Concurrency

ConcurrentHashMap: nullキーとnull値を許可しない

HashMapと異なり、ConcurrentHashMapはnullキー・null値のputを即座に例外にします。

Java 8+

pitfallconcurrencyconcurrenthashmapmapnull

01 / 01 proof

SourceConcurrentMapNulls.java

Java 21

package examples.concurrency;

import java.util.concurrent.ConcurrentHashMap;

public final class ConcurrentMapNulls {
  private ConcurrentMapNulls() {}

  public static void putNullKey() {
    new ConcurrentHashMap<String, String>().put(null, "value");
  }

  public static void putNullValue() {
    new ConcurrentHashMap<String, String>().put("key", null);
  }
}

Source✓Test

TestConcurrentMapNullsTest.java

Java 21

package examples.concurrency;

import static org.junit.jupiter.api.Assertions.assertThrows;

import org.junit.jupiter.api.Test;

class ConcurrentMapNullsTest {
  @Test
  void ConcurrentHashMapはnullキーとnull値を許可しない() {
    assertThrows(NullPointerException.class, ConcurrentMapNulls::putNullKey);
    assertThrows(NullPointerException.class, ConcurrentMapNulls::putNullValue);
  }
}

01Assertion

このテストで確認できること

nullキーのputはNullPointerExceptionになる
null値のputもNullPointerExceptionになる

HashMapからConcurrentHashMapへ置き換えると、nullの扱いも互換ではありません。欠損はOptionalなど別の明示的な表現で扱います。

OBSERVED RESULT

expected exception: null key → NullPointerException
actual:   NullPointerException
expected exception: null value → NullPointerException
actual:   NullPointerException

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