enum

1. enum public enum Week {Sunday, Moday, Tuesday, Wednesday, Thursday, Friday, Saturday} Week.Sunday.ordinal() 0 Week.Sunday “Sunday”

2. enum • 不可繼承 • public enum Suites extenEnum{ • } • 可實作 interface • 不能放在方法中 • 所有成員為static final • 可以用==或equals來比較 • Compiler會自動加入private final 於列值中

3. Enum • An enum specifies a list of constant values assigned to a type. • An enum is NOT a String or an int. • An enum constant's type is the enum type. • An enum can be declared outside or inside a class, but NOT in a method. • You can NEVER invoke an enum constructor directly.

4. Enum • The enum constructor is invoked automatically, with the arguments you define after the constant value. • You can define more than one argument to the constructor, and you can overload the enum constructors • An enum declared outside a class must NOT be marked static, final, abstract, protected, or private.

5. Enum • enum constants can send arguments to the enum constructor, using the syntax BIG(8). • Compiling an enum generates a .class file whose name is derived from the enum's name. • The semicolon at the end of an enum declaration is optional. These are legal: • enumFoo { ONE, TWO, THREE} • enumFoo { ONE, TWO, THREE}; • Foo.values() returns an array of MyEnum's values.

6. Declaring Enums enumProblem { BIG, HUGE, SMALL }   // this cannot be private or protectedclass MyClass {   Problem size;}public class MainClass {public static void main(String[] args) {MyClass drink = new MyClass();drink.size = Problem.BIG;           }}

7. An example of declaring an enum inside a class • class MyClass2 {enumProblem {BIG, HUGE, SMALL }  Problem size;}public class MainClass {public static void main(String[] args) {    MyClass2 drink = new MyClass2();drink.size = MyClass2.Problem.BIG;   // enclosing class name required  }}

8. You cannot declare enums in a method • enum Problem {BIG, HUGE, SMALL } • class MyClass { • Problem size; • } • public class MainClass { • public static void main(String[] args) { • enum Problem { BIG, HUGE, SMALL } // WRONG! • MyClass drink = new MyClass(); • drink.size = Problem.BIG; • } • } • Exception in thread "main" java.lang.Error: Unresolved compilation problem: • The member enum Problem cannot be local at MainClass.main(MainClass.java:9)

9. 8 BIG 8 HUGE 10 SMALL 16 • enum Problem { • // 8, 10 & 16 are passed to the constructor • BIG(8), HUGE(10), SMALL(16); • Problem(int ounces) { // constructor • this.ounces = ounces; • } • private int ounces; • public intgetOunces() { • return ounces; • } • } • class MyClass { • Problem size; • public static void main(String[] args) { • MyClass drink1 = new MyClass(); • drink1.size = Problem.BIG; • MyClass drink2 = new MyClass(); • drink2.size = Problem.SMALL; • System.out.println(drink1.size.getOunces()); • for(Problem cs: Problem.values()) • System.out.println(cs + " " + cs.getOunces()); • } • }

10. Enums may have main() methods and can be invoked as applications. • enumLightState { • RED, YELLOW, GREEN;public static void main(String[] argv) {System.out.println(LightState.RED.toString());  }}

11. Enums have built-in name() and toString() methods, both of which return the name of the current instance. public class MainClass {public static void main(String[] argv) {System.out.println(LightState.RED.name());  }}enumLightState {  RED, YELLOW, GREEN;}

12. You can add data, methods, and constructors to an enum. • enumSuit {  DIAMOND(true), HEART(true), CLUB(false), SPADE(false);private booleanred;  Suit(booleanb) {    red = b;  }public booleanisRed() {return red;  }public String toString() {    String s = name();    s += red ? ":red" : ":black";return s;  }}