#10.en Lambda, method reference and stream

Exercice 1 - 'The math adds up'

1. We are trying to count the number of occurrences of a word in a list.

        var list = List.of("hello", "world", "hello", "lambda");
        System.out.println(count(list, "hello"));  // 2
       

   Write the code of the count method knowing that the counter is a long integer.
2. We are looking to write a semantically equivalent count2 method (which does the same thing) to count but using the API of Stream.
   How do I get a Stream from an object of type List ?
   The idea here is to filter the stream to keep only equal words to the word passed in parameter then to count these.
   What are the methods for filtering a stream and counting the number of elements, respectively?
   The method used to filter takes an object of type Predicate<T> as a parameter. In our case, what is the type corresponding to T ?
   Indicate the code allowing to create a lambda filtering on the word passed in parameter which can be declared as Predicate
   Write the code for the count2 method.

Exercice 2 - UpperCase

1. We are trying to write a method taking as a parameter a list of character strings and returning a new list containing the character strings in uppercase.

         var list = List.of("hello", "world", "hello", "lambda");
         System.out.println(upperCase(list));  // [HELLO, WORLD, HELLO, LAMBDA]
        

   Write the upperCase method (first) without using the Stream API.
2. We are now looking to write a method upperCase2 doing the same thing but with a Stream .
   How can we use the Stream.map method here?
   To store the result in a new list, the idea is to create list then add each word to the list.

       public static List<String> upperCase2(List<String> words) {
         var uppercases = new ArrayList<String>();
         ...
       

   to request the addition, we will use the forEach method on the stream.
   Write the code for the upperCase 2 method using lambdas.
3. In fact, instead of using lambdas, it is possible in this example to use the syntax of method references with the operator :: (coloncolon).
   Write a method upperCase3 which uses the syntax of method reference.
4. In fact, instead of using forEach , it is better (no side effect) to use the collect method with the Collector returned by the method Collectors.toList().
   Write an upperCase4 method using the collector Collectors.toList().

Exercice 3 - Counting on a discount

1. Explain why we are not going to use the map method but the mapToInt method?
2. Write the code for the method method count3 .

Exercice 4 - Speed evaluation

    var list2 =
      new Random(0)
        .ints(1_000_000, 0, 100)
        .mapToObj(Integer::toString)
        .collect(Collectors.toList());
   

1. Explain what the local variable list2 contains.
2. We are looking to write a printAndTime method allowing to calculate the execution time of the call to the count method on the list2 .
   To calculate the execution time, we ask the time before then the time after and if we subtract the two times, we find the execution time.

       var start = System.nanoTime();
       ... // faire le calcul
       var end = System.nanoTime();
       System.out.println("result " + result);
       System.out.println(" elapsed time " + (end - start));
        

   Write the code of printAndTime.
3. We also want to calculate the execution time with other methods, like count2 for example. How not to duplicate the code for calculating the execution time?
   Which functional interface should we use knowing that we are going to call printAndTime as follows?

        printAndTime(() -> count(list2, "33"));
        printAndTime(() -> count2(list2, "33"));
        printAndTime(() -> count3(list2, "33"));
       

   
   Write the new code of printAndTime.
4. Explain the results you get.

Exercice 5 - Games Of Streams