# Chapter 6 Concurrency Enhancements

## Parallel Array Operations

Arrays class now has a number of parallelized operations.

Arrays.parallelSort: sort an array of primitive values or objects.

You can supply a Comparator.

You can supply the bounds of a range.

parallelSetAll: fills an array with values that are computed from a function. The function receives the element index. There are versions for all primitive type arrays and for object arrays.

parallelPrefix: replaces each array element with the accumulation of the prefix for a given associative operation.

This can be parallelized in this way: join neighboring elements first, then update the indicated elements by multiplying them with elements that are one or two positions below.

## Completable Futures

The java.util.concurrent library provides a Future<T> interface to denote a value of type T that will be available at some point in the future.

Completable futures make it possible to compose asynchronous operations.

### Futures

Consider a method that reads a web page in a separate thread, which is going to take a while. When you call it, the method returns right away, and you have a Future<String>.

Suppose we want to extract all URLs from the page in order to build a web crawler. We have a class Parser with a method

How can we apply it to the future object? First, call the get method on the future to get its value when it becomes available. Then, process the result:

No better off than with a method public String readPage(URL url) that blocks until the result is available.

### CompletableFuture

Provides the feature to set “when the result becomes available, here is how to process it”. A CompletableFuture has a method thenApply to which you can pass the post-processing function.

The thenApply method doesn’t block either. It returns another future. When the first future has completed. its result is fed to the getLinks method, and the return value of that method becomes the final result.

This composability is the key aspect of the CompletableFuture class. Composing future actions solves a problem in programming asynchronous applications.

### The Composition Pipeline

Pipeline of futures starts out by generating a CompletableFuture, usually with the static method supplyAsync. That method requires a Supplier<T>, that is, a function with no parameters yielding a T. The function is called on a separate thread.

runAsync: takes a Runnable, yielding a CompletableFuture<Void>. Useful if you simply want to schedule one action after another, without passing data between them.

All methods ending in Async have two variants. One runs the provided action on the common ForkJoinPool. The other has a parameter of type java.util.concurrent.Executor, and it uses the given executor to run the action.

Next, you can call thenApply or thenApplyAsync to run another action, either in the same thread or another. With either method, you supply a function and you get a CompletableFuture<U>, where U is the return type of the function.

You can have additional processing steps. Eventually, you will be done.
thenAccept: takes a Consumer, a function with return type void. Ideally, you would never call get on a future. The last step in the pipeline simply deposits the result where it belongs.

You don’t explicitly start the computation. The static supplyAsync method starts it automatically, and the other methods cause it to be continued.

### Composing Asynchronous Operations

Adding an Action to a CompletableFuture Object

Method Parameter Description
thenApply T -> U Apply a function to the result
thenCompose T -> CompletableFuture Invoke the function on the result and execute the returned future
handle (T, Throwable) -> U Process the result or error
thenAccept T -> void Like thenApply, but with void result
whenComplete (T, Throwable) -> void Like handle, but with void result
thenRun Runnable Execute the Runnable with void result

For each method shown, there are also two Async variants.

T -> U is Function<? super T, U>

The calls CompletableFuture<U> future.thenApply(f) and CompletableFuture<U> future.thenApplyAsync(f) return a future that applies f to the result of future when it is available. The second call runs f in another thread.

thenCompose: takes a function T -> CompletableFuture<U>.

Here we have two functions: T -> CompletableFuture and U -> CompletableFuture. Clearly, they compose to a function T -> CompletableFuture by calling the second function when the first one has completed. That is exactly what thenCompose does.

handle: handles an exception thrown in a CompletableFuture. The supplied function is called with the result (or null if none) and the exception (or null if none), and it gets to make sense of the situation.

Combining Multiple Composition Objects

Method Parameters Description
thenCombine CompletableFuture, (T, U) -> V Execute both and combine the results with the given function.
theAcceptBoth CompletableFuture, (T, U) -> void Like thenCombine, but with void result.
runAfterBoth CompletableFuture<?>, Runnable Execute the runnable after both complete.
applyToEither CompletableFuture, T -> V When a result is available from one or the other, pass it to the given function
acceptEither CompletableFuture, T -> void Like applyToEither, but with void result.
runAfterEither CompletableFuture<?>, Runnable Execute the runnable after one or the other completes.
static allOf CompletableFuture<?>… Complete with void result after all given futures complete.
static anyOf CompletableFuture<?>… Complete with void result after any of the given futures completes.

The first three methods run a CompletableFuture<T> and a CompletableFuture<U> action in parallel and combine the results.

The next three methods run two CompletableFuture<T> actions in parallel. As soon as one of them finishes, its result is passed on, and the other result is ignored.

The static allOf and anyOf methods take a variable number of completable futures and yield a CompletableFuture<Void> that completes when all of them, or any one of them, completes. No results are propagated.

Technically speaking, the methods accept parameters of CompletionStage, not CompletableFuture. That is an interface type with almost forty abstract methods, currently implemented only by CompletableFuture.