Question

我是Iterators的新手，不知道我做错了什么。在我的项目中，车站有汽车，有乘客，汽车也有乘客。我的目标是检查汽车是否已经到达目标站，如果没有，则将其从当前站移除并将其添加到下一站，将其移动到下一站。

        Iterator<Station> stations = allStations.iterator();
        while(stations.hasNext())
        {
            Station currentStation = (Station)stations.next();
            ArrayList<Car> currentStationCars = currentStation.getCarList();
            Iterator cars = currentStationCars.iterator();
            Car currentCar = (Car)cars.next();
            while(cars.hasNext())
            {

最初，我在这里声明了currentCar，但是这导致了NoSuchElement异常 - 我猜是因为我在每次迭代时都继续向前移动光标。不确定，我一小时前才刚刚了解到这一点。现在，这段代码导致无限循环。

                //original position of currentCar declaration
                int stepper = 0;
                if(currentCar.getCurrentLocation() < currentCar.getDestination())
                {
                    stepper = 1;
                }
                else if(currentCar.getCurrentLocation() > currentCar.getDestination())
                {
                    stepper = -1;
                }
                while(stepper != 0)
                {
                    currentCar.setCurrentLocation(currentCar.getCurrentLocation() + stepper);
                    currentStation.removeCar(currentCar);
                    if(currentCar.getCurrentLocation() < currentCar.getDestination())
                    {
                        stepper = 1;
                    }
                    else if(currentCar.getCurrentLocation() > currentCar.getDestination())
                    {
                        stepper = -1;                       
                    }
                    else { 
                        stepper = 0; 
                    }
            }
        }

Answer 1

你只是在循环之前推进迭代器一次。这意味着如果列表为空（例如，在检查cars.next()之前调用cars.hasNext()），并且如果它至少有一个元素则调用Car，您将获得异常，因为您没有＆＃39; t前进到循环中的下一个while (cars.hasNext()) { Car currentCar = (Car)cars.next(); .... }。

你应该只在循环内推进迭代器：

Iterator<Car> cars = currentStationCars.iterator();
...
while(cars.hasNext())
{
    Car currentCar = cars.next();
    ....
}

请注意，如果使用泛型类型，则可以避免转换：

Exception in thread "main" java.lang.NoClassDefFoundError: 
"com/google/protobuf/MessageOrBuilder
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$100(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$100(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at AddPerson.main(AddPerson.java:69)
Caused by: java.lang.ClassNotFoundException: 
com.google.protobuf.MessageOrBuilder
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 25 more"

使用迭代器的无限循环

1 个答案: