Java ConcurrentHashMap: reduce() Example
February 13, 2022
reduce() method of Java ConcurrentHashMap class.
Find the Java doc of
reduce() method.
<U> U reduce(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer, BiFunction<? super U,? super U,? extends U> reducer)
1. parallelismThreshold - The number of elements needed for this operation to be executed in parallel.
2. transformer - A
BiFunction that transforms the element. It means the BiFunction has key/value as input and combine it into one output.
3. reducer - A
BiFunction that accumulates all the outputs of transformer function.
Returns : The result of accumulating the given transformation of all key/value pairs.
The
reduce method returns the result of accumulating the given transformation of all key/value pairs using the given reducer to combine values, or null if none.
Example-1
ReduceDemo1.java
package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;
public class ReduceDemo1 {
public static void main(String[] args) {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("Mohan", 20);
map.put("Sohan", 22);
map.put("Vishal", 25);
String output = map.reduce(1, (k, v) -> k + "-" + v,
(s1, s2) -> s1 + ", " + s2);
System.out.println(output);
}
}
Mohan-20, Sohan-22, Vishal-25
Example-2
ReduceDemo2.java
package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;
public class ReduceDemo2 {
public static void main(String[] args) {
ConcurrentHashMap<Integer, Integer> map = new ConcurrentHashMap<>();
map.put(2, 20);
map.put(3, 30);
map.put(4, 40);
Integer output = map.reduce(1, (k, v) -> k * v,
(i1, i2) -> i1 + i2);
// 2*20 + 3*30 + 4*40
System.out.println(output); // Output: 290
}
}
