Herb Schildt s Java Prog ramming Cookbook in Java

Printing Data Matrix 2d barcode in Java Herb Schildt s Java Prog ramming Cookbook

// Start the second thread new MyThread(syncCol); try { synchronized(syncCol) { for(String str : syncCol) { Systemoutprintln("Main thread: " + str); // Let the second thread run, if it can Threadsleep(500); } } } catch(InterruptedException exc) { Systemoutprintln("Main thread interrupted"); } } }

Main thread: Alpha Main thread: Beta Main thread: Gamma Second thread: Alpha Second thread: Beta Second thread: Gamma Second thread: Omega

Here is how the program works SyncDemo begins by creating a TreeSet called tsStr This set is then wrapped in a synchronized collection called syncCol and three elements are added to it Next, main( ) starts a second thread of execution, which is defined by MyThread The MyThread constructor is passed a reference to syncCol This thread begins by sleeping for a short period of time, which lets the main thread resume running MyThread then adds another element to the collection and then begins iterating through the collection After creating MyThread, the main thread starts iterating over the set, delaying 500 milliseconds between iterations This delay enables the second thread to run, if it can However, because syncCol is synchronized, the second thread can t run until the main thread finishes Therefore the iteration of the elements in main( ) finishes before MyThread adds another element to the set, thus avoiding a ConcurrentModificationException To prove the necessity of using a synchronized collection when multithreading, try this experiment In the example, comment out these lines:

Collection<String> syncCol = CollectionssynchronizedCollection(tsStr);

Then, remove the comment from this line:

// Collection<String> syncCol = tsStr;

Now, recompile and run the program Because syncCol is no longer synchronized, both the main thread and MyThread can access it simultaneously This results in a ConcurrentModificationException when MyThread attempts to add an element to it

5:

In addition to synchronizedCollection( ), Collections provides several other methods that return synchronized views tailored for specific types of collections They are shown here:

static <T> List<T> synchronizedList(List<T> col) static <T> Set<T> synchronizedSet(Set<T> col) static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> col) Returns a thread-safe list backed by col Returns a thread-safe set backed by col Returns a thread-safe sorted set backed by col

Collections also supplies the following methods that return synchronized views of maps:

static <K, V> Map<K, V> synchronizedMap(Map<K, V> map) static <K, V> SortedMap<K, V> synchronizedSortedMap( SortedMap<K, V> map) Returns a thread-safe map backed by map Returns a thread-safe sorted map backed by map

Classes javautilCollections Methods static <T> Collection<T> unmodifiableCollection( Collection< extends T> col)

Collections supplies a set of methods that create an immutable view of a collection These methods begin with the name unmodifiable The underlying collection cannot be changed through such a view This is useful in cases in which you want to ensure that a collection will not be modified, such as when it is passed to third-party code

To create an immutable view of a collection involves these steps: 1 Create a collection that will be read-only 2 Obtain an immutable view of the collection by calling the unmodifiableCollection( ) method 3 Operate on the collection through the read-only view

To create an immutable view of a collection, use one of the unmodifiable methods defined by the Collections class It defines four methods that return immutable views of a collection The one used by this recipe is unmodifiableCollection( ) It is shown here static <T> Collection<T> unmodi ableCollection(Collection< extends T> col) It returns a reference to a read-only view of the collection passed to col This reference can then be passed to any code that is not allowed to modify the collection Any attempt to modify the collection through this reference will result in an UnsupportedOperationException