This question already has answers here:
Why Hashtable does not allow null keys or values?
(10 answers)
Closed 4 years ago.
I know about null key is not allowed in Hashtable because to store element in Hashtable hash code must required. But if key is null it will unable to calculate hash code for null key. But I don't understand but what is the exact reason in mind for Sun developers not to allow null value.
Someone says there is null check for value inside put method implementation and that's why it throws NullPointerException. But my question is why that null value check. Is there any specific reason behind it.
I went through lots of read but no got satisfied answer. Some one says there is ambiguity if there is null value and if you try to retrieve value using get() method it will return null and this null is because of actual value is null or key is missing that's why null and could not predict reason. So i need pin point answer with proof.
You will get NULL for value if you do
hashtable.get("key")
and "key" is not in the map, then you don't need to store null values.
If you would be able to store null, you will never know what you had: null mapping or that is a missing mapping.
Hashtable is considered legacy code. You should use HashMap and it allow null for values and also one key can be null.
EDIT
After deeper search I may have argument for such decision. Hashtable is synchronized (and HashMap isn't).
From JavaDoc:
Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.
As you can see successor of Hashtable is not HashMap as I previously write but ConcurrentHashMap. I was surprised that ConcurrentHashMap does not allows null. I start digging and found this:
From the author of ConcurrentHashMap himself (Doug Lea):
The main reason that nulls aren't allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. The main one is that if map.get(key) returns null, you can't detect whether the key explicitly maps to null vs the key isn't mapped. In a non-concurrent map, you can check this via map.contains(key), but in a concurrent one, the map might have changed between calls.
So maybe authors of Hashtable have the same reason as authors of ConcurrentHashMap
Having a null value is still considered a bad decision in HashMap and the new Map classes and the static factory methods in java-9 prove that:
Map.of("test", null)
will throw a NulPointerException
From Java Documentation
To successfully store and retrieve objects from a hash table, the
objects used as keys must implement the hashCode method and the equals
method
Null is not an object, so can not call .equals() or .hashCode() on it, so the Hashtable can't compute a hash to use it as a key
Hashtable containsValue(Object value) function throw NullPointerException if the value is null so for the value also not allowed null
Related
I'm trying to work with Java's Junit Test. My goal is to create a hashtable as
Hashtable< String , Hashtable<String,String> > student =
new Hashtable<String, Hashtable<String, String>>();
Next, I want to add at first only the key. After that, when I have the value then add it to the key in hashtable.
Example
student.put("student1",null) ;
I tried to work with null, but during test process I get a
java.lang.NullPointerException
Is it not possible? Any ideas to how adding only the key without value?
Is it possible to add a key without a value in hashtable Java?
Short answer: No
Why?
Because it will throw and Exception if
* #exception NullPointerException if the key or value is
* <code>null</code>
And in the same source code has a validation
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
...
You can use a Map Instead.
HashTable is an older implementation and HashMap came as an advanced version with more capabilities. You can't call .equals() or .hashCode() on it as null isn't an object.
HashMap is a better replacement for single threaded applications or any time synchronization is not a requirement, because of the performance impact synchronization introduces. If you need a Threadsafe option you can also use ConcurrentHashMap
The documentation says that you can't:
Maps the specified key to the specified value in this hashtable.
Neither the key nor the value can be null.
The best that you can do, if you need the key, is to use an empty HashTable as the value:
student.put("student1", new Hashtable<>());
In most use cases you would not care about putting null into a HashMap. You can check which keys have a value assigned via the keySet() and treat everything else as null (since it was never assigned).
However, your question to me indicates that you need to distinguish three states: Key-Value pair, key that were never assigned a value, and keys that were explicitly assigned a null value.
Since the default HashMap does not support this behavior, you could implement your own class that implements the Map interface. It basically only wraps a HashMap, with a few important exceptions:
When you assign a value of null to a key, you put the key into a Set of "NullKeys" instead of into the HashMap.
When you retrieve a key, check whether it is in the "NullKey" Set. If yes, return null, otherwise look into the HashMap.
There are some special cases (overwriting an existing value with null etc.) but this would be the basic strategy.
(I am not sure whether this is a useful class to have other than in very specialized scenarios but it would meet your requirements.)
The JavaDoc of ConcurrentHashMap says this:
Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.
My question: Why?
2nd question: Why doesn't Hashtable allow null?
