Java Doc for ConcurrentHashMap.java in  » Apache-Harmony-Java-SE » java-package » java » util » concurrent » Java Source Code / Java DocumentationJava Source Code and Java Documentation

Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException . However, iterators are designed to be used by only one thread at a time.

The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read.

This class implements all of the optional methods of the Map and Iterator interfaces.

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

This class is a member of the Java Collections Framework.
since:
   1.5
author:
   Doug Lea<
Parameters:
  K - > the type of keys maintained by this map<
Parameters:
  V - > the type of mapped values

 if (!map.containsKey(key)) 
 return map.put(key, value);
 else
 return map.get(key);
 

 
 if (map.get(key).equals(value)) {
 map.remove(key);
 return true;
 } else return false;
 

 
 if (map.get(key).equals(oldValue)) {
 map.put(key, newValue);
 return true;
 } else return false;
 

 
 if ((map.containsKey(key)) {
 return map.put(key, value);
 } else return null;
 

The value can be retrieved by calling the get method with a key that is equal to the original key.
Parameters:
  key - the table key.
Parameters:
  value - the value. the previous value of the specified key in this table,or null if it did not have one.
throws:
  NullPointerException - if the key or value isnull.

 if (!map.containsKey(key)) 
 return map.put(key, value);
 else
 return map.get(key);
 

 
 if (map.get(key).equals(value)) {
 map.remove(key);
 return true;
 } else return false;
 

 
 if (map.get(key).equals(oldValue)) {
 map.put(key, newValue);
 return true;
 } else return false;
 

 
 if ((map.containsKey(key)) {
 return map.put(key, value);
 } else return null;