Effective Java

《Effective Java》

6. enum & annotation

6.1 enum basics

int枚举模式 -> String枚举模式 -> enum枚举模式

    1. Java的枚举本质上是int值
    1. Java枚举类型是通过公有的static final域为每个枚举常量导出实例类
    1. 因为没有可以访问的构造器, 所以枚举类型是真正的final
    1. 单元素枚举类型已经成为实现singleton的最佳方法, 它更简洁, 并且无偿提供了序列化机制, 绝对防止多次序列化
    1. 枚举类型是实例受控的, 它是singleton的泛型化, singleton本质上是单元素的枚举.

6.2 Constant-specific method implementaton(特定于常量的方法实现)

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public enum Operation {

    PLUS("+") {
        double apply(double x, double y) {
            return x + y;
        }
    },
    MINUS("-") {
        double apply(double x, double y) {
            return x - y;
        }
    },
    TIMES("*") {
        double apply(double x, double y) {
            return x * y;
        }
    },
    DIVIDE("/") {
        double apply(double x, double y) {
            return x / y;
        }
    };
    private final String symbol;

    @Override
    public String toString() {
        return symbol;
    }
    //默认是private, 且不能为protected和public
    Operation(String sym) {
        this.symbol = sym;
    }

    abstract double apply(double x, double y);
}

public class Test {
    public static void main(String[] ar) {
        double x = 2, y = 5;
        for (Operation op : Operation.values()) {
            System.out.printf("%f %s %f = %f%n", x, op, y, op.apply(x, y));
        }
    }
}

6.3 Strategy enum(策略枚举)

如果多个枚举常量同时共享相同的行为, 则考虑策略枚举.

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/***策略枚举模式****/
public enum PayrollDay {
    MONDAY(PayType.WEEKDAY),
    TUESDAY(PayType.WEEKDAY),
    WEDNESDAY(PayType.WEEKDAY),
    THURSDAY(PayType.WEEKDAY),
    FRIDAY(PayType.WEEKDAY),
    SATURDAY(PayType.WEEKEND),
    SUNDAY(PayType.WEEKEND);

    private final PayType payType;

    PayrollDay(PayType type) {
        this.payType = type;
    }

    double pay(double workedHours, double payRate) {
        return this.payType.pay(workedHours, payRate);
    }


    private enum PayType {
        WEEKDAY {
            @Override
            double overtimePay(double hrs, double payRate) {
                return hrs <= HOURS_PER_SHIFT ? 0 : (hrs - HOURS_PER_SHIFT) * payRate / 2;
            }
        },
        WEEKEND {
            @Override
            double overtimePay(double hrs, double payRate) {
                return hrs * payRate / 2;
            }
        };

        private static final int HOURS_PER_SHIFT = 8;

        abstract double overtimePay(double hrs, double payRate);

        double pay(double workedHour, double payRate) {
            return workedHour * payRate + overtimePay(workedHour, payRate);
        }
    }
}