stringList.stream().forEach(strElement -> System.out.println(strElement));

Week 7: Enum, Generic Type, Streams, write to file, class diagram/08A. Streams+++

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+From version 8 onwards, Java has supported so-called "streams".
+With streams, it is easy to HANDLE ALL (OR SUITABLY LIMITED) ELEMENTS of an ENTIRE DATA STRUCTURE at once. 
+By combining streams with LAMBDA EXPRESSIONS, Java's features can be stretched towards functional programming languages.
+
+In this section, we will look at the operation of streams through a few simple examples. 
+The basic idea is that a stream can be created from 'any Collection', such as a list, using the stream method. 
+
+Various operations can then be targeted at the stream. 
+
+Then a so-called terminal operation ends the stream, and typically produces 
+- a single VALUE or 
+- a new COLLECTION of elements as a result.
+https://ville.utu.fi/APP/connector/0/215/source/0c9e196c-6ab5-41d6-9f2c-f4a02c05d1db.png
+
+
+
+========================================================
+
+
+Processing All Elements: forEach
+
+Let's start with an example where we want to apply the SAME OPERATION to ALL ELEMENTS of a collection. 
+A typical example would be a situation where we have a list of strings 
+and we want to print them one by one. 
+
+An implementation using a stream would look like this:
+
+public static void main(String[] args) {
+    ArrayList<String> strings = new ArrayList<>();
+    strings.add("first");
+    strings.add("second");
+    strings.add("third");
+    strings.add("fourth");
+    
+    strings.stream().forEach(string -> System.out.println(string));
+}
+ 
+
+The program prints
+first
+second
+third
+fourth
+ 
+
+
+
+
+
+
+========================================================
+
+
+
+
+
+
+The forEach method takes the operation to be performed as a parameter. 
+A lambda expression is a handy way to define a single operation to be performed.
+
+The LAMBDA EXPRESSION is in the form
+parameter -> operation, which uses the parameter
+
+...in the example, it's
+string -> System.out.println(string)
+ 
+
+
+
+
+
+
+
+
+
+========================================================
+
+
+In practice, it means that the print statement is executed for all elements of the stream.
+Let's look at another example where the method printVowels is called for all elements of the list:
+
+public static void main(String[] args) {
+    ArrayList<String> ducks = new ArrayList<>();
+    ducks.add("Huey");
+    ducks.add("Dewey");
+    ducks.add("Louie");
+    ducks.add("Scrooge");
+    ducks.add("Gyro Gearloose");
+    
+    ducks.stream().forEach(duck -> printVowels(duck));
+}
+
+public static void printVowels(String string) {
+    for (int i=0; i<string.length(); i++) {
+        if ("aeiouy".contains("" + string.charAt(i))) {
+            System.out.print(string.charAt(i));
+        }
+    }
+    System.out.println();
+}
+ 
+
+The program prints:
+ue
+eey
+ouie
+ooe
+yoeaooe
+
+
+
+
+
+========================================================
+
+
+A third example prints the names
+
+ArrayList<Person> hlot = new ArrayList<>();
+persons.add(new Person("Heikki Person", "heikki@example.com"));
+persons.add(new Teacher("Oliver Teacher", "oliver@example.com", 123));
+persons.add(new Student("Oscar Student", "oscar@example.com", 23));
+
+persons.stream().forEach(person -> 
+    System.out.println("name: " + person.getName() + ", email: " + person.getEmail()));
+ 
+
+Ohjelma tulostaa:
+Nimi: Heikki Person, email: heikki@example.com
+Nimi: Oliver Teacher, email: oliver@example.com
+Nimi: Oscar Student, email: oscar@example.com
+
+