content: /*
   *  Licensed to the Apache Software Foundation (ASF) under one
   *  or more contributor license agreements.  See the NOTICE file
   *  distributed with this work for additional information
   *  regarding copyright ownership.  The ASF licenses this file
   *  to you 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.apache.causeway.commons.collections;
  
  import java.util.Collection;
  import java.util.NoSuchElementException;
  import java.util.Objects;
  import java.util.Optional;
  import java.util.stream.Stream;
  import java.util.stream.StreamSupport;
  
  import org.jspecify.annotations.Nullable;
  
  import org.apache.causeway.commons.internal.exceptions._Exceptions;
  
  /**
   * Provides a subset of the functionality that the Java {@link Collection}
   * interface has, focusing on immutability.
   */
  public interface ImmutableCollection<E>
  extends Iterable<E> {
  
      /**
       * Returns the number of elements in this collection.  If this collection
       * contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * @return the number of elements in this collection
       */
      int size();
  
      /**
       * Returns {@code true} if this collection contains no elements.
       *
       * @return {@code true} if this collection contains no elements
       */
      boolean isEmpty();
  
      /**
       * @return either 'empty', 'one' or 'multi'
       */
      Cardinality getCardinality();
  
      /**
       * @return whether this Can contains given {@code element}, that is, at least one contained element
       * passes the {@link Objects#equals(Object, Object)} test with respect to the given element.
       */
      boolean contains(@Nullable E element);
  
      /**
       * @return this collection's single element or an empty Optional,
       * if this collection has any cardinality other than ONE
       */
      Optional<E> getSingleton();
  
      /**
       * Shortcut for {@code getSingleton().orElseThrow(_Exceptions::noSuchElement)}
       * @throws NoSuchElementException if result is empty
       */
      default E getSingletonOrFail() {
          return getSingleton().orElseThrow(_Exceptions::noSuchElement);
      }
  
      /**
       * @return Stream of elements this collection contains
       */
      default Stream<E> stream() {
          return StreamSupport.stream(spliterator(), false);
      }
  
      /**
       * @return possibly concurrent Stream of elements this collection contains
       */
      default Stream<E> parallelStream() {
          return StreamSupport.stream(spliterator(), true);
      }
  
  }
  