Error in Converting a Map to ConcurrentMap in Java

Question

I have a Map<String,Object> to be converted to a ConcurrentMap<String,Object>

    Map<String,Object> testMap = new HashMap<String,Object>();
    testMap.put("test", null); //This null causing issues in conversion
    testMap.put("test2","123");
    testMap.put("test3",234);
    ConcurrentMap<String,Object> concMap = new ConcurrentHashMap<>(testMap);

I get a null pointer exception. If I copy to a new HashMap<String,Object>

    Map<String,Object> testMap = new HashMap<String,Object>();
    testMap.put("test", null);
    testMap.put("test2","123");
    testMap.put("test3",234);
    Map<String,Object> concMap = new HashMap<>(testMap);

I don't get any errors. Is there a safe way to a Map<String,Object> to a ConcurrentMap<String,Object> without the NullPointerException

The question was if there is a safe way to convert a Map to a ConcurrentMap so I don't see why this was closed a a duplicate — Joakim Danielson, Jan 15 '19 at 08:00
You can make use of `Optional.empty()` for null values if you want to convert to ConcurrentMap — Joakim Danielson, Jan 15 '19 at 14:52

score 2 · Answer 1 · answered Jan 15 '19 at 07:46

2

It's in the documentation for ConcurrentHashMap:

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

As for other ConcurrentMaps, the only other implementation ConcurrentSkipListMap, does not allow null either.

answered Jan 15 '19 at 07:46

daniu

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Please do not edit other answers to include your answer in theirs, thanks. – Zabuzard Jan 15 '19 at 07:50
@Zabuza Accidentally edited yours, undid it already in case you haven't noticed. There wasn't really a good reason to "include my answer in yours" since your answer was already the same. – daniu Jan 15 '19 at 08:19

score 2 · Accepted Answer · answered Jan 15 '19 at 07:47

If you look into the source code of ConcurrentHashMap you will see, it doesn't allow null key or value -

java.util.concurrent.ConcurrentHashMap#putVal

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();

score 0 · Answer 3 · edited Jan 15 '19 at 07:47

0

The issue is not about converting a HashMap into ConcurrentHashMap. It is more about ConcurrentHashMap being null-safe, it does not allow null values. From its documentation:

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

edited Jan 15 '19 at 07:47

daniu

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answered Jan 15 '19 at 07:47

Zabuzard

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Error in Converting a Map to ConcurrentMap in Java

3 Answers3