I've used a lot of HashMaps for storing data. But when changing to ConcurrentHashMap I got several times into trouble because of NullPointerExceptions.
From the author of ConcurrentHashMap himself (Doug Lea):
The main reason that nulls aren't allowed in ConcurrentMaps
(ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that
may be just barely tolerable in non-concurrent maps can't be
accommodated. The main one is that if map.get(key) returns null, you
can't detect whether the key explicitly maps to null vs the key isn't
mapped. In a non-concurrent map, you can check this via
map.contains(key), but in a concurrent one, the map might have changed
between calls.
I believe it is, at least in part, to allow you to combine containsKey and get into a single call. If the map can hold nulls, there is no way to tell if get is returning a null because there was no key for that value, or just because the value was null.
Why is that a problem? Because there is no safe way to do that yourself. Take the following code:
if (m.containsKey(k)) {
return m.get(k);
} else {
throw new KeyNotPresentException();
}
Since m is a concurrent map, key k may be deleted between the containsKey and get calls, causing this snippet to return a null that was never in the table, rather than the desired KeyNotPresentException.
Normally you would solve that by synchronizing, but with a concurrent map that of course won't work. Hence the signature for get had to change, and the only way to do that in a backwards-compatible way was to prevent the user inserting null values in the first place, and continue using that as a placeholder for "key not found".
Josh Bloch designed HashMap; Doug Lea designed ConcurrentHashMap. I hope that isn't libelous. Actually I think the problem is that nulls often require wrapping so that the real null can stand for uninitialized. If client code requires nulls then it can pay the (admittedly small) cost of wrapping nulls itself.
You can't synchronize on a null.
Edit: This isn't exactly why in this case. I initially thought there was something fancy going on with locking things against concurrent updates or otherwise using the Object monitor to detect if something was modified, but upon examining the source code it appears I was wrong - they lock using a "segment" based on a bitmask of the hash.
In that case, I suspect they did it to copy Hashtable, and I suspect Hashtable did it because in the relational database world, null != null, so using a null as a key has no meaning.
I guess that the following snippet of the API documentation gives a good hint:
"This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details."
They probably just wanted to make ConcurrentHashMap fully compatible/interchangeable to Hashtable. And as Hashtable does not allow null keys and values..
ConcurrentHashMap is thread-safe. I believe that not allowing null keys and values was a part of making sure that it is thread-safe.
I don't think disallowing null value is a correct option.
In many cases, we do want do put a key with null value into the con-current map. However, by using ConcurrentHashMap, we cannot do that.
I suggest that the coming version of JDK can support that.
This question already has answers here:
Why does ConcurrentHashMap prevent null keys and values?
(7 answers)
Closed 9 years ago.
Most of the synchronized and concurrent collections like HashTable, ConcurrentHashMap etc do not allow null values. Is there any specific issue with null elements?
Hashtable is somewhat obsolete so I won't comment on it. As for ConcurrentHashMap one of the important additions of the API vs. a standard HashMap are a few atomic methods such as putIfAbsent. Javadoc:
Returns the previous value associated with the specified key, or null if there was no mapping for the key
In particular, if the map allowed null keys, the method would be a lot more complicated to use. A typical pattern, where you need to make sure that values can't be overwritten, is:
ConcurrentMap<K,V> map = new ConcurrentHashMap<> ();
V value = map.get(key);
if (value == null) {
value = new V();
V previousValue = map.putIfAbsent(key, value);
if (previousValue != null) { //Here you need to be sure what that means
value = previousValue;
}
}
useValue(value);
Another example is how you check if a key is in a HashMap (and you need the return value):
V value = map.get(key);
if (value == null && !map.containsKey(key)) {
}
The problem in a concurrent environment is that the whole thing is not atomic.
See also this post and these comments by the author of CHM.
HashTable.get(key) method returns null if the specified key is not present in the HashTable. If HashTable allows null as values, there can be two possibilities if I am getting a null from HashTable.get(key) method.
The key is not present in HashTable
The key is present, but the value set was null
It may be confusing for the user of the API.
I believe they don't allow null values just to prevent this ambiguity.
This is a synchronized map that accepts null key and values
Map m = Collections.synchronizedMap(new HashMap());
From the Hashtable JavaDoc:
To successfully store and retrieve objects from a hashtable, the
objects used as keys must implement the hashCode method and the
equals method.
In a nutshell, since null isn't an object, you can't call .equals() or
.hashCode() on it, so the Hashtable can't compute a hash to use it as
a key.
