code: package it.unibz.inf.ontop.model.term.functionsymbol.db.impl;
  
  import com.google.common.collect.ImmutableList;
  import com.google.common.collect.ImmutableSet;
  import it.unibz.inf.ontop.iq.node.VariableNullability;
  import it.unibz.inf.ontop.model.term.*;
  import it.unibz.inf.ontop.model.term.functionsymbol.db.DBConcatFunctionSymbol;
  import it.unibz.inf.ontop.model.term.functionsymbol.db.DBFunctionSymbolSerializer;
  import it.unibz.inf.ontop.model.type.DBTermType;
  import it.unibz.inf.ontop.model.type.TermType;
  import it.unibz.inf.ontop.utils.ImmutableCollectors;
  import it.unibz.inf.ontop.utils.VariableGenerator;
  
  import java.util.Optional;
  import java.util.function.Function;
  import java.util.stream.Collectors;
  import java.util.stream.IntStream;
  
  public abstract class AbstractDBConcatFunctionSymbol extends AbstractTypedDBFunctionSymbol implements DBConcatFunctionSymbol {
  
      private final DBFunctionSymbolSerializer serializer;
  
      protected AbstractDBConcatFunctionSymbol(String nameInDialect, int arity, DBTermType dbStringType,
                                               DBTermType rootDBTermType, DBFunctionSymbolSerializer serializer) {
          super(nameInDialect + arity,
                  // No restriction on the input types TODO: check if OK
                  IntStream.range(0, arity)
                          .mapToObj(i -> (TermType) rootDBTermType)
                          .collect(ImmutableCollectors.toList()), dbStringType);
          this.serializer = serializer;
      }
  
      @Override
      public Optional<ImmutableFunctionalTerm.FunctionalTermDecomposition> analyzeInjectivity(
              ImmutableList<? extends ImmutableTerm> arguments, ImmutableSet<Variable> nonFreeVariables,
              VariableNullability variableNullability, VariableGenerator variableGenerator, TermFactory termFactory) {
  
          if (arguments.stream()
                  .filter(t -> (!(t instanceof GroundTerm)) || (!((GroundTerm) t).isDeterministic()))
                  .filter(t -> !nonFreeVariables.contains(t))
                  .count() <= 1) {
              return Optional.of(decomposeInjectiveTopFunctionalTerm(arguments, nonFreeVariables, variableNullability,
                      variableGenerator, termFactory));
          } else
              return Optional.empty();
  
      }
  
      @Override
      public String getNativeDBString(ImmutableList<? extends ImmutableTerm> terms,
                                      Function<ImmutableTerm, String> termConverter, TermFactory termFactory) {
          return serializer.getNativeDBString(terms, termConverter, termFactory);
      }
  
      @Override
      protected ImmutableTerm buildTermAfterEvaluation(ImmutableList<ImmutableTerm> newTerms, TermFactory termFactory,
                                                       VariableNullability variableNullability) {
          if (newTerms.stream().allMatch(t -> t instanceof Constant)) {
              // NB: for null rejecting, all the terms are expected be non-null
              return termFactory.getDBStringConstant(
                      newTerms.stream()
                              .map(t -> extractString((Constant) t))
                              .collect(Collectors.joining()));
          }
          else
              return super.buildTermAfterEvaluation(newTerms, termFactory, variableNullability);
      }
  
      protected abstract String extractString(Constant constant);
  }
  