New check: PreferMapEntryToIterateWholeMap

[romani]

Consider user Map.Entry for whole map iteration instead of
getKeys() and then get value by key.
check could simply check while/for for such case. This check will not find all
weird cases but will help significantly with major cases, that are done as C++ habit, refactoring, .....

http://sourceforge.net/p/checkstyle/feature-requests/586/

from Check JavaDoc:
This check can help you to write the whole for-each map iteration more correctly:
If you iterate over a map using map.keySet() or map.entrySet(), but your code uses only map values, Check will propose you to use map.values() instead of map.keySet() or map.entrySet(). Replacing map.keySet() or map.entrySet() with map.values() for such cases can a bit improve an iteration performance.
Bad:

for (Map.Entry; entry : map.entrySet()){
   System.out.println(entry.getValue());
}
for (String key : map.keySet(){
   System.out.println(map.get(key));
}

Good:

for (String value : map.values()){
   System.out.println(value);
}

If you iterate over a map using map.entrySet(), but never call entry.getValue(), Check will propose you to use map.keySet() instead of map.entrySet(). to iterate over map keys only.
Bad:

for (Map.Entry entry : map.entrySet()){
   System.out.println(entry.getKey());
}

Good:

for (String key : map.keySet()){
   System.out.println(key);
}

If you iterate over a map with map.keySet() and use both keys and values, check will propose you to use map.entrySet() to improve an iteration performance by avoiding search operations inside a map. For this case, iteration can significantly grow up a performance.
Bad:

for (String key : map.keySet()){
   System.out.println(key + "  " + map.get(key));
}

Good:

for (Map.Entry entry : map.entrySet()){
   System.out.println(entry.getValue() + "   " + entry.getKey());
}

[daniilyar]

I think, this is a useful check (idea was taken from the same-named FindBugs Check).

The main check idea is:

1. To force developer to iterate over maps using Map.Entry instead of [ keyset() + getValue(key) ] for every map entry in case if developer needs both keys and values.
2. To force developer to use keySet() method in case if developer does not use the appropriate values ( == if map.getValue(key) method is never executed inside a loop).

Such type of refactoring can improve performance when it is done in frequently-used code.

But if developer needs keys only for his algorithm, we should not prevent this, or show any warnings.

Code examples:
http://www.sergiy.ca/how-to-iterate-over-a-map-in-java/

http://stackoverflow.com/questions/12639259/findbugs-warning-inefficient-use-of-keyset-iterator-instead-of-entryset-iterato

and:

        Map<String, String> map = Maps.newLinkedHashMap();
        map.put("key1", "value1");
        map.put("key2", "value2");
        map.put("key3", "value3");

        System.out.println("\nUsing map.keySet():");
        for (String key : map.keySet()) {
            System.out.println(key + " --> " + map.get(key));
        }

        System.out.println("\nUsing Map.Entry:");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " --> " + entry.getValue());
        }