ConcurrentHashMap
- Java course
- Advanced
- Lesson 64/82
- 5 min
Learn parallel maps with ConcurrentHashMap.
ConcurrentHashMap
ConcurrentHashMap is a thread-safe map. Unlike HashMap, many threads can read and write concurrently; no synchronized lock is needed.
ConcurrentHashMap<String, Integer> map =
new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);Internally it locks in segments, so most operations run in parallel. Methods like computeIfAbsent and merge provide atomic updates. This is ideal for high-volume caches and counters.
Atomic updates
computeIfAbsent computes and inserts the value if the key is absent, atomically. merge combines with the existing value.
map.computeIfAbsent("a", k -> 0);
map.merge("a", 1, Integer::sum); // 0 + 1 = 1These methods do insert-if-absent and update patterns without locks. They prevent race conditions; the code is both safer and shorter. This is the standard approach for high-volume counter applications.
Practice Counter with ConcurrentHashMap
Write main in a CounterDemo class; create a ConcurrentHashMap<String, Integer>. Call map.merge("a", 1, Integer::sum) three times. Finally print map.get("a").
Starter code
import java.util.concurrent.*;
// Write main in CounterDemo
// Create a ConcurrentHashMap; increment "a" three times with merge; print getSample solution
import java.util.concurrent.*;
class CounterDemo {
public static void main(String[] args) {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.merge("a", 1, Integer::sum);
map.merge("a", 1, Integer::sum);
map.merge("a", 1, Integer::sum);
System.out.println(map.get("a"));
}
}Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the difference between ConcurrentHashMap and HashMap?
- It is slower
- It only accepts String keys
- It is sorted
- It is thread-safe Correct answer
Explanation: ConcurrentHashMap is thread-safe.
2 Match ConcurrentHashMap methods
| Term | Meaning |
|---|---|
put |
Adds a value |
get |
Reads a value |
computeIfAbsent |
Computes if missing |
merge |
Combines |
3 Complete the computeIfAbsent call
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.____("a", k -> 0);ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.computeIfAbsent("a", k -> 0);Explanation: computeIfAbsent computes if absent.