列表过程中的无限循环

Question

I have the following block of code:

ArrayList<ELPERouteStop> candidates = new ArrayList<>();
    for (int i=0; i < metric.length;i++) 
    {

        /*
         * We get a from object for each row (customer) and a to object for the element
         * of the matrix with the highest metric from the ith customer.
         */
        ELPERouteStop from = all_stops.get(i);
        ELPERouteStop to = all_stops.get(getMaxValue(metric[i]));
        if(i==0)
        {   
            candidates.add(from);
            candidates.add(to);
        }
        for (int j=0; j < candidates.size();j++)
        {
            int k=0;
            if (candidates.get(j) == from || candidates.get(j)==to) 
            {
                k++;
            }
            if (k == 0)
            {
                candidates.add(from);
                candidates.add(to);
            }
        }
    }

What seems to be the problem is the change of size of the candidates list at each iteration. I need to insert the from and to objects to the candidates list, only if they don't exist already. Hence, I do the check in the second for loop. When I run the code it appears to enter an infinite loop and I cannot find why. Thank you in advance!

Answer 1

You can use Collection#contains instead of iterating the collection each time.

List<ELPERouteStop> candidates = new ArrayList<>();
for (int i=0; i < metric.length;i++) {

    ELPERouteStop from = all_stops.get(i);
    ELPERouteStop to = all_stops.get(getMaxValue(metric[i]));

    if (!candidates.contains(from) && !candidates.contains(to)) {
        candidates.add(from);
        candidates.add(to);
    }
}

Or, slightly more efficient - depending on how fast getMaxValue is:

for (int i=0; i < metric.length;i++) {

    ELPERouteStop from = all_stops.get(i);
    if (!candidates.contains(from)) {
        ELPERouteStop to = all_stops.get(getMaxValue(metric[i]));
        if (!candidates.contains(to)) {
            candidates.add(from);
            candidates.add(to);
        }
    }
}

Answer 2

Reason for infinite loop is this conditional statement if (candidates.get(j) == from || candidates.get(j)==to) . As you are using == operator to compare the objects it always returns false because == operator always used to compare the reference of the objects not the values. So k value is increasing exponentially on every iteration so that it leads to infinte looping. for more details about the == and equals() method

Solution:

To compare the objects in collection you can use contains() . But for comparing objects you need to override the equals method in ELPERouteStop class.

For sample refer:

class Test{

String a;

Test(String a){
    this.a = a;
}
@Override
public boolean equals(Object obj) {
        return (this.a.equals( ((Test)obj).a));
}

}

public static void main(String[] args) {

    ArrayList<Test> arr = new ArrayList<Test>(4); 

    Test a = new Test("abc");

    arr.add(new Test("abc")); 

    boolean test = a == new Test("abc"); // Returns false due to reference checking

    boolean ans = arr.contains(a); // returns true as this checks with the value  

}

Answer 3

I need to insert the from and to objects to the candidates list, only if they don't exist already. This seems like a good candidate for a Set . You could try to use a Set instead of a List and let the data structure itself implement that constraint for you. If you go with that approach, make sure you override the equals and hashCode methods for your objects.

Answer 4

In case order matters:

ArrayList<ELPERouteStop> candidates = new ArrayList<>();
        for (int i=0; i < metric.length;i++) 
        {

            /*
             * We get a from object for each row (customer) and a to object for the element
             * of the matrix with the highest metric from the ith customer.
             */
            ELPERouteStop from = all_stops.get(i);
            ELPERouteStop to = all_stops.get(getMaxValue(metric[i]));
            if(i==0)
            {   
                candidates.add(from);
                candidates.add(to);
            }
            else if(!candidates.contains(from)&&!candidates.contains(to))
            {
                    candidates.add(from);
                    candidates.add(to);
            }
        }

In case order doesn't matter:

Set<ELPERouteStop> candidates = new hashset<>();
        for (int i=0; i < metric.length;i++) 
        {

            /*
             * We get a from object for each row (customer) and a to object for the element
             * of the matrix with the highest metric from the ith customer.
             */
            ELPERouteStop from = all_stops.get(i);
            ELPERouteStop to = all_stops.get(getMaxValue(metric[i]));
       //comment out to optimize
            //if(i==0)
            //{   
            //  candidates.add(from);
            //  candidates.add(to);
            //}
              candidates.add(from);
             candidates.add(to);
        }

Answer 5

Since both the loops are for-loop, it's impossible that you enter an infinite loop.

are you sure that the program was stopped in this block ?

I guess it is perhaps just because of low performance making it seems like an infinite loop, try to decrease the size of metric and run again to see if the program can stop.

Also you can use Set to optimize your code like this:

// first declare a set:
Set<int> candidatesSet = new HashSet<>();

// and after added each from/to to `candidates`
candidatedSet.add(from.hashCode())

// replace you second loop with
if (!candidatedSet.contains(from.hashCode()) && !candidatedSet.contains(to.hashCode())) {
    candidates.add(from);
    candidates.add(to);
    candidatedSet.add(from.hashCode());
    candidatedSet.add(to.hashCode());
}

Of course, you need to make sure that you hashCode returns distinct int for each different candidate