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.formulas;
  
  import org.logicng.datastructures.Assignment;
  import org.logicng.datastructures.Substitution;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.stream.Stream;
  
  /**
   * Super class for Boolean constants.
   * @version 2.2.0
   * @since 1.0
   */
  public abstract class Constant extends Formula {
  
      private static final Iterator<Formula> ITERATOR = new Iterator<Formula>() {
          @Override
          public boolean hasNext() {
              return false;
          }
  
          @Override
          public Formula next() {
              throw new NoSuchElementException();
          }
  
          @Override
          public void remove() {
              throw new UnsupportedOperationException();
          }
      };
  
      /**
       * Constructor.
       * @param type    the constant type
       * @param factory the factory which created this instance
       */
      Constant(final FType type, final FormulaFactory factory) {
          super(type, factory);
      }
  
      @Override
      public int numberOfOperands() {
          return 0;
      }
  
      @Override
      public boolean isConstantFormula() {
          return true;
      }
  
      @Override
      public boolean isAtomicFormula() {
          return true;
      }
  
      @Override
      public boolean containsVariable(final Variable variable) {
          return false;
      }
  
      @Override
      public Formula restrict(final Assignment assignment) {
          return this;
      }
  
      @Override
      public boolean containsNode(final Formula formula) {
          return this == formula;
      }
  
      @Override
      public Formula substitute(final Substitution substitution) {
          return this;
      }
  
      @Override
      public Iterator<Formula> iterator() {
          return ITERATOR;
      }
  
      @Override
      public Stream<Formula> stream() {
          return Stream.empty();
      }
  }
  