เมธอดนามธรรมในอีนัม

พฤติกรรมเฉพาะค่าคงที่

public class Main {
    enum Operation {
        PLUS { public int apply(int a, int b) { return a + b; } },
        MINUS { public int apply(int a, int b) { return a - b; } };
        public abstract int apply(int a, int b);
    }
    public static void main(String[] args) {
        System.out.println(Operation.PLUS.apply(3, 4));
        System.out.println(Operation.MINUS.apply(10, 6));
    }
}

การทำงานของรูปแบบนี้

public class Main {
    enum Operation {
        TIMES { public int apply(int a, int b) { return a * b; } };
        public abstract int apply(int a, int b);
    }
    public static void main(String[] args) {
        // The constant is an instance of a generated subclass
        System.out.println(Operation.TIMES.getClass().getSimpleName().isEmpty()
            ? "anonymous subclass" : Operation.TIMES.apply(6, 7));
    }
}

เครื่องคำนวณครบชุด

public class Main {
    enum Operation {
        PLUS  { public double apply(double a, double b) { return a + b; } },
        MINUS { public double apply(double a, double b) { return a - b; } },
        TIMES { public double apply(double a, double b) { return a * b; } },
        DIVIDE{ public double apply(double a, double b) { return a / b; } };
        public abstract double apply(double a, double b);
    }
    public static void main(String[] args) {
        double x = 6, y = 3;
        for (Operation op : Operation.values()) {
            System.out.printf("%s -> %.1f%n", op, op.apply(x, y));
        }
    }
}

การผสานฟิลด์และพฤติกรรม

public class Main {
    enum Operation {
        PLUS("+")  { public double apply(double a, double b) { return a + b; } },
        MINUS("-") { public double apply(double a, double b) { return a - b; } };
        private final String symbol;
        Operation(String symbol) { this.symbol = symbol; }
        public abstract double apply(double a, double b);
        @Override public String toString() { return symbol; }
    }
    public static void main(String[] args) {
        Operation op = Operation.PLUS;
        System.out.println("2 " + op + " 3 = " + op.apply(2, 3));
    }
}

เหตุใดจึงไม่ใช้คำสั่งสวิตช์

public class Main {
    enum Op {
        SQUARE { public int apply(int n) { return n * n; } },
        CUBE   { public int apply(int n) { return n * n * n; } };
        public abstract int apply(int n);
    }
    public static void main(String[] args) {
        System.out.println(Op.SQUARE.apply(5));
        System.out.println(Op.CUBE.apply(3));
    }
}

เมธอดแบบรูปธรรมร่วมกับเมธอดนามธรรม

public class Main {
    enum Shape {
        CIRCLE { public double area(double x) { return Math.PI * x * x; } },
        SQUARE { public double area(double x) { return x * x; } };
        public abstract double area(double x);
        public String report(double x) { return this + " area = " + area(x); }
    }
    public static void main(String[] args) {
        System.out.println(Shape.CIRCLE.report(2));
    }
}

อีนัมกลยุทธ์

public class Main {
    enum PayType {
        WEEKDAY { int overtime(int mins) { return mins; } },
        WEEKEND { int overtime(int mins) { return mins * 2; } };
        abstract int overtime(int mins);
        int pay(int mins) { return mins + overtime(Math.max(0, mins - 480)); }
    }
    public static void main(String[] args) {
        System.out.println("Weekend pay: " + PayType.WEEKEND.pay(540));
    }
}

อีนัมที่มีอินเทอร์เฟซและบอดี

public class Main {
    interface Command { String run(); }
    enum Action implements Command {
        START { public String run() { return "starting..."; } },
        STOP  { public String run() { return "stopping..."; } };
    }
    public static void main(String[] args) {
        for (Action a : Action.values()) System.out.println(a.run());
    }
}

การเข้าถึงฟิลด์จากบอดี

public class Main {
    enum Tax {
        STANDARD(0.20) { double apply(double amt) { return amt * (1 + rate); } },
        REDUCED(0.05)  { double apply(double amt) { return amt * (1 + rate); } };
        protected final double rate;
        Tax(double rate) { this.rate = rate; }
        abstract double apply(double amt);
    }
    public static void main(String[] args) {
        System.out.printf("%.2f%n", Tax.STANDARD.apply(100));
    }
}

กำหนดค่าเริ่มต้นแล้วเขียนทับ

public class Main {
    enum Notifier {
        EMAIL,
        SMS { String send(String m) { return "SMS: " + m; } };
        String send(String m) { return "EMAIL: " + m; }
    }
    public static void main(String[] args) {
        System.out.println(Notifier.EMAIL.send("hi"));
        System.out.println(Notifier.SMS.send("hi"));
    }
}

เครื่องจักรสถานะขนาดย่อม

public class Main {
    enum State {
        IDLE    { State next() { return RUNNING; } },
        RUNNING { State next() { return DONE; } },
        DONE    { State next() { return DONE; } };
        abstract State next();
    }
    public static void main(String[] args) {
        State s = State.IDLE;
        for (int i = 0; i < 3; i++) { System.out.println(s); s = s.next(); }
        System.out.println(s);
    }
}

ตรวจสอบความเข้าใจ

ทบทวน

public class Main {
    enum E { A { int v() { return 1; } }; abstract int v(); }
    public static void main(String[] args) {
        System.out.println("Abstract enum recap: " + E.A.v());
    }
}