HashMap is newer, and has more advanced capabilities, which are
basically just an improvement on the Hashtable functionality. As such,
when HashMap was created, it was specifically designed to handle null
values as keys and handles them as a special case.
Specifically, the use of null as a key is handled like this when
issuing a .get(key):
(key==null ? k==null : key.equals(k))
Source:Why does Hashtable not take null key?
When you synchronize it means you will obtain lock on that object or at some portion. If your object is null then how will you decide that in which portion you will obtain lock?
Say in concurrenthashmaps, you can divide it into 16 different locks. Now how will you decide where null will be placed?
Null has no value attached to it, it simply means no object. So to avoid ambiguity its better not to have it.
I have some questions about Java Collection objects...
When we add objects to a collection like HashSet or HashMap, how the the objects internally stored?
Why doesn't Hashtable allow null values?
You're not adding an object to the collection. You're adding a reference.
As for why HashTable doesn't allow null keys and values - it's just a design decision; in some cases it's helpful to disallow null keys, while in others it's annoying. HashMap does allow for both null keys and values, for example. There are various reasons for prohibiting nulls:
Usually a null key or a null value indicates a programming error in the calling code. Rejecting this at the point of insertion makes it far easier to find the error than waiting until the code fetches a value and then has an unexpected null.
If you know that values in a map can't be null, then you don't need to do a separate check for containment and then fetch: you can fetch, and if the result is null, you know the key was missing.
It takes a bit more work to handle null keys in the map implementation. While null values can sometimes be useful, null keys almost never are.
Hashtable does not allow null values because it uses the equals and hashcode methods of the objects added to it
http://en.wikipedia.org/wiki/Hash_function
The JavaDoc of ConcurrentHashMap says this:
Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.
My question: Why?
2nd question: Why doesn't Hashtable allow null?
I've used a lot of HashMaps for storing data. But when changing to ConcurrentHashMap I got several times into trouble because of NullPointerExceptions.
From the author of ConcurrentHashMap himself (Doug Lea):
The main reason that nulls aren't allowed in ConcurrentMaps
(ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that
may be just barely tolerable in non-concurrent maps can't be
accommodated. The main one is that if map.get(key) returns null, you
can't detect whether the key explicitly maps to null vs the key isn't
mapped. In a non-concurrent map, you can check this via
map.contains(key), but in a concurrent one, the map might have changed
between calls.
I believe it is, at least in part, to allow you to combine containsKey and get into a single call. If the map can hold nulls, there is no way to tell if get is returning a null because there was no key for that value, or just because the value was null.
Why is that a problem? Because there is no safe way to do that yourself. Take the following code:
if (m.containsKey(k)) {
return m.get(k);
} else {
throw new KeyNotPresentException();
}
Since m is a concurrent map, key k may be deleted between the containsKey and get calls, causing this snippet to return a null that was never in the table, rather than the desired KeyNotPresentException.
Normally you would solve that by synchronizing, but with a concurrent map that of course won't work. Hence the signature for get had to change, and the only way to do that in a backwards-compatible way was to prevent the user inserting null values in the first place, and continue using that as a placeholder for "key not found".
Josh Bloch designed HashMap; Doug Lea designed ConcurrentHashMap. I hope that isn't libelous. Actually I think the problem is that nulls often require wrapping so that the real null can stand for uninitialized. If client code requires nulls then it can pay the (admittedly small) cost of wrapping nulls itself.
You can't synchronize on a null.
Edit: This isn't exactly why in this case. I initially thought there was something fancy going on with locking things against concurrent updates or otherwise using the Object monitor to detect if something was modified, but upon examining the source code it appears I was wrong - they lock using a "segment" based on a bitmask of the hash.
In that case, I suspect they did it to copy Hashtable, and I suspect Hashtable did it because in the relational database world, null != null, so using a null as a key has no meaning.
I guess that the following snippet of the API documentation gives a good hint:
"This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details."
They probably just wanted to make ConcurrentHashMap fully compatible/interchangeable to Hashtable. And as Hashtable does not allow null keys and values..
ConcurrentHashMap is thread-safe. I believe that not allowing null keys and values was a part of making sure that it is thread-safe.
I don't think disallowing null value is a correct option.
In many cases, we do want do put a key with null value into the con-current map. However, by using ConcurrentHashMap, we cannot do that.
I suggest that the coming version of JDK can support that.