OptionalInt; chk ArrayList not empty -> then run getAsInt()

Week 7: Enum, Generic Type, Streams, write to file, class diagram/09A. sum/average/min/max(), OptionalInt+++

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+A typical example of an operation applied to a collection is calculating the SUM of all elements. 
+If we have a list containing numbers as strings, we can easily calculate the sum of its elements by forming a stream from the list:
+
+ArrayList<String> numbers = new ArrayList<>();
+
+numbers.add("23");
+numbers.add("9");
+numbers.add("7");
+numbers.add("3");
+
+int sum = numbers.stream().mapToInt(number -> Integer.valueOf(number)).sum();
+System.out.println("The sum of the numbers: " + sum);
+ 
+
+The program prints:
+42
+ 
+
+In this stream, we need the operation 'mapToInt', which converts the pieces into int type. 
+Here again, we use a lambda expression and give the operation as Integer's valueOf.
+
+
+
+
+
+
+==========================================================
+
+
+Somewhat confusingly, the operation is also needed when the list is already of integer type. 
+This is because certain terminal operations only work when the elements are of type 'int' ('Integer' is not acceptable):
+
+ArrayList<Integer> numbers = new ArrayList<>();
+for (int i=1; i<20; i+=2) {
+    numbers.add(i);
+}
+
+System.out.println(numbers);
+
+// The confusing-looking "change" number -> number converts
+// an Integer stream into an int stream
+System.out.println("The sum of the numbers: " + numbers.stream().mapToInt(number -> number).sum());
+ 
+
+The program prints:
+[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
+The sum of the numbers: 100
+ 
+
+
+==========================================================
+
+
+Other handy operations include average, min, and max, 
+which return the average, minimum, and maximum of a list of numbers:
+
+ 
+
+ArrayList<Integer> numbers = new ArrayList<>();
+for (int i=1; i<10; i++) {
+    numbers.add(i);
+}
+System.out.println(numbers);
+
+System.out.println("Maximum:");
+System.out.println(numbers.stream().mapToInt(number -> number).max().getAsInt());
+
+System.out.println("Minimum:");
+System.out.println(numbers.stream().mapToInt(number -> number).min().getAsInt());
+
+System.out.println("Average:");
+System.out.println(numbers.stream().mapToInt(number -> number).average().getAsDouble());
+ 
+
+The program prints:
+[1, 2, 3, 4, 5, 6, 7, 8, 9]
+Maximum:
+9
+Minimum:
+1
+Average:
+5.0
+
+
+
+
+ 
+
+Now, a terminal operation is needed to convert the result into a number, for example, getAsInt(). 
+This is because the output of the preceding max operation is not an integer but a value of type 'OptionalInt'.
+
+An 'extended type' from a normal integer (such as 'OptionalInt') is needed because the list might not contain any values 
+- in which case the maximum value of the list cannot be presented as an integer.
+
+
+
+
+
+
+
+
+If the stream is empty when it comes to the max operation, the getAsInt() operation will throw an exception:
+
+ArrayList<Integer> numbers = new ArrayList<>();
+
+// In an empty stream, there is no maximum...
+int max = numbers.stream().mapToInt(number -> number).max().getAsInt();
+
+The programs throws an exception:
+Exception in thread "main" java.util.NoSuchElementException: No value present
+ 
+
+
+
+
+
+
+The problem can be circumvented either by
+1- CHECKING that the LIST is NOT EMPTY or
+2- by performing the 'getAsInt' operation only AFTER it has been confirmed that the VALUE EXISTS:
+
+OptionalInt max = numbers.stream().mapToInt(number -> number).max();
+
+if (!max.isEmpty()) {
+    System.out.println("Maximum: " + max.getAsInt());
+
+
+
+
+