Puts the given value in the given ConcurrentHashMap for the given key if no mapping exists, and returns the new value. - Java java.util.concurrent

Java examples for java.util.concurrent:ConcurrentHashMap

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Description

Puts the given value in the given ConcurrentHashMap for the given key if no mapping exists, and returns the new value.

Demo Code


//package com.java2s;
import java.util.concurrent.ConcurrentHashMap;

public class Main {
    /**/*ww w  .j  a v a 2 s .c  o  m*/
     * Puts the given value in the given map for the given key if no mapping exists,
     * and returns the new value.
     * @param map The map to store the value in.
     * @param key The key in the map to store the value in.
     * @param absentValue The value to put if no mapping exists.
     * @return The value that is now stored in the map. This will be absentValue if
     * no mapping existed and the absentValue was stored, or the existing value if
     * one existed. 
     */
    public static <K, V> V putIfAbsent(ConcurrentHashMap<K, V> map, K key,
            V absentValue) {
        // First quickly try to get the value
        V value = map.get(key);
        // If we failed to retrieve one, try to put the absentValue
        if (value == null) {
            value = map.putIfAbsent(key, absentValue);
            // If there was not an old mapping, it is now mapped to absent value
            if (value == null) {
                value = absentValue;
            }
        }
        // Return the current mapping.
        return value;
    }
}

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