Reentrant use of HashMap::computeIfAbsent leaves the map in an inconsistent state

DoesNotCompute.java

import java.util.HashMap;
import java.util.Map;

public class DoesNotCompute {

    public static void main(String[] args) {
        System.out.println(System.getProperty("java.version"));

        Map<String, Integer> map = new HashMap<>();
        map.computeIfAbsent("spud", s -> calculate(s, map));

        System.out.println("map = " + map);
        System.out.println("map.size() = " + map.size());
        System.out.println("new HashMap(map).size() = " + new HashMap<>(map).size());
    }

    private static Integer calculate(String key, Map<String, Integer> map) {
        if (key.isEmpty()) {
            return 0;
        } else {
            String left = key.substring(0, key.length() - 1);
            String right = key.substring(1);
            return 1
                   + map.computeIfAbsent(left, s -> calculate(s, map))
                   + map.computeIfAbsent(right, s -> calculate(s, map));
        }
    }

}

output_1.8.0_131.txt

1.8.0_131
map = {p=1, =0, s=1, d=1, u=1, pu=3, spud=15, sp=3, pud=7, ud=3}
map.size() = 12
new HashMap(map).size() = 10

Abysmally, this also happens with a ConcurrentHashMap, and potentially puts the thread in an infinite loop.

At least now it crashes:

11
Exception in thread "main" java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1134)
	at DoesNotCompute.calculate(DoesNotCompute.java:24)
	at DoesNotCompute.lambda$calculate$1(DoesNotCompute.java:24)
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1133)
	at DoesNotCompute.calculate(DoesNotCompute.java:24)
	at DoesNotCompute.lambda$calculate$1(DoesNotCompute.java:24)
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1133)
	at DoesNotCompute.calculate(DoesNotCompute.java:24)
	at DoesNotCompute.lambda$main$0(DoesNotCompute.java:10)
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1133)
	at DoesNotCompute.main(DoesNotCompute.java:10)