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it is possible to iterate Map by using enhanced loop?

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posted 4 years ago

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Enthuware Question:

Which of the following are true about the enhanced for loop?

1. It can iterate over an array or a Collection but not a Map.
2. Using an enhanced for loop prevents the code from going into an infinite loop.
3. Using an enhanced for loop on an array may cause infinite loop.
4. An enhanced for loop can iterate over a Map.
5. You cannot find out the number of the current iteration while iterating.

Correct Answers is :1,2,5
My Answer is : 2,4,5

My doubt is 1 Question it possible to iterate Map or not by using enhanced loop.

but i will try this example it's fine to run.

Campbell Ritchie

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posted 4 years ago

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You aren't iterating a Map, but a Set.
Why did you think it is impossible to go into an infinite loop? An infinite loop remains an infinite loop however you write it, and it is possible to go into an infinite loop if you manage to write a circular linked list. Ordinary arrays or linear linked lists won't go into an infinite loop if you iterate them.

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Tim Holloway

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posted 4 years ago

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You can iterate any Collection. Maps are Collections, Collections are Iterable. Q.E.D.

Now some Collections may not be idempotently iterable, and unless a Collection is explicitly defined as being an Ordered Collection, the sequence in which the elements may iterate can be unpredictable, but you can nonetheless iterate them.

Furthermore, in the case of a Map, the Entries as key/value pairs may be iterated, as well as the keySet and EntrySet Collections defined within the Map.

The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.

You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.

Adrian Grabowski

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posted 4 years ago

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Tim Holloway wrote:Maps are Collections

Are you sure?

Tim Holloway

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posted 4 years ago

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Well, it seems that in Java, a Map is a container for Collections, rather than a direct implementation or descendant of Collection itself. So literally speaking, a Map isn't itself iterable. But its aspects are.

Ron McLeod

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posted 4 years ago

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Map is part of the Collections Framework, and like Tim mentioned, does provide iterable views for the keys, values, and entries.

Javadoc for Interface Map<K,V> wrote:The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

Campbell Ritchie

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But Map itself isn't Iterable, which it probably why question 1 was shown as right.

Tim Holloway

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posted 4 years ago

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Technically the truth, but have you ever carelessly said "I'll iterate over this Map" as a shorthand for walking the keys or values?

Actually, an occasional question I've dealt with in the JSF forum has been when people tried to use a Map as a Model object and I had to explain to them that maps were not iterable because they were not ordered collections. I'm not entirely sure but what EL doesn't find something to iterate when the item in question is a Map, just not something that's got a guaranteed order.

As is so often the case, the "right" answers on exams are frequently determined by what context you consider the questions under.

Also, question 4 seems redundant after question 1.

Paul Anilprem

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As per Section 14.4.2 of JLS 8, "The type of the Expression must be Iterable or an array type (§10.1), or a compile-time error occurs.". Map does not implement Iterable and is not a Collection either. (Yes, it is part of the Collections Framework but is not a Collection itself).
Therefore, the given answer is correct.
Option 1 and option 4 may look the same but are actually different.

BTW, the following explanation is already given with option 1:

An enhanced for loop needs either an array or an object of a class that implements java.lang.Iterable. Map does not implement Iterable, though you can use keySet() or values() methods to get a Collection (which extends Iterable) and then iterate over that Collection.

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Campbell Ritchie

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Tim Holloway wrote:. . . have you ever carelessly said "I'll iterate over this Map" . . .? . . . .

Of course we have; we say all sorts of things carelessly. I once carelessly thought to myself, “I'll put some water from the kettle on my coffee,” and just as carelessly didn't put the kettle on first. Fortunately cold coffee still tastes like coffee and I could microwave it. Tea would have tasted like water

Tim Holloway

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It took me a minute to figure out what sort of preparation method you were using until I realized it's probably pour-over. I'm too used to machines or pots.

But the analogy was inexact, since when you got done, you only had "partial" coffee, but when I iterate a Map, the entire Map is indeed iterated. It's only that the details of the process are implicit.

Campbell Ritchie

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Pour‑over with a nylon filter.