Question

我只是在学习Java枚举。当我在下面运行代码时，我得到一个错误，下面也会重现。基本上，我的问题是：当我在枚举中定义一个方法时，我想在该方法中检查枚举的值，以便可以基于该值执行某些操作，如何执行此检查？下面，我有一个带有三个可能值的Enum，在方法getNext中，我有三个if语句，将这个Enum的值与三个可能值中的每一个进行比较。但是我仍然收到一条错误消息，说存在一条没有回报的道路。

package enumerations;

enum TrafficLightColor2 {
    RED(12), GREEN(10), YELLOW(2);

    private int waitTime;

    TrafficLightColor2(int waitTime) {
        this.waitTime = waitTime;
    }

    int getWaitTime() {
        return waitTime;
    }

    TrafficLightColor2 getNext() {
        if (this.equals(TrafficLightColor2.GREEN)) {
            return TrafficLightColor2.YELLOW;
        }
        if (this.equals(TrafficLightColor2.YELLOW)) {
            return TrafficLightColor2.RED;
        }
        if (this.equals(TrafficLightColor2.RED)) {
            return TrafficLightColor2.GREEN;
        }
    }
}

// A computerized traffic light.
class TrafficLightSimulator2 implements Runnable {
    private Thread thrd; // holds the thread that runs the simulation
    private TrafficLightColor2 tlc; // holds the traffic light color
    boolean stop = false; // set to true to stop the simulation
    boolean changed = false; // true when the light has changed

    TrafficLightSimulator2(TrafficLightColor2 init) {
        tlc = init;
        thrd = new Thread(this);
        thrd.start();
    }

    TrafficLightSimulator2() {
        tlc = TrafficLightColor2.RED;
        thrd = new Thread(this);
        thrd.start();
    }

    // Start up the light.
    public void run() {
        while (!stop) {
            try {
                Thread.sleep(tlc.getWaitTime());
            } catch (InterruptedException exc) {
                System.out.println(exc);
            }
            changeColor();
        }
    }

    // Change color.
    synchronized void changeColor() {
        tlc = tlc.getNext();
        changed = true;
        notify(); // signal that the light has changed
    }

    // Wait until a light change occurs.
    synchronized void waitForChange() {
        try {
            while (!changed)
                wait(); // wait for light to change
            changed = false;
        } catch (InterruptedException exc) {
            System.out.println(exc);
        }
    }

    // Return current color.
    synchronized TrafficLightColor2 getColor() {
        return tlc;
    }

    // Stop the traffic light.
    synchronized void cancel() {
        stop = true;
    }
}


class TrafficLightDemo2 {
    public static void main(String args[]) {
        TrafficLightSimulator tl =
                new TrafficLightSimulator(TrafficLightColor.GREEN);
        for (int i = 0; i < 9; i++) {
            System.out.println(tl.getColor());
            tl.waitForChange();
        }

        tl.cancel();
    }
}

我得到了错误

$ javac enumerations/TrafficLightDemo2.java
enumerations/TrafficLightDemo2.java:26: error: missing return statement
    }
    ^
1 error

Answer 1

TrafficLightColor2 getNext() {
    if (this.equals(TrafficLightColor2.GREEN)) {
        return TrafficLightColor2.YELLOW;
    }
    if (this.equals(TrafficLightColor2.YELLOW)) {
        return TrafficLightColor2.RED;
    }
    if (this.equals(TrafficLightColor2.RED)) {
        return TrafficLightColor2.GREEN;
    }
}

如果所有3个if为假，则此方法不返回值。

在处添加return或更好地抛出错误，例如

throw new IllegalArgumentException("Unsupported enum")

Answer 2

在枚举类中使用实例字段的优点是，您可以轻松地将实现详细信息与独立于API的常量相关联。换句话说，您可以轻松地将数据与枚举常量关联，这样就可以接受一种优雅的解决方案，例如在您需要添加新的枚举常量的情况下，您并不会永远结婚。

因此，您可以在履行以下相同合同的同时大大简化实施：

enum TrafficLightColor2 {
    RED(2, 12), 
    GREEN(0, 10), 
    YELLOW(1, 2);


    private int order;   // implementation detail; non-exported
    private int waitTime;

    TrafficLightColor2(int ord, int waitTime) {
        this.order = ord;
        this.waitTime = waitTime;
    }

    int getWaitTime() {
        return waitTime;
    }

    TrafficLightColor2 getNext() {
        final int nextColor = (this.order + 1) % 3;  // magic numbers introduce fragility
        return Arrays.stream(TrafficLight2.values())
                .filter(e -> e.order == nextColor)
                .findAny()
                .get();
    }
}

此版本与原始实现相比有一些优点：维护更容易，因为如果添加了枚举常量，则编译器将强制您添加订单值。在原始版本中，如果您在添加常量后忘记修改if-else-block，则程序将继续运行，但不会提供正确的行为。并且由于order的实现是隐藏的，因此您可以随时将其删除或将其更改为其他实现，而不会影响API的正确性。

Answer 3

您是否考虑过将下一个状态与声明的值一起包含？

public enum TrafficLightColor2 {
    RED(12, "GREEN"), GREEN(10, "YELLOW"), YELLOW(2, "RED");

    int waitTime;
    String nextState;
    Configurations(int waitTime, String nextState) {
        this.waitTime = waitTime;
        this.nextState = nextState;
    }
    public int getWaitTime() {
        return waitTime;
    }
    public String getNextState() {
        return nextState;
    }
}

有了这个，你可以得到下一个状态

TrafficLightColor2 trafficLightColor = TrafficLightColor2.GREEN;
System.out.println(TrafficLightColor2.valueOf(trafficLightColor.getNextState()));

如何在Enum方法中将Enum的值与Enum的所有可能值进行比较，并避免丢失return语句？

3 个答案: