Java 面向对象（七）

枚举

枚举的引入（模拟枚举）

class Student {
    private int restDay;

    public int getRestDay() {
        return restDay;
    }

    public void setRestDay(int restDay) {
        this.restDay = restDay;
    }
}

public class Test {
    public static void main(String[] args) {
        Student stu = new Student();
        stu.setRestDay(1);
        int res = stu.getRestDay();
        if (res == 6 || res == 7) {
            System.out.println("放假");
        } else {
            System.out.println("上课");
        }
    }
}

上面的代码，存在一些问题：

（1）数据不安全，输入8

stu.setRestDay(8);  // 输出：上课（没有星期八）

（2）业务逻辑不合理：中外一周的第一天不一样，中国第一天是周一，外国第一天是周天。

改进后：

class WeekDay {
    public static final int MONDAY = 1;
    public static final int TUESDAY = 2;
    public static final int WEDNESDAY = 3;
    public static final int THURSDAY = 4;
    public static final int FRIDAY = 5;
    public static final int SATURDAY = 6;
    public static final int SUNDAY = 7;
}

class Student {
    private int restDay;

    public int getRestDay() {
        return restDay;
    }

    public void setRestDay(int restDay) {
        this.restDay = restDay;
    }
}

public class Test {
    public static void main(String[] args) {
        Student stu = new Student();
        stu.setRestDay(WeekDay.MONDAY);
        int res = stu.getRestDay();
        if (res == 6 || res == 7) {
            System.out.println("放假");
        } else {
            System.out.println("上课");
        }
    }
}

改进后的代码，还存在问题：

（1）数据不安全，输入8

stu.setRestDay(8);  // 输出：上课（没有星期八）

再次改进：

class WeekDay {
    private WeekDay() {}
    public static final WeekDay MONDAY = new WeekDay();
    public static final WeekDay TUESDAY = new WeekDay();
    public static final WeekDay WEDNESDAY = new WeekDay();
    public static final WeekDay THURSDAY = new WeekDay();
    public static final WeekDay FRIDAY = new WeekDay();
    public static final WeekDay SATURDAY = new WeekDay();
    public static final WeekDay SUNDAY = new WeekDay();
}

class Student {
    private WeekDay restDay;

    public WeekDay getRestDay() {
        return restDay;
    }

    public void setRestDay(WeekDay restDay) {
        this.restDay = restDay;
    }
}

public class Test {
    public static void main(String[] args) {
        Student stu = new Student();
        stu.setRestDay(WeekDay.MONDAY);
        WeekDay res = stu.getRestDay();
        if (res == WeekDay.SATURDAY || res == WeekDay.SUNDAY) {
            System.out.println("放假");
        } else {
            System.out.println("上课");
        }
    }
}

上面的代码，问题解决了，但是代码不好看，太冗余。

什么是枚举

枚举：表示一个事物固定状态。

比如：季节（春，夏，秋，冬）、星期（周一到周日）、性别（男，女）

枚举的定义

格式：

[修饰符] enum 枚举的名称  {
    常量1，常量2，常量3……
}

比如：

enum Sex {
    MAN, FEMALE
}

枚举的本质

java 枚举是一个语法糖。是一个特殊的类，是多个常量对象的集合。

当我们定义一个枚举后，它的本质还是一个类，一个继承了Enum的类。

enum Sex {
    MAN, FEMALE
}

反编译后：

final class Sex extends Enum
{

    public static final Sex MAN;
    public static final Sex FEMALE;
    private static final Sex ENUM$VALUES[];

    private Sex(String s, int i)
    {
        super(s, i);
    }

    public static Sex[] values()
    {
        Sex asex[];
        int i;
        Sex asex1[];
        System.arraycopy(asex = ENUM$VALUES, 0, asex1 = new Sex[i = asex.length], 0, i);
        return asex1;
    }

    public static Sex valueOf(String s)
    {
        return (Sex)Enum.valueOf(Test/Sex, s);
    }

    static 
    {
        MAN = new Sex("MAN", 0);
        FEMALE = new Sex("FEMALE", 1);
        ENUM$VALUES = (new Sex[] {
            MAN, FEMALE
        });
    }
}