相关知识点：《Effective Java》 第33条：用EnumMap代替序数索引

第33条中提到了水的三态转化的例子。实际工作中常见的OA审核，也是相似的场景。

public enum Status {
    CANCEL,
    NOT_PASS,
    ONE,
    TWO,
    THREE,
    PASS,
    ;

    public enum Transition {
        ONE_CANCEL(ONE, CANCEL),
        TWO_CANCEL(TWO, CANCEL),
        THREE_CANCEL(THREE, CANCEL),
        ONE_NOT_PASS(ONE, NOT_PASS),
        TWO_NOT_PASS(TWO, NOT_PASS),
        THREE_NOT_PASS(THREE, NOT_PASS),
        ONE_TWO(ONE, TWO),
        TWO_THREE(TWO, THREE),
        THREE_PASS(THREE, PASS),
        ;

        private final Status from;
        private final Status to;

        Transition(Status from, Status to) {
            this.from = from;
            this.to = to;
        }
    }

    private static final Map<Status, Map<Status, Transition>> m = new EnumMap<>(Status.class);

    static {
        for (Status status : Status.values())
            m.put(status, new EnumMap<>(Status.class));
        for (Transition trans : Transition.values())
            m.get(trans.from).put(trans.to, trans);
    }

    public static Transition from(Status from, Status to) {
        return m.get(from).get(to);
    }

    public static Boolean isTransitionable(Status from, Status to) {
        Transition trans = from(from, to);
        return trans != null;
    }
}

m存储的数据如下：

image.png

审核时，一般会传入nextStatus，和currentStatus比较，以判断状态是否可转换，只需调用isTransitionable方法。
测试用例：

public class StatusTest {

    @Test
    public void testIsUpdatable() {

        // cancel
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.ONE));
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.TWO));
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.THREE));
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.CANCEL));
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.NOT_PASS));
        Assert.assertFalse(Status.isTransitionable(Status.CANCEL, Status.PASS));

        // not pass
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.ONE));
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.TWO));
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.THREE));
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.CANCEL));
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.NOT_PASS));
        Assert.assertFalse(Status.isTransitionable(Status.NOT_PASS, Status.PASS));

        // one
        Assert.assertFalse(Status.isTransitionable(Status.ONE, Status.ONE));
        Assert.assertTrue(Status.isTransitionable(Status.ONE, Status.TWO));
        Assert.assertFalse(Status.isTransitionable(Status.ONE, Status.THREE));
        Assert.assertTrue(Status.isTransitionable(Status.ONE, Status.CANCEL));
        Assert.assertTrue(Status.isTransitionable(Status.ONE, Status.NOT_PASS));
        Assert.assertFalse(Status.isTransitionable(Status.ONE, Status.PASS));

        // two
        Assert.assertFalse(Status.isTransitionable(Status.TWO, Status.ONE));
        Assert.assertFalse(Status.isTransitionable(Status.TWO, Status.TWO));
        Assert.assertTrue(Status.isTransitionable(Status.TWO, Status.THREE));
        Assert.assertTrue(Status.isTransitionable(Status.TWO, Status.CANCEL));
        Assert.assertTrue(Status.isTransitionable(Status.TWO, Status.NOT_PASS));
        Assert.assertFalse(Status.isTransitionable(Status.TWO, Status.PASS));

        // three
        Assert.assertFalse(Status.isTransitionable(Status.THREE, Status.ONE));
        Assert.assertFalse(Status.isTransitionable(Status.THREE, Status.TWO));
        Assert.assertFalse(Status.isTransitionable(Status.THREE, Status.THREE));
        Assert.assertTrue(Status.isTransitionable(Status.THREE, Status.CANCEL));
        Assert.assertTrue(Status.isTransitionable(Status.THREE, Status.NOT_PASS));
        Assert.assertTrue(Status.isTransitionable(Status.THREE, Status.PASS));

        // pass
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.ONE));
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.TWO));
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.THREE));
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.CANCEL));
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.NOT_PASS));
        Assert.assertFalse(Status.isTransitionable(Status.PASS, Status.PASS));
    }
}

如果不用EnumMap，一般想到的判断逻辑，可能是下面的这种：

public static Boolean isUpdate(Status from, Status to) {
    switch (from) {
        case ONE:
            switch (to) {
                case CANCEL:
                case NO_PASS:
                case TWO:
                    return true;
                default:
                    return false;
            }
        case TWO:
            switch (to) {
                case CANCEL:
                case NO_PASS:
                case THREE:
                    return true;
                default:
                    return false;
            }
        case THREE:
            switch (to) {
                case CANCEL:
                case NO_PASS:
                case PASS:
                    return true;
                default:
                    return false;
            }
        default:
            return false;
    }
}

相比这种方法，使用EnumMap的方法代码更简洁。

一般需要把状态值写入数据库中，只需要对上面代码进行改造即可：

Status增加2个属性，一个value，一个desc，并增加一构造函数

CANCEL(1, "取消"),

Status.CANCEL.getValue()