enum Topic1 {VALUE1, ...}; "someStr" == Topic1.VALUE1;

Week 7: Enum, Generic Type, Streams, write to file, class diagram/01A. Enum Classes+++

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+Last week we saw a few examples of storing constant values as static variables in a class. 
+In older versions of Java, it was common to use static variables in this way:
+
+class Suit {
+    public static final int SPADES = 1;
+    public static final int HEARTS = 2;
+    public static final int CLUBS = 3;
+    public static final int DIAMONDS = 4;
+}
+
+
+
+
+
+Now, the playing card class could look something like this:
+
+class PlayingCard {
+    private int suit;
+    private int number;
+    
+    public PlayingCard(int suit, int number) {
+        this.suit = suit;
+        this.number = number;
+    }
+
+    public int getSuit() {
+        return suit;
+    }
+
+    public int getNumber() {
+        return number;
+    } 
+}
+
+
+
+
+...and a new playing card could be created like this:
+
+public static void main(String[] args) {
+   PlayingCard sevenOfSpades = new PlayingCard(Suit.SPADES, 7);
+}
+ 
+
+
+
+
+===============================================================
+
+
+
+However, the method has a clear shortcoming: 
+the constant numerical values do not automatically associate with their meanings. 
+
+Even if we agreed that the numerical value 1 corresponds to the suit of spades in a deck of cards, 
+requesting the suit will nevertheless return an integer:
+
+public static void main(String[] args) {
+   PlayingCard sevenOfSpades = new PlayingCard(Suit.SPADES, 7);
+   System.out.println("Card's suit: " + sevenOfSpades.getSuit());
+   System.out.println("Card's number: " + sevenOfSpades.getNumber());
+}
+
+The program prints:
+Card's suit: 1
+Card's number: 7
+ 
+
+A better solution is to use the enum class introduced in Java 5.
+
+
+
+
+
+
+
+
+
+==============================================
+
+
+
+
+What is an enum class?
+
+The purpose of an enum class is to define a SET of VALUES. 
+Typical examples could be 
+the suits in a card game, 
+compass directions or 
+the houses at Hogwarts.
+
+The class is defined with the keyword 'enum'. 
+In its simplest form, we just write the values that belong to the set 
+inside the class definition, separated by commas:
+
+enum Suit {
+    SPADES, HEARTS, CLUBS, DIAMONDS
+}
+ 
+
+
+
+
+
+Now we can change the definition of the card game class so that the suit is of type 'Suit', not an integer. 
+This also limits the possible values: the suit of the card cannot be anything other than one of the four defined in the enum class Suit.
+
+class PlayingCard {
+    private Suit suit;
+    private int number;
+    
+    public PlayingCard(Suit suit, int number) {
+        this.suit = suit;
+        this.number = number;
+    }
+
+    public Suit getSuit() {
+        return suit;
+    }
+
+    public int getNumber() {
+        return number;
+    } 
+}
+ 
+
+
+
+
+
+
+
+
+
+
+
+
+This is now seen not only when creating a card, but also when printing the card's suit:
+
+public static void main(String[] args) {
+   PlayingCard sevenOfClubs = new PlayingCard(Suit.CLUBS, 7);
+   System.out.println("Card's suit: " + sevenOfClubs.getSuit());
+   System.out.println("Card's number: " + sevenOfClubs.getNumber());
+}
+
+Now the program prints:
+Card's suit: CLUBS
+Card's number: 7
+
+
+
+
+
+
+
+
+
+==============================================
+
+
+
+
+
+
+
+
+Using Enum Values
+
+Enum type values can be directly compared with Java's equality and inequality operators. 
+// ==
+// !=
+
+If we want to check whether a card is a diamond, we can write:
+
+public static void main(String[] args) {
+    PlayingCard card = new PlayingCard(Suit.DIAMONDS, 9);
+    if (card.getSuit() == Suit.DIAMONDS) {
+        System.out.println("The card is a diamond!");
+    }
+}
+
+
+
+
+
+
+==============================================
+
+
+
+As another example, let's define another enum class, which includes subjects taught:
+
+enum Subject {
+    MATHEMATICS, PHYSICS, CHEMISTRY, BIOLOGY, SWEDISH
+}
+
+
+
+
+ 
+
+Now we can use the value as the type of the 'mainSubject' field in the 'Student' class, for example:
+
+class Student extends Person {
+    private int credits;
+    private Subject mainSubject;
+
+    public Student(String name, String email, int credits, Subject mainSubject) {
+        super(name, email);
+        this.credits = credits;
+        this.mainSubject = mainSubject;
+    }
+
+    public int getCredits() {
+        return credits;
+    }
+    
+    public Subject getMainSubject() {
+        return mainSubject;
+    }
+}
+
+
+
+
+
+
+Now it's easy to write a method, for example, that picks out the mathematics students from the student list:
+
+public static ArrayList<Student> mathematicians(ArrayList<Student> students) {
+    ArrayList<Student> al = new ArrayList<>();
+    for (Student student : students) {
+        if (student.getMainSubject() == Subject.MATHEMATICS) {
+            al.add(student);
+        }
+    }
+    return al;
+}
+ 
+
+Enum classes also support more versatile (and complex) features, which are not covered in this course. 
+You can find more information at
+https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
+