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Java 1.8+ Library with tons of utility classes required in all projects

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/*
 * Copyright (C) 2014-2022 Philip Helger (www.helger.com)
 * philip[at]helger[dot]com
 *
 * 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 com.helger.commons.hierarchy;

import javax.annotation.Nonnegative;
import javax.annotation.Nullable;

import com.helger.commons.collection.impl.ICommonsCollection;

/**
 * This interface can be used to generically resolved children of a certain
 * object.
 *
 * @author Philip Helger
 * @param 
 *        The type of the children to retrieve.
 */
public interface IChildrenProvider 
{
  /**
   * Check if an item has children.
   *
   * @param aCurrent
   *        The object to determine the children of. No null or
   *        non- null constraint possible.
   * @return true if this item has children, false
   *         otherwise.
   */
  default boolean hasChildren (final CHILDTYPE aCurrent)
  {
    return getChildCount (aCurrent) > 0;
  }

  /**
   * Check if an item has no children.
   *
   * @param aCurrent
   *        The object to determine the children of. No null or
   *        non- null constraint possible.
   * @return true if this item has no children, false
   *         otherwise.
   */
  default boolean hasNoChildren (final CHILDTYPE aCurrent)
  {
    return !hasChildren (aCurrent);
  }

  /**
   * @param aCurrent
   *        The object to determine the children count of. No null
   *        or non- null constraint possible.
   * @return The number of contained direct children. Always ≥ 0.
   */
  @Nonnegative
  int getChildCount (CHILDTYPE aCurrent);

  /**
   * Get the children of the passed object.
   *
   * @param aCurrent
   *        The object to determine the children of. No null or
   *        non-null constraint possible.
   * @return The child objects, or null if there are no children.
   *         If null is passed, the resolver is expected to return
   *         any possible top level (root) elements. This method may NOT return
   *         null if the call to {@link #hasChildren(Object)} with
   *         the same object returned true.
   */
  @Nullable
  ICommonsCollection  getAllChildren (CHILDTYPE aCurrent);
}