Because int is not an object type we write integer

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returns an object of whatever (object) type we want. Because int is not an object type, we write Integer instead. Java implicitly converts an int to/from an Integer in all the places it is needed in this program. • The join method also returns the value returned by compute in the thread that is being joined to. Therefore, we do not need an ans field: we write int leftAns = left.join() to get the value that the thread bound to left returned from its compute method. • The invoke method works similarly when passed a subclass of a RecursiveTask class: It returns the value of the compute method that is called. • The class declaration of SumArray indicates that it extends RecursiveTask<Integer> . More generally, always put the same type between the angle brackets as the return type of compute . • Note that because Java expressions are always evaluated left-to-right, we could replace the last 3 lines of the else-branch in compute with just return right.compute() + left.join(); . But this is rather subtle — left.join() + right.compute() would destroy the parallelism — so we do not advise writ- ing this shorter but less clear code. If you are familiar with Java’s generic types, this use of them should not be particularly perplexing. The library is also using static overloading for the invoke method. But as users of the library, it suffices just to follow the pattern in the example above. CPEN 221 – Fall 2016
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Fork-Join Parallelism 21 Why is there RecursiveAction and RecursiveTask ? The former is better style when there really is nothing to return; otherwise RecursiveTask is better. Both are actually implemented in terms of the same superclass inside the library. You can use that class directly, but it is less convenient. Given the convenience of not needing a field for returning values, why not also provide the convenience of not needing fields to pass arguments ( arr , lo , and hi )? That would be nice, but there is no particularly pleasant way to write such a library in Java. The ForkJoin framework is just a library; it cannot make any changes/extensions to the Java language. It uses some advanced Java features to be as convenient as it can be. 6 Reductions and Maps It may seem that given all the work we did to implement something as conceptually simple as summing an array that fork/join programming is too complicated. To the con- trary, it turns out that many, many problems can be solved very much like we solved this one. Just like regular for-loops took some getting used to when you started pro- gramming but now you can recognize exactly what kind of loop you need for all sorts of problems, divide-and-conquer parallelism often follows a small number of patterns. Once you know the patterns, most of your programs are largely the same. For example, here are several problems for which efficient parallel algorithms look al- most identical to summing an array: • Count how many array elements satisfy some property (e.g., how many elements are the number 42).
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