Is there any Relation between Iterator.hasNext and for-each loop

Question

I was using JProfiler for profiling of my application, as it is a huge application so I am very aware of its performance and efficiency.
It was taking too long so I replace all Iterator.hasNext with for-each but when I am seeing in the JProfilers CPU view it is showing me Iterator.hasNext method called where I am using for-each.
Why does so? Is there any relation between these two? Here is the example code :

    List<Map<String, Object>> mapList = jdbcTemplate
                        .queryForList(MAP.SELECT_ALL);
                for (Map<String, Object> map : mapList) {
            list.add(fillPreferenceMaster(preferenceMasterMap));
}

http://stackoverflow.com/questions/2113216/which-is-more-efficient-a-for-each-loop-or-an-iterator they are the same thing when generated to bytecode — Sean, Feb 20 '12 at 06:00

score 10 · Accepted Answer · edited Jun 20 '20 at 09:12

Yes, the enhanced for statement uses Iterator under-the-covers for iterable collections, see Section 14.14.2 of the JLS:

If the type of Expression is a subtype of Iterable, then let I be the type of the expression Expression.iterator(). The enhanced for statement is equivalent to a basic for statement of the form:
for (I #i = Expression.iterator(); #i.hasNext(); ) {
        VariableModifiersopt Type Identifier = #i.next();
   Statement
}
Where #i is a compiler-generated identifier that is distinct from any other identifiers (compiler-generated or otherwise) that are in scope (§6.3) at the point where the enhanced for statement occurs.

score 2 · Answer 2 · answered Feb 20 '12 at 06:03

2

The for-each loop in Java uses the underlying iterator mechanism. So to answer your question, the code is identical.

This is most likely not a bottleneck in your code. You should identify the top 10 method hotspots using JProfiler and find out how to optimize those method calls.

answered Feb 20 '12 at 06:03

Amir Afghani

37,814
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I am doing that but most of the time is taken by jsp methods and they are visible in very strange manner. I don't know which tag is taking too much time...... – NIVESH SENGAR Feb 20 '12 at 06:33

score 0 · Answer 3 · answered Feb 20 '12 at 06:01

0

The for-each statement uses the iterator to iterate over your collection. The compiler replaces your for-each with a while loop.

answered Feb 20 '12 at 06:01

Alex Calugarescu

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Is there any Relation between Iterator.hasNext and for-each loop

3 Answers3