map then filter OR filter (directly)

Week 7: Enum, Generic Type, Streams, write to file, class diagram/10A. Filtering Elements+++

@@ -0,0 +1,117 @@
+The examples presented above have all targeted all operations of a collection. 
+However, filtering operations, which can be used to determine which elements we want to keep in the stream, are essential to streams.
+
+The filter operation also takes a LAMBDA EXPRESSION as a PARAMETER. 
+Only those elements for which the operation defined in the lambda expression returns 'true' REMAIN IN THE STREAM.
+
+
+The example calculates the sum of only the even elements:
+ArrayList<Integer> numbers = new ArrayList<>();
+numbers.add(1);
+numbers.add(2);
+numbers.add(3);
+numbers.add(4);
+
+int sumOfEven = numbers.stream().mapToInt(number -> number).filter(number -> number % 2 == 0).sum();
+System.out.println("The Sum of even numbers: " + sumOfEven);
+ 
+
+The program prints:
+The sum of even numbers: 6
+
+So after the filter, only even numbers remain in the stream - all subsequent operations only apply to them.
+
+
+
+
+
+=============================================================
+
+
+
+Let's look at another example where only the names with more than 4 characters are printed from the stream of STRINGS:
+
+ArrayList<String> names = new ArrayList<>();
+names.add("Bob");
+names.add("Elisa");
+names.add("Mia");
+names.add("Marco");
+names.add("Tia");
+names.add("Anneli");
+
+names.stream().filter(name -> name.length() > 4).forEach(name -> System.out.println(name));
+ 
+
+Ohjelma tulostaa:i
+
+Elisa
+Marco
+Anneli
+ 
+
+
+
+
+=============================================================
+
+
+
+
+Simple conditions are easy to define directly in the lambda expression, 
+but for more complex ones, we can write our OWN FILTER METHOD. 
+The method should return true or false depending on whether we want to keep the element in the stream or not.
+
+public static void main(String[] args) {
+    ArrayList<String> names = new ArrayList<>();
+    names.add("Kirsi");
+    names.add("Anne");
+    names.add("Mia");
+    names.add("Aune");
+    names.add("Tina");
+    names.add("Ester");
+
+    System.out.println("Names starting with a vowel:");
+    names.stream().filter(name -> startsWithVowel(name)).forEach(name -> System.out.println(name));
+}
+
+
+public static boolean startsWithVowel(String name) {
+    if ("AEIOU".contains("" + name.charAt(0))) {
+        return true;
+    } 
+    else {
+        return false;
+    }
+}
+ 
+
+Ohjelma tulostaa:
+Anne
+Aune
+Ester
+
+
+
+
+
+
+=============================================================
+
+
+ 
+
+Let's look at one more example. 
+The method picks only those students from the list whose number of credits is less than 25. 
+The average of these points is then calculated.
+
+
+Note how the program uses the operation mapToInt: 
+it is now used to pick the number of credits from the student object.
+
+public static double averageCredits(ArrayList<Student> students) {
+    // Split into two lines due to length
+    return students.stream().mapToInt(s -> s.getCredits()).
+            filter(credits -> credits < 25).average().getAsDouble();
+    
+}
+