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.exception.MinorOntopInternalBugException;
  import it.unibz.inf.ontop.iq.node.VariableNullability;
  import it.unibz.inf.ontop.model.term.*;
  import it.unibz.inf.ontop.model.term.functionsymbol.db.DBIsNullOrNotFunctionSymbol;
  import it.unibz.inf.ontop.model.type.DBTermType;
  
  import java.util.Optional;
  
  public abstract class AbstractDBIsNullOrNotFunctionSymbol extends DBBooleanFunctionSymbolImpl
          implements DBIsNullOrNotFunctionSymbol {
  
      private static final String IS_NULL_NAME = "IS_NULL";
      private static final String IS_NOT_NULL_NAME = "IS_NOT_NULL";
  
      private final boolean isNull;
  
      protected AbstractDBIsNullOrNotFunctionSymbol(boolean isNull, DBTermType dbBooleanTermType, DBTermType rootDBTermType) {
          super(isNull ? IS_NULL_NAME : IS_NOT_NULL_NAME, ImmutableList.of(rootDBTermType), dbBooleanTermType);
          this.isNull = isNull;
      }
  
      @Override
      public boolean blocksNegation() {
          return false;
      }
  
      @Override
      public ImmutableExpression negate(ImmutableList<? extends ImmutableTerm> subTerms, TermFactory termFactory) {
          ImmutableTerm subTerm = subTerms.get(0);
          return isNull
                  ? termFactory.getDBIsNotNull(subTerm)
                  : termFactory.getDBIsNull(subTerm);
      }
  
      @Override
      public boolean isAlwaysInjectiveInTheAbsenceOfNonInjectiveFunctionalTerms() {
          return false;
      }
  
      @Override
      public boolean canBePostProcessed(ImmutableList<? extends ImmutableTerm> arguments) {
          return true;
      }
  
      @Override
      protected ImmutableTerm buildTermAfterEvaluation(ImmutableList<ImmutableTerm> newTerms,
                                                       TermFactory termFactory, VariableNullability variableNullability) {
          ImmutableTerm newTerm = newTerms.get(0);
  
          IncrementalEvaluation incrementalEvaluation = newTerm.evaluateIsNotNull(variableNullability);
          switch (incrementalEvaluation.getStatus()) {
              case SAME_EXPRESSION:
                  return super.buildTermAfterEvaluation(newTerms, termFactory, variableNullability);
              case SIMPLIFIED_EXPRESSION:
                  return incrementalEvaluation.getNewExpression()
                          .map(e -> isNull ? e.negate(termFactory) : e)
                          .orElseThrow(() -> new MinorOntopInternalBugException("A simplified expression was expected"));
              case IS_NULL:
                  return termFactory.getNullConstant();
              case IS_FALSE:
                  return termFactory.getDBBooleanConstant(isNull);
              case IS_TRUE:
                  return termFactory.getDBBooleanConstant(!isNull);
              default:
                  throw new MinorOntopInternalBugException("Unexpected status");
          }
      }
  
      @Override
      public boolean isNullable(ImmutableSet<Integer> nullableIndexes) {
          return false;
      }
  
      @Override
      protected boolean tolerateNulls() {
          return true;
      }
  
      @Override
      public boolean isTrueWhenNull() {
          return isNull;
      }
  
      @Override
      public IncrementalEvaluation evaluateIsNotNull(ImmutableList<? extends ImmutableTerm> terms, TermFactory termFactory,
                                                     VariableNullability variableNullability) {
          return IncrementalEvaluation.declareIsTrue();
      }
  }
  