I'm reading the java official doc regarding wrappers implementation, which are static methods in Collections used to get synchronized collection, for example : List<Type> list = Collections.synchronizedList(new ArrayList<Type>());
...
the thing that I did not understand is the following (I quote from the java doc ) :
A collection created in this fashion is every bit as thread-safe as a normally synchronized collection, such as a Vector.
In the face of concurrent access, it is imperative that the user manually synchronize on the returned collection when iterating over it. The reason is that iteration is accomplished via multiple calls into the collection, which must be composed into a single atomic operation...
how it could be every bit as thread-safe an need to manually synchronize when iterating ??

It is thread safe in the sense that each of it's individual methods are thread safe, but if you perform compound actions on the collection, then your code is at risk of concurrency issues.
ex:
List<String> synchronizedList = Collections.synchronizedList(someList);
synchronizedList.add(whatever); // this is thread safe
the individual method add() is thread safe but if i perform the following:
List<String> synchronizedList = Collections.synchronizedList(someList);
if(!synchronizedList.contains(whatever))
synchronizedList.add(whatever); // this is not thread safe
the if-then-add operation is not thread safe because some other thread might have added whatever to the list after contains() check.

There is no contradiction here: collections returned from synchronizedXyz suffer the same shortcoming as synchronized collections available to you directly, namely the need to manually synchronize on iterating the collection.
The problem of external iteration cannot be solved by a better class design, because iterating a collection inherently requires making multiple calls to its methods (see this Q&A for detailed explanation).
Note that starting with Java 1.8 you can iterate without additional synchronization using forEach method of your synchronized collection*. This is thread-safe, and comes with additional benefits; see this Q&A for details.
The reason this is different from iterating externally is that forEach implementation inside the collection takes care of synchronizing the iteration for you.

Difference between CopyOnWriteArrayList and synchronizedList

As per my understanding concurrent collection classes preferred over synchronized collections because the concurrent collection classes don't take a lock on the complete collection object. Instead they take locks on a small segment of the collection object.
But when I checked the add method of CopyOnWriteArrayList, we are acquiring a lock on complete collection object. Then how come CopyOnWriteArrayList is better than a list returned by Collections.synchronizedList? The only difference I see in the add method of CopyOnWriteArrayList is that we are creating copy of that array each time the add method is called.
public boolean add(E e) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
Object[] newElements = Arrays.copyOf(elements, len + 1);
newElements[len] = e;
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}

As per my understanding concurrent collection classes preferred over synchronized collection because concurrent collection classes don't take lock on complete collection object. Instead it takes lock on small segment of collection object.
This is true for some collections but not all. A map returned by Collections.synchronizedMap locks the entire map around every operation, whereas ConcurrentHashMap locks only one hash bucket for some operations, or it might use a non-blocking algorithm for others.
For other collections, the algorithms in use, and thus the tradeoffs, are different. This is particularly true of lists returned by Collections.synchronizedList compared to CopyOnWriteArrayList. As you noted, both synchronizedList and CopyOnWriteArrayList take a lock on the entire array during write operations. So why are the different?
The difference emerges if you look at other operations, such as iterating over every element of the collection. The documentation for Collections.synchronizedList says,
It is imperative that the user manually synchronize on the returned list when iterating over it:
List list = Collections.synchronizedList(new ArrayList());
...
synchronized (list) {
Iterator i = list.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
Failure to follow this advice may result in non-deterministic behavior.
In other words, iterating over a synchronizedList is not thread-safe unless you do locking manually. Note that when using this technique, all operations by other threads on this list, including iterations, gets, sets, adds, and removals, are blocked. Only one thread at a time can do anything with this collection.
By contrast, the doc for CopyOnWriteArrayList says,
The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException. The iterator will not reflect additions, removals, or changes to the list since the iterator was created.
Operations by other threads on this list can proceed concurrently, but the iteration isn't affected by changes made by any other threads. So, even though write operations lock the entire list, CopyOnWriteArrayList still can provide higher throughput than an ordinary synchronizedList. (Provided that there is a high proportion of reads and traversals to writes.)

For write (add) operation, CopyOnWriteArrayList uses ReentrantLock and creates a backup copy of the data and the underlying volatile array reference is only updated via setArray(Any read operation on the list during before setArray will return the old data before add).Moreover, CopyOnWriteArrayList provides snapshot fail-safe iterator and doesn't throw ConcurrentModifficationException on write/ add.
But when I checked add method of CopyOnWriteArrayList.class, we are acquiring lock on complete collection object. Then how come CopyOnWriteArrayList is better than synchronizedList. The only difference I see in add method of CopyOnWriteArrayList is we are creating copy of that array each time add method get called.
No, the lock is not on the entire Collection object. As stated above it is a ReentrantLock and it is different from the intrinsic object lock.
The add method will always create a copy of the existing array and do the modification on the copy and then finally update the volatile reference of the array to point to this new array. And that's why we have the name "CopyOnWriteArrayList" - makes copy when you write into it.. This also avoids the ConcurrentModificationException

1) get and other read operation on CopyOnWriteArrayList are not synchronized.
2) CopyOnWriteArrayList's iterator never throws ConcurrentModificationException while Collections.synchronizedList's iterator may throw it.

