There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code.Flon's Law

A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates from multiple threads. Iterators are designed to be used by only one thread at a time. ConcurrentHashMap does not guarantee the runtime of its operations as part of its contract. It also allows tuning for certain load factors.

Full support for accessing from concurrent threads | |

Do not throw ConcurrentModificationException. | |

Iterators can be accessed from only one thread. | |

Does not allow null to be used as a key or value. | |

When multiple threads try to retrieve and update, most recently completed update take to retrieve the data | |

The concurrent update based on concurrencyLevel constructor argument. If only one thread is updating and others only read, you can start concurrencyLevel with one. |

package com.careerdrill.collections.map; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ConcurrentHashMapUse { private static final String CONCURRENCY_LEVEL_DEFAULT = "2"; private static final String CONCURRENCY_KEY = "ConcurrentHashMap"; private ConcurrentMap< Double, Double > sqrtCache = new ConcurrentHashMap< Double, Double >(); public double getSqrt(Double d) { Double sqrt = sqrtCache.get(d); if(sqrt == null) { sqrt = Math.sqrt(d); System.out.printf("calculated sqrt of %s = %s%n", d, sqrt); Double existing = sqrtCache.putIfAbsent(d, sqrt); if(existing != null) { System.out.printf("Remove the calculated sqrt %s and use the cached sqrt %s", sqrt, existing); sqrt = existing; } } return sqrt; } public static void main(String[] args) { final ConcurrentHashMapUse test = new ConcurrentHashMapUse(); final int concurrencyLevel = Integer.parseInt(System.getProperty(CONCURRENCY_KEY, CONCURRENCY_LEVEL_DEFAULT)); final ExecutorService executor = Executors.newCachedThreadPool(); String []strarray={ "2", "4"}; try { for(int i = 0; i < concurrencyLevel; i++) { for(String s : strarray) { final Double d = Double.valueOf(s); executor.submit(new Runnable() { @Override public void run() { System.out.printf("sqrt of %s = %s in thread %s%n", d, test.getSqrt(d), Thread.currentThread().getName()); } }); } } } finally { executor.shutdown(); } } }

Calculated sqrt of 2.0 = 1.4142135623730951 Calculated sqrt of 2.0 = 1.4142135623730951 Calculated sqrt of 4.0 = 2.0 Calculated sqrt of 4.0 = 2.0 sqrt of 4.0 = 2.0 in thread pool-1-thread-4 Remove the calculated sqrt 1.4142135623730951 and use the cached sqrt 1.4142135623730951sqrt of 2.0 = 1.4142135623730951 in thread pool-1-thread-3 sqrt of 2.0 = 1.4142135623730951 in thread pool-1-thread-1 Remove the calculated sqrt 2.0 and use the cached sqrt 2.0sqrt of 4.0 = 2.0 in thread pool-1-thread-2

ConcurrentHashMap | ConcurrentSkipListMap |
---|---|

Does not guarantee the runtime of its operations | Guarantees average O(log(n)) performance |

Does not allow null to be used as a key or value | Null value and null keys are not allowed |

Do not maintain insertion order | Elements fetched in natural order |

Support Only Serializable | Support cloneable and Serializable |

Do not support ceilingEntry, ceilingKey, firstEntry, floorEntry operations | Support ceilingEntry, ceilingKey, firstEntry, floorEntry operations |

If you need to allow multiple threads using a hash map, you can use ConcurrentHashMap.

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