ConcurrentHashmap is a thread-safe version of HashMap, but there are some differences.
Basic: Map Somehow the difference in Map
HashMap
ConcurrentHashMap
[Added from java 1.5] to achieve exclusive control [https://qiita.com/YanHengGo/items/21ca9408fb59ddb7a566). But does not allow nulls.
ConcurrentHashMap.java
/**
* Maps the specified key to the specified value in this table.
* Neither the key nor the value can be null.
*
* <p> The value can be retrieved by calling the <tt>get</tt> method
* with a key that is equal to the original key.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>
* @throws NullPointerException if the specified key or value is null
*/
public V put(K key, V value) {
if (value == null)
throw new NullPointerException();
int hash = hash(key.hashCode());
return segmentFor(hash).put(key, hash, value, false);
}
Example:
Sample1.java
ConcurrentHashMap map = new ConcurrentHashMap();
map.put("test", null);
Exception in thread "main" java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:881)
at playground.Sample1.main(Sample1.java:9)
Sample2.java
ConcurrentHashMap map = new ConcurrentHashMap();
map.put(null, "test");
Exception in thread "main" java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:882)
at playground.Sample2.main(Sample2.java:12)
There is a source that does this when replacing HashMap → ConcurrentHashmap.
Realcode.java
if (inputvalue == null) {
return;
}
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