In Java, do I need to declare my collection synchronized if it's read-only?

Normally no because you are not changing the internal state of the collection in this case. When you iterate over the collection a new instance of the iterator is created and the state of the iteration is per iterator instance.

Aside note: Remember that by keeping a read-only collection you are only preventing modifications to the collection itself. Each collection element is still changeable.

class Test {    public Test(final int a, final int b) {        this.a = a;        this.b = b;    }    public int a;    public int b;}public class Main {    public static void main(String[] args) throws Exception {        List<Test> values = new ArrayList<Test>(2);        values.add(new Test(1, 2));        values.add(new Test(3, 4));        List<Test> readOnly = Collections.unmodifiableList(values);        for (Test t : readOnly) {            t.a = 5;        }        for (Test t : values) {            System.out.println(t.a);        }    }}

This outputs:

55

Important considerations from @WMR answser.

It depends on if the threads that are reading your collection are started before or after you're filling it. If they're started before you fill it, you have no guarantees (without synchronizing), that these threads will ever see the updated values.

The reason for this is the Java Memory Model, if you wanna know more read the section "Visibility" at this link: http://gee.cs.oswego.edu/dl/cpj/jmm.html

And even if the threads are started after you fill your collection, you might have to synchronize because your collection implementation could change its internal state even on read operations (thanks Michael Bar-Sinai, I didn't know such collections existed).

Another very interesting read on the topic of concurrency which covers topics like publishing of objects, visibility, etc. in much more detail is Brian Goetz's book Java Concurrency in Practice.

In the general case, you should. This is because some collections change their internal structure during reads. A LinkedHashMap that uses access order is a good example. But don't just take my word for it:

In access-ordered linked hash maps, merely querying the map with get is a structural modification The Linked hash map's javadoc

If you are absolutely sure that there are no caches, no collection statistics, no optimizations, no funny stuff at all - you don't need to sync. In that case I would have put a type constraint on the collection: Don't declare the collection as a Map (which would allow LinkedHashMap) but as HashMap (for the purists, a final subclass of HashMap, but that might be taking it too far...).

It depends on if the threads that are reading your collection are started before or after you're filling it. If they're started before you fill it, you have no guarantees (without synchronizing), that these threads will ever see the updated values.

The reason for this is the Java Memory Model, if you wanna know more read the section "Visibility" at this link: http://gee.cs.oswego.edu/dl/cpj/jmm.html

And even if the threads are started after you fill your collection, you might have to synchronize because your collection implementation could change its internal state even on read operations (thanks Michael Bar-Sinai, I didn't know such collections existed in the standard JDK).

Another very interesting read on the topic of concurrency which covers topics like publishing of objects, visibility, etc. in much more detail is Brian Goetz's book Java Concurrency in Practice.