add assignment

u07-2/README.md

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+# Problem 7.2: Implementation of `ExecutorCompletionService`
+
+a) Implement the following simplified version of class `java.util.concurrent.ExecutorCompletionService`:
+
+```java
+public class SimpleExecutorCompletionService<ResultType> {
+	public SimpleExecutorCompletionService(ExecutorService ex) { … }
+
+	// Submit task to ex for execution:
+	public Future<ResultType> submit(Callable<ResultType> task) { … }
+
+	// Fetch result of some finished task if available (else null):
+	public Future<ResultType> poll() { … }
+}
+
+```
+
+Use class `SimpleExecutorCompletionServiceTester` oin the file server to test your class. If your
+implementation is correct, the numbers printed with "… received result" should appear in mostly[^1]
+ascending order. Method `Future.isDone()` could be helpful for your implementation.
+
+[^1]: This order is not really guaranteed: While the "internal" length of each task would indeed produce this order,
+the total execution time of a task may be longer due to external delays by the thread pool (e.g. how long it takes
+to start a submitted task) and by other threads (e.g. preemption).