```markdown ### How to Copy a TreeMap to Another You can copy a `TreeMap` in Java using several methods. Here are a few examples: 1. **Using the Constructor**: ```java TreeMap original = new TreeMap<>(); original.put("Zafrul", 9); original.put("Java", 27); TreeMap copiedmap = new TreeMap<>(original); System.out.println(copiedmap); ``` 2. **Using `putAll` Method**: ```java TreeMap copiedmap1 = new TreeMap<>(original); copiedmap1.putAll(original); System.out.println(copiedmap1); ``` 3. **Using Streams**: ```java TreeMap copy = original.entrySet().stream().collect(Collectors.toMap( Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> v1, TreeMap::new )); System.out.println(copy); ``` ### Interface Abstract Method Example | Interface | Abstract Method | Example Use | |----------------|------------------|--------------------------------------| | Runnable | `void run()` | Threads | | Supplier | `T get()` | Lazy values | | Consumer | `void accept(T t)` | Printing, logging, etc. | | Function | `R apply(T t)` | Transformations | | Predicate | `boolean test(T t)` | Filtering logic | | BiFunction | `R apply(T,U)` | Combine two inputs | ### `map()` vs `flatMap()` | Feature | `map()` | `flatMap()` | |-----------------|----------------------------------|--------------------------------------| | Transforms | One-to-one mapping | One-to-many mapping, then flattens it | | Output | `Stream>` (if mapping to Stream) | `Stream` | | Use-case | Modify/transform each element | Flatten nested structure (e.g., `List>`) | ### Example of `map()` and `flatMap()` ```java List words = Arrays.asList("java", "spring"); // Using map() List> mapped = words.stream().map(word -> Arrays.asList(word.split(""))).collect(Collectors.toList()); // Output: [[j, a, v, a], [s, p, r, i, n, g]] // Using flatMap() List flatMapped = words.stream().flatMap(word -> Arrays.stream(word.split(""))).collect(Collectors.toList()); // Output: [j, a, v, a, s, p, r, i, n, g] ``` ### Return All Duplicate Elements from an Integer Array Using Java 8 Streams ```java List numbers = Arrays.asList(1, 2, 3, 2, 4, 5, 1, 6, 7, 5); Set duplicates = numbers.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting())).entrySet().stream().filter(entry -> entry.getValue() > 1).map(Map.Entry::getKey).collect(Collectors.toSet()); ``` ### Calculate the Average Salary per Department ```java double averageSalary = employees.stream().mapToDouble(Employee::getSalary).average().orElse(0.0); // fallback for empty list // Calculate the average salary per department Map averageSalaryByDepartment = employees.stream().collect(Collectors.groupingBy(Employee::getDepartment, Collectors.averagingDouble(Employee::getSalary))); ``` ### Given an Integer Array, Return the Most Frequent Even Element ```java int arr[] = {1, 2, 3, 2, 4, 5, 1, 6, 7, 5}; Optional mostFrequentEven = Arrays.stream(arr).boxed().collect(Collectors.groupingBy(Function.identity(), Collectors.counting())).entrySet().stream().filter(entry -> entry.getKey() % 2 == 0).sorted(Comparator.comparing(Map.Entry::getValue).reversed().thenComparing(Map.Entry::getKey)) // tie-breaker: smaller value wins.map(Map.Entry::getKey).findFirst(); ``` ### Executor – Task Dispatcher The basic interface to run asynchronous tasks. You submit a task, and it gets executed on a thread. ```java Executor executor = Executors.newSingleThreadExecutor(); executor.execute(() -> System.out.println("Cooking dish 🍝")); ``` ### ExecutorService – Advanced Thread Pool Manager It's an extension of `Executor`. Offers more control: - Submit tasks - Shutdown pool - Await completion - Schedule tasks - Get results via `Future` ```java ExecutorService service = Executors.newFixedThreadPool(4); service.submit(() -> System.out.println("Processing report 📊")); ``` ### Future – A Ticket for the Result When you submit a task and want to get the result later, you use a `Future`. ```java Future future = service.submit(() -> { Thread.sleep(1000); // simulate delay return 42; }); System.out.println("Doing other work..."); System.out.println("Result: " + future.get()); // blocks until result is ready ``` ### Use Cases for Future - File processing in parallel - Long-running batch jobs - REST API calls in parallel - Data transformation (large datasets) - Background tasks (emailing, SMS, etc.) ### Sum of Reverse ```java int number = 5; int sum = IntStream.rangeClosed(0, number).map(i -> number - i).sum(); ``` ### Sort Employees If age is equal, then sort by salary: ```java employees.sort(Comparator.comparingInt((Employee e) -> e.age).thenComparingDouble((Employee e) -> e.salary)); ``` ```

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