Iterating issue of java.lang.Iterable instance [duplicate]

Question

My Code

import java.util.*;

public class Test<T> implements Iterable<T> {

    private List<T> persons = new ArrayList<T>();


    public static void main(String[] args) {

        Test<Test> ob = new Test<Test>();
        ob.persons.add(ob);

        for (Test t : ob.iterator()) {

            System.out.println();

        }

    }

    @Override
    public Iterator<T> iterator() {

        return persons.iterator();
    }

}

Error:

Can only iterate over an array or an instance of java.lang.Iterable

What's adifference between two, Why A can't be legal and B is working ?

    for (Test t : ob.iterator()) {   ----------------------A

        System.out.println();

    }


while (ob.iterator().hasNext()) { -------------------------- B 

        System.out.println();

   }

Want to know why A underlying code is not working ? Is something different happening in for loop ? While and for both are provided the same iterator state to be iterated.

Does this answer your question? Enhanced for-loop does not accept Iterator — Savior
– Savior, Commented Apr 13, 2020 at 14:05

Andy Turner · Accepted Answer · 2020-04-13 13:46:11Z

2

Drop the iterator call:

for (Test t : ob) {

You have to give the for loop an Iterable (or an array), not an Iterator.

This:

while (ob.iterator().hasNext()) {

Almost certainly doesn't do what you mean. Each time the loop guard is evaluated, this creates a new iterator, at the start of the iteration, and checks if there is a "next" element. Unless ob is empty initially, this will continue to be true on each iteration (unless you remove an element from ob inside the loop).

The (probably) correct way to do this is:

Iterator<T> iterator = ob.iterator();
while (iterator.hasNext()) { // use the same iterator instance throughout iteration.
  T next = iterator.next();
}

answered Apr 13, 2020 at 13:46

Andy Turner

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3 Comments

Maria Over a year ago

ob in for loop, how its iterable ? Just becasue its implemented Iterable interface. How ob did return iterable ? In while, it gets called explicitly.

Andy Turner Over a year ago

@Maria ob is iterable because it's a variable of type Test<T>, a class which implements Iterable<T>. The enhanced for loop is syntactic sugar which gets translated into a for loop: the compiler inserts the iterator() call. See the language spec.

Maria Over a year ago

Is it a different ways of doing same thing ? Am I wrong ?

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