guava/src/com/google/common/io/MoreFiles.java
private static Collection<IOException> addException(
      @Nullable Collection<IOException> exceptions, IOException e) {
    if (exceptions == null) {
      exceptions = new ArrayList<>(); // don't need Set semantics
    }
    exceptions.add(e);
    return exceptions;
  }
private static Collection<IOException> addException(
      @Nullable Collection<IOException> exceptions, IOException e) {
    if (exceptions == null) {
      exceptions = new ArrayList<>(); // don't need Set semantics
    }
    exceptions.add(e);
    return exceptions;
  }
Adds the given exception to the given collection, creating the collection if it's null. Returns
the collection.