code: /*
   * Copyright (c) 2019 the Eclipse Milo Authors
   *
   * This program and the accompanying materials are made
   * available under the terms of the Eclipse Public License 2.0
   * which is available at https://www.eclipse.org/legal/epl-2.0/
   *
   * SPDX-License-Identifier: EPL-2.0
   */
  
  package org.eclipse.milo.opcua.sdk.server.events.operators;
  
  import org.eclipse.milo.opcua.sdk.server.events.FilterContext;
  import org.eclipse.milo.opcua.sdk.server.events.OperatorContext;
  import org.eclipse.milo.opcua.sdk.server.events.ValidationException;
  import org.eclipse.milo.opcua.sdk.server.model.nodes.objects.BaseEventTypeNode;
  import org.eclipse.milo.opcua.stack.core.StatusCodes;
  import org.eclipse.milo.opcua.stack.core.UaException;
  import org.eclipse.milo.opcua.stack.core.types.structured.FilterOperand;
  import org.jetbrains.annotations.Nullable;
  
  public class Not implements Operator<Boolean> {
  
      Not() {}
  
      @Override
      public void validate(FilterContext context, FilterOperand[] operands) throws ValidationException {
          if (operands.length < 1) {
              throw new ValidationException(StatusCodes.Bad_FilterOperandCountMismatch);
          }
      }
  
      @Nullable
      @Override
      public Boolean apply(
          OperatorContext context,
          BaseEventTypeNode eventNode,
          FilterOperand[] operands) throws UaException {
  
          validate(context, operands);
  
          FilterOperand op0 = operands[0];
  
          Object value0 = context.resolve(op0, eventNode);
  
          if (value0 instanceof Boolean) {
              return !(Boolean) value0;
          } else {
              return null;
          }
      }
  
  }
  