code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.io.parsers;
  
  import org.antlr.v4.runtime.BailErrorStrategy;
  import org.antlr.v4.runtime.CharStream;
  import org.antlr.v4.runtime.CharStreams;
  import org.antlr.v4.runtime.CommonTokenStream;
  import org.antlr.v4.runtime.Lexer;
  import org.antlr.v4.runtime.misc.ParseCancellationException;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.FormulaFactory;
  
  import java.io.IOException;
  import java.io.InputStream;
  
  /**
   * Super class for a formula parser.
   * @version 1.2
   * @since 1.2
   */
  public abstract class FormulaParser {
  
      private final FormulaFactory f;
      private Lexer lexer;
      private ParserWithFormula parser;
  
      /**
       * Constructor.
       * @param f the formula factory
       */
      protected FormulaParser(final FormulaFactory f) {
          this.f = f;
      }
  
      /**
       * Sets the internal lexer and the parser.
       * @param lexer  the lexer
       * @param parser the parser
       */
      protected void setLexerAndParser(final Lexer lexer, final ParserWithFormula parser) {
          this.lexer = lexer;
          this.parser = parser;
          this.parser.setFormulaFactory(this.f);
          this.lexer.removeErrorListeners();
          this.parser.removeErrorListeners();
          this.parser.setErrorHandler(new BailErrorStrategy());
      }
  
      /**
       * Parses and returns a given input stream.
       * @param inputStream an input stream
       * @return the {@link Formula} representation of this stream
       * @throws ParserException if there was a problem with the input stream
       */
      public Formula parse(final InputStream inputStream) throws ParserException {
          if (inputStream == null) {
              return this.f.verum();
          }
          try {
              final CharStream input = CharStreams.fromStream(inputStream);
              this.lexer.setInputStream(input);
              final CommonTokenStream tokens = new CommonTokenStream(this.lexer);
              this.parser.setInputStream(tokens);
              return this.parser.getParsedFormula();
          } catch (final IOException e) {
              throw new ParserException("IO exception when parsing the formula", e);
          } catch (final ParseCancellationException e) {
              throw new ParserException("Parse cancellation exception when parsing the formula", e);
          } catch (final LexerException e) {
              throw new ParserException("Lexer exception when parsing the formula.", e);
          }
      }
  
      /**
       * Parses and returns a given string.
       * @param in a string
       * @return the {@link Formula} representation of this string
       * @throws ParserException if the string was not a valid formula
       */
      public Formula parse(final String in) throws ParserException {
          if (in == null || in.isEmpty()) {
              return this.f.verum();
          }
          try {
              final CharStream input = CharStreams.fromString(in);
              this.lexer.setInputStream(input);
              final CommonTokenStream tokens = new CommonTokenStream(this.lexer);
              this.parser.setInputStream(tokens);
              return this.parser.getParsedFormula();
          } catch (final ParseCancellationException e) {
              throw new ParserException("Parse cancellation exception when parsing the formula", e);
          } catch (final LexerException e) {
              throw new ParserException("Lexer exception when parsing the formula.", e);
          }
      }
  
      /**
       * Returns the factory of this parser.
       * @return the factory of this parser
       */
      public FormulaFactory factory() {
          return this.f;
      }
  
      @Override
      public String toString() {
          return getClass().getSimpleName();
      }
  }
  