Synchronizing ArrayList

In ArrayList api we have:
Note that this implementation is not synchronized. If multiple threads
access an ArrayList instance concurrently, and at least one of the
threads modifies the list structurally, it must be synchronized
externally. (A structural modification is any operation that adds or
deletes one or more elements, or explicitly resizes the backing array;
merely setting the value of an element is not a structural
modification.) This is typically accomplished by synchronizing on some
object that naturally encapsulates the list. If no such object exists,
the list should be "wrapped" using the Collections.synchronizedList
method.
Here what is meant by "This is typically accomplished by synchronizing on some object that naturally encapsulates the list"? How this related to concurrent modification exception?

from ArrayList
This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the
list should be "wrapped" using the Collections.synchronizedList
method. This is best done at creation time, to prevent accidental
unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(...));
By "naturally encapsulates" it means that if the list is a field of an object, but the list is not publically accessible suppose the following:
class ParkingLot{
private ArrayList<Cars> spots;
public boolean park(int spotNumber, Car car){
if( spots.get(spotNumber)==null){
spot.set(spotNumber,car);
return true;
}
return false;
}
}
In this case ParkinLot would encapulate the list spot. If you were to try and call park(), you'd want to synchronize on the ParkingLot object to prevent two threads trying to park a car in the same spot at the same time.
It is related to a ConcurrentModificationException in that it prevents you from changing the list from separate threads simultaneously (by synchronizing), which could leave the list in an inconsistent state (ie Two cars parking at the same time thinking they've successfully parked).

Here what is meant by "This is typically accomplished by synchronizing
on some object that naturally encapsulates the list"
If you have a class which encapsulates an ArrayList then if you synchronize on the wrapper object than the underlying ArrayList will also be synchronized. e.g.
class MyClasss{
private final ArrayList list;
......
......
......
}
If you synchronize on the instance of MyClass than the underlying list is also synchronized and the all the read / write will be serialized e.g.
class MyClasss{
private final ArrayList list;
......
......
......
public void fun(){
synchronized(this){
list.add(....)
}
}
How this related to concurrent modification exception?
Before answering the above question you need to understand how ConcurrentModificationException is thrown by JVM.
ConcurrentModificationException is implemented in java by checking the modification count of each Collection. Every time you do an operation it compares the modification count before doing the operation and after doing the operation.
So if you synchronize the Collection then simultaneous modification of the modification count will not happen resulting in not throwing the ConcurrentModificationException.
Hope it helps.

Why not use Vector instead? It's already synchronized.

Need to manually synchronize the Synchronized list while iteration when it could be avoided?

My question is about synchronizedList method Collections Class.
Javadocs say:
It is imperative that the user manually synchronize on the returned list when iterating over it:
List list = Collections.synchronizedList(new ArrayList());
...
synchronized(list) {
Iterator i = list.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
Though manually synchroniziation is not required for other methods. I looked into the source code of Collections class
and found shyncronization has already been taken care for all methods like add
public boolean add(E e) {
synchronized(list) {return c.add(e);}
}
but not for iterator method. I think iterator method could have also handled synchronization in the same fashion
as above method (it would have avoided the extra work i.e manual synchronization for programmers). i am sure there
must be some concrete reason behind it but i am missing it?
public Iterator<E> iterator() {
return c.iterator(); // Must be manually synched by user!
}
A way to avoid manual synchronization from Programmer
public Iterator<E> iterator() {
synchronized(list) {
return c.iterator(); // No need to manually synched by user!
}
}

I think iterator method could have also handled synchronization in the same fashion as above method
No, it absolutely couldn't.
The iterator has no control over what your code does between calls to the individual methods on it. That's the point. Your iteration code will call hasNext() and next() repeatedly, and synchronization during those calls is feasible but irrelevant - what's important is that no other code tries to modify the list across the whole time you're iterating.
So imagine a timeline of:
t = 0: call iterator()
t = 1: call hasNext()
t = 2: call next()
// Do lots of work with the returned item
t = 10: call hasNext()
The iterator can't synchronize between the end of the call to next() at t=2 and the call to hasNext() at t=10. So if another thread tries to (say) add an item to the list at t=7, how is the iterator meant to stop it from doing so?
This is the overall problem with synchronized collections: each individual operation is synchronized, whereas typically you want a whole chunky operation to be synchronized.

If you don't synchronize the entire iteration, another thread could modify the collection as you iterate, leading to a ConccurentModificationException.
Also, the returned iterator is not thread-safe.
They could fix that by wrapping the iterator in a SynchronizedIterator that locks every method in the iterator, but that wouldn't help either – another thread could still modify the collection between two iterations, and break everything.
This is one of the reasons that the Collections.synchronized*() methods are completely useless.
For more information about proper thread-safe collection usage, see my blog.

If you want to avoid manual synchronization, you have to use a Collection like java.util.concurrent.CopyOnWriteArrayList. Every time an object is added to the list, the underlying datastructure is copyied to avaoid a concurrent modification exception.
The reason why you need manual serialization on the Iterator in your example is that the Iterator uses the same internal datastructure as the list but they are independend objects and both Iterator and list can be accessed by different threads at any arbitrary moment in time.
Another aproach would be to make a local copy of the list and iterate over the copy.

usability of synchronized... methods in java.util.Collections

I'm looking at the static method
Collections.synchronizedList(List<T> list)
Javadoc says
It is imperative that the user manually synchronize on the returned list when iterating over it...
What's the purpose of creating a synchronized list if I still have to manually synchronize it?

The reason why you would use
Collections.synchronizedList(List<T> list)
is because all the methods but the iterator are synchronized using the list itself as the mutex so you don't have to do
synchronized(list) {
list.add(type);
}
Instead you can just do
list.add(type);
and it will be thread safe.
The only method which isn't synchronized is when iterating the list. The list iterator can't be return in a synchronized fashion, since you will be iterating through it afterwards which is why it is required to manually synchronize the list. So in conclusion, you only have to synchronize the list when iterating over it, everything else you don't have to.

The java collections tutorial explains this. Basically, each iteration reauires multiple calls to the underlying collection. These calls muct be atomicised as a single 'transaction'.