Trouble with iterator in Java

Question

First of all, and in order for you to understand this question, i'm going to briefly explain my project:

I have a class named Pair, which basically creates the type

Pair<String,Double>

and has a getFirst() and getSecond() methods for returning the String and Double values respectively.

Then i have another class, named Package, which basically consists in a list of Pairs, and implements the Iterable interface, so i can iterate trough the list:

Package<Pair<String,Double>> package;
List <Pair<String,Double>> list;

What i want is to sum the doubles on each Pair, using the iterator().

The iterator is defined like this for the Package class:

public Iterator<E> iterator() {

        return this.iterator();
    }

I've tried two different approaches, which in both cases resulted in a:

Exception in thread "main" java.lang.StackOverflowError
    at Package.iterator(Package.java:98)

Here's the first one:

public static double packageWeight(Package<Pair<String, Double>> package) {

    double sum = 0;
    Pair<String, Double> pair;

    while (package.iterator().hasNext()) {

        pair = package.iterator().next();
        sum = sum + pair.getSecond();

    }

    return sum;
}

And the second one:

public static double packageWeight(Package<Pair<String, Double>> package) {

    double sum = 0;
    Pair<String, Double> pair;

    Iterator<Pair<String,Double>> it = Package.iterator();

    while (it.hasNext()) {

        pair = it.next();
        sum = sum + pair.getSecond();

    }

    return sum;
}

My question is: What am i doing wrong in order to get this error?

Also, don't you need to pass the iterator an object to iterate over? In the packageWeight method... — holtc
– holtc, Commented Apr 7, 2015 at 14:05
You can't have a variable called package, surely? It's a java keyword. — Andy Turner
– Andy Turner, Commented Apr 7, 2015 at 14:06
You also can't be invoking Package.iterator() unless you make it static. — Andy Turner
– Andy Turner, Commented Apr 7, 2015 at 14:11
The first code is fundamentally wrong anyway. You'll always get a new iterator (at least if its correctly implemented), so you should have an infinite loop. Also since you perform a read-only operation, let your class implement the Iterable interface and use a for-each. That will be less error prone... — Alexis C.
– Alexis C., Commented Apr 7, 2015 at 14:11

Andy Turner · Accepted Answer · 2015-04-07 14:13:02Z

5

The immediate cause of the StackOverflowError is that you are invoking the method in itself:

public Iterator<E> iterator() {

        return this.iterator();
    }

You need to invoke a different method here, e.g. return list.iterator() or package.iterator() (except that package is not a valid identifier in Java, since it is a keyword).

answered Apr 7, 2015 at 14:13

Andy Turner

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2 Comments

laker001 Over a year ago

yes! Indeed the problem was solved using my second approach and changing the return status to list.iterator(), thanks!

laker001 Over a year ago

about the package.iterator() problem, i was just translating the question to english, and it turned out to look like i had a problem there! the original name is different though

Community · Accepted Answer · 2017-05-23 12:22:11Z

2

Your first attempt is flawed because you are requesting a new iterator each time you call package.iterator() - I think you knew that because you then wrote the second one.

The fault with the second is more subtle - you need to use package.iterator() not Package.iterator(). The second should not work anyway.

And what Andy Turner said.

edited May 23, 2017 at 12:22

CommunityBot

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answered Apr 7, 2015 at 14:12

OldCurmudgeon

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