Object-Oriented Programming - Lecture 3-bis: Enum types - Lê Hồng Phương
Enum types ● An enum type is a type whose fields consist of a fixed set of constants. – Compass directions – The days of the week – The suits of a card ● Because they are constants, the names of an enum type's fields are in uppercase letters.
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Lecture 3-bis: Enum types
Lê Hồng Phương
phuonglh@gmail.com
Department of Mathematics, Mechanics and Informatics,
Vietnam National University, Hanoi
2012-2013 Object-Oriented Programming: Enum types 2
Enum types
● An enum type is a type whose fields consist of
a fixed set of constants.
– Compass directions
– The days of the week
– The suits of a card
● Because they are constants, the names of an
enum type's fields are in uppercase letters.
2012-2013 Object-Oriented Programming: Enum types 3
Enum types
public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}
public enum CompassDirection {
NORTH, SOUTH, EAST, WEST
}
public enum Suit {
SPADE, CLUB, DIAMOND, HEART
}
2012-2013 Object-Oriented Programming: Enum types 4
The Card class revisited
public class Card {
private int rank;
private String suit;
public Card(int rank, String suit) {
this.rank = rank;
this.suit = suit;
}
// ...
}
public class CardTester {
public static void main(String[] args) {
Card c1 = new Card(2, "Spade");
Card c2 = new Card(2, "Undefined");
}
} We should preventuser from doing this.
2012-2013 Object-Oriented Programming: Enum types 5
The Card class revisited
public class Card {
private int rank;
private Suit suit;
public Card(int rank, Suit suit) {
this.rank = rank;
this.suit = suit;
}
// ...
}
public class CardTester {
public static void main(String[] args) {
Card c1 = new Card(1, Suit.SPADE);
}
} There are only 4 possible
suits that a card can take.
2012-2013 Object-Oriented Programming: Enum types 6
Using an enum type
public class DayTester {
private Day day;
public DayTester(Day day) {
this.day = day;
}
public void tell() {
switch (day) {
case MONDAY:
System.out.println("Mondays are bad.");
break;
case FRIDAY:
System.out.println("Fridays are better.");
break;
case SATURDAY:
case SUNDAY:
System.out.println("Weekends are best.");
break;
default:
System.out.println("Midweek days are so-so.");
break;
}
}
2012-2013 Object-Oriented Programming: Enum types 7
Using an enum type
public static void main(String[] args) {
DayTester firstDay = new DayTester(Day.MONDAY);
firstDay.tell();
DayTester thirdDay = new DayTester(Day.WEDNESDAY);
thirdDay.tell();
DayTester fifthDay = new DayTester(Day.FRIDAY);
fifthDay.tell();
DayTester sixthDay = new DayTester(Day.SATURDAY);
sixthDay.tell();
DayTester seventhDay = new DayTester(Day.SUNDAY);
seventhDay.tell();
}
} // end of the class DayTester
Mondays are bad.
Midweek days are so-so.
Fridays are better.
Weekends are best.
Weekends are best.
2012-2013 Object-Oriented Programming: Enum types 8
Using an enum type
● In Java, an enum type also defines a class.
– The enum class body can include methods and other
fields.
● The compiler automatically adds some special
methods when it creates an enum.
– The static values() method that returns an array
containing all of the values of the enum in the order
they are declared.
Day[] days = Day.values();
for (Day d : days) {
System.out.println(d);
}
SUNDAY
MONDAY
TUESDAY
WEDNESDAY
THURSDAY
FRIDAY
SATURDAY
