Iterating over keys and values in a Map

Question

Sometimes I find myself duplicating code to extract both key and value entries from a Map (when testing/debugging a third-party API, for example).

Map<String, String> someMap;
Set<String> keys = someMap.keySet();
for(int j=0;j<someMap.size();j++){
    String key = (String) keys.toArray()[j];
    System.out.println("key > " + key + "  : value = " + someMap.get(key));
}

I know Groovy has some great abstractions for this (e.g. Get key in groovy maps), but I'm constrained to POJ. Surely there must be a more elegant and less verbose way to do this, in Java I mean?

amit · Answer 1 · 2012-07-18T08:32:12.983

9

You can use Entry<String,String> and iterate over it with a for-each loop. You get the Entry object by using Map.entrySet().

The key and value from an entry can be extracted using Entry.getKey() and Entry.getValue()

Code example:

Map<String, String> someMap;
for (Entry<String,String> entry : someMap.entrySet()) {
    System.out.println("key > " + entry.getKey()+ "  : value = " + entry.getValue());
}

edited Jul 18 '12 at 08:32

answered Jul 18 '12 at 08:21

amit

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score 5 · Accepted Answer · answered Jul 18 '12 at 08:27

5

You can simplify the code by using the for each loop:

Map<String, String> someMap;
for(String key : someMap.keySet()){
    System.out.println("key > " + key + "  : value = " + someMap.get(key));
}

Or you do this with the entry set. Amit provided some code for that while I was still editing my answer ;-)

answered Jul 18 '12 at 08:27

André Stannek

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This solution is indeed more elegant then the original, but note that for iterating both keys and values - usually it is unadvised to iterate over keys and then get the value using the `get()` method - due to the performance overhead of an extra look up. When you iterate over the `Entry` set, it is expected to perform faster. – amit Jul 18 '12 at 08:31
1

After reading http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Map.Entry.html, I must agree with Amit. But for implementation purposes, this is my preffered answer. Thanks guys – raven Jul 18 '12 at 08:37

score 0 · Answer 3 · edited May 23 '17 at 11:55

0

Depending on what you're trying to do, Google collections has a Maps API that provides some helper functions to transform maps etc.

If you're looking to just pretty print the map, this post may be useful

edited May 23 '17 at 11:55

Community

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answered Jul 18 '12 at 08:25

qwerty

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Iterating over keys and values in a Map

3 Answers3