apache/beam · error · IllegalArgumentException

List of SerializableFunction must be the same size as the…

Error message

List of SerializableFunction must be the same size as the PCollectionList

What it means

PAssert's PCollectionListContentsAssert.satisfies(List<SerializableFunction>) requires one checker function per PCollection in the list. When the list of checkers has a different size than the PCollectionList, it throws this IllegalArgumentException, because there would be no defined pairing between checkers and collections.

Solutions

Provide exactly one SerializableFunction per PCollection in the PCollectionList, in order
Derive the checker list from the PCollectionList itself so sizes can't diverge
Assert each PCollection separately with PAssert.that(pcollection).satisfies(...) instead of the list variant

Example fix

// before
assertThat(list).satisfies(Collections.singletonList(checker)); // list has 2 PCollections
// after
assertThat(list).satisfies(Arrays.asList(checker1, checker2));

Defensive patterns

Strategy: validation

Validate before calling

if (checkerFnList.size() != pcollectionList.size()) {
  throw new IllegalStateException("Need " + pcollectionList.size() + " checkers, got " + checkerFnList.size());
}

Type guard

boolean checkersMatchList(List<?> checkers, PCollectionList<?> list) {
  return checkers != null && checkers.size() == list.size();
}

Try / catch

try {
  assertThat(pcollectionList).satisfies(checkers);
} catch (IllegalArgumentException e) {
  if (e.getMessage().contains("same size as the PCollectionList")) {
    throw new IllegalStateException("Add/remove checkers to match PCollectionList size", e);
  }
  throw e;
}

Prevention

Generate the checker list from the same code that builds the PCollectionList so sizes stay in sync
Update checker lists whenever the pipeline adds PCollections to the list
Prefer per-PCollection PAssert.that(...).satisfies(...) for lists that change shape

When it happens

Trigger: Calling PAssert.that(pcollectionList).satisfies(checkers) where checkers.size() != pcollectionList.size() — e.g. building checkers for a subset of PCollections, or the PCollectionList gains an element while the checker list does not.

Common situations: Tests whose pipeline is modified to add another PCollection to the list but whose checker list wasn't updated; dynamically generated lists where counts drift.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/665dbd174b19ed68. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java:622

      for (int i = 0; i < pCollectionList.size(); i++) {
        PAssert.that(pCollectionList.get(i)).satisfies(checkerFn);
      }
      return this;
    }

    /**
     * Takes list of {@link SerializableFunction}s of the same size as {@link #pCollectionList}, and
     * applies each matcher to the {@code PCollection} with the identical index in the {@link
     * #pCollectionList}.
     *
     * <p>Returns this {@code PCollectionListContentsAssert}.
     */
    public PCollectionListContentsAssert<T> satisfies(
        List<SerializableFunction<Iterable<T>, Void>> checkerFnList) {
      if (checkerFnList == null) {
        throw new IllegalArgumentException("List of SerializableFunction must not be null");
      } else if (checkerFnList.size() != pCollectionList.size()) {
        throw new IllegalArgumentException(
            "List of SerializableFunction must be the same size as the PCollectionList");
      }
      for (int i = 0; i < pCollectionList.size(); i++) {
        PAssert.that(pCollectionList.get(i)).satisfies(checkerFnList.get(i));
      }
      return this;
    }
  }

  /**
   * An {@link IterableAssert} about the contents of a {@link PCollection}. This does not require
   * the runner to support side inputs.
   */
  protected static class PCollectionContentsAssert<T> implements IterableAssert<T> {
    private final PCollection<T> actual;
    private final AssertionWindows rewindowingStrategy;
    private final SimpleFunction<Iterable<ValueInSingleWindow<T>>, Iterable<T>> paneExtractor;
    private final PAssertionSite site;

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