为每个循环嵌套？

Question

        for ( Route r : timetable.keySet())
        {
            for (Station s: r?)   //compile error here.
        }

Hi all, basically I'm trying to write a method that adds stations to a combobox from a timetable. The timetable is essentially a hashmap that returns a Route, List(of type service), and a route is definined by a string representing the name of the route, and

// a list of stations visited on this route 
private ArrayList<Station> stations;

Basically I need to access the stations in the route so I'm using a foreach loop to get each route in the timetable and then trying to get all the stations in that route, but I'm not sure how I would do the second nested loop, as in which list should I be referring to as I am getting a compile error if I refer the route.

I cannot modify the Route class, as that has been provided and I need to work around that but it does have this method to return the stations

   public Station getStop(int i) {
    if (i < 1 || i > stations.size()) {
        throw new NoSuchStopException("No " + i + "th stop on route" + this);
    }
    return stations.get(i - 1);
}

Answer 1

You will have to modify the class Route and add a getter method that returns the ArrayList (because it's private ):

public class Route {
    // ...

    public List<Station> getStations()
    {
        return stations;
    }
}

Then you can use that method in the for loop:

for (Route r : timetable.keySet()) {
    for (Station s : r.getStations()) {
        // ...
    }   
}

Edit: As @DavidWallace mentioned, if you don't want someone to modify the ArrayList through the getter method, you can return an unmodifiableList :

return Collections.unmodifiableList(stations);

Answer 2

You could achieve a solution as follows but it doesn't use nested for-each loops since it requires catching the exception to know when there are no more stations for that route (this solution does not require any changes to the Route class):

for ( Route r : timetable.keySet())
{
    int stationIndex = 1;
    try {
        do {
            Station station = route.getStop(stationIndex++);
            // todo - put the station in the combo box
        } while(true);
    } catch(NoSuchStopException nsse) {
        // we have reached the index where there wasn't a stop, so move to the next route
    }
}

Answer 3

You could write this as

for ( Route r : timetable.keySet()) {
    for (Station s: r.getStations()) {

        // ...

    }
}

provided the Route class has a method like

public List<Station> getStations() {
    return Collections.unmodifiableList(stations);
}

Note the use of unmodifiableList . This prevents someone from writing code that calls getStations and then modifies the result. Basically, it should be impossible to use a "getter" method to modify a class; hence using the various "unmodifiable" wrapper methods in the Collections class is best practice for a getter that returns a collection.

Edit

Dealing with the new requirement of not being able to change Route , and assuming that there is a method getNumberOfStops as well as the getStop of the question, you could write

for (Route r : timetable.keySet()) {
    for (int stopNumber = 1; stopNumber <= r.getNumberOfStops(); stopNumber++) {
        Station s = r.getStop(stopNumber);
        // ...
    }
}

Answer 4

for-each loop iterate collection or array type and Route object r is not iterable type that is why it raised compiler error .

for ( Route r : timetable.keySet()){
     for (Station s: r.getStations()){ // Get the station List from route object
       ...
     }   
}

Note: assumed there must be getter method of stations in Route class.

Answer 5

Explanation of for each loop

for(XXX a : YYY)    
YYY --- > should be any class that implements iterable  
and XXX objects should be in YYY