List, boxed()

Week 7: Enum, Generic Type, Streams, write to file, class diagram/14A. Creating List, convert to ArrayList+++

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+Creating a list with predefined initial values is made easier, 
+for example, with the static method 'of' from the List class:
+
+List<Integer> numbers = List.of(1, 3, 2, 4, 3, 6, 4, 3, 2, 1);
+System.out.println(numbers);
+
+List<String> names = List.of("Kai", "Jack", "Kirsi", "Joanna");
+names.stream().forEach(name -> System.out.println(name));
+ 
+
+Ohjelma tulostaa:
+[1, 3, 2, 4, 3, 6, 4, 3, 2, 1]
+Kai
+Jack
+Kirsi
+Joanna
+
+
+
+ 
+
+Note that the method returns a list of type 'List' - not 'ArrayList'. 
+The difference is that 'List' is IMMUTABLE (i.e., you cannot add elements to it, for example).
+
+If a mutable list is needed, the List-type object can be given as a parameter to the ArrayList class constructor:
+
+List<Integer> numbers = List.of(1, 3, 2, 4, 3, 6, 4, 3, 2, 1);
+ArrayList<Integer> mutableList = new ArrayList<>(numbers);
+
+// this can be normally modified
+mutableList.add(23);
+System.out.println(mutableList);
+
+
+// Initialization can also be conveniently done on one line
+ArrayList<Double> measurements = new ArrayList<>(List.of(1.0, 2.0, 3.0, 2.5, 5.25));
+measurements.add(10.75);
+System.out.println(measurements);
+ 
+
+
+=================
+
+
+You can conveniently create an 'ArrayList' from an array with the 'Array' class method 'asList', 
+if it's an object type:
+
+String[] np = {"Huey", "Dewey", "Louie"};
+ArrayList<String> nephews = new ArrayList<>(Arrays.asList(np));
+ 
+
+
+=================
+
+
+
+However, forming an Integer type ArrayList from an int type array does not work this way. 
+This can be circumvented (though a bit clumsily) for example by using a stream.
+
+A stream can be created from an array with the 'Arrays' class method 'stream', 
+and as we saw earlier, 
+the boxed() method converts the stream from a primitive type to an object type.
+
+int[] luvut = {10, 20, 30, 40, 50, 60, 70};
+int[] numbers = {10, 20, 30, 40, 50, 60, 70};
+
+
+// ArrayList<Integer> => ArrayList of 'Integer' objects
+// ArrayList can only hold objects (eg Integer), not primitives (eg int)
+// .boxed() -> convert each 'int' value to 'Integer' object
+ArrayList<Integer> numberList = Arrays.stream(numbers).
+                                       // after boxed() --we get--> Stream<Integer>
+                                       boxed().
+                                       collect(Collectors.toCollection(ArrayList::new));
+
+
+
+
+