001/* -*- mode: Java; c-basic-offset: 2; indent-tabs-mode: nil; coding: utf-8-unix -*-
002 *
003 * Copyright © 2025 microBean™.
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
006 * the License. You may obtain a copy of the License at
007 *
008 *     http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
011 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
012 * specific language governing permissions and limitations under the License.
013 */
014package org.microbean.assign;
015
016import java.util.Comparator;
017
018import javax.lang.model.element.ElementKind;
019
020/**
021 * A {@link Comparator} establishing a <dfn>partial order</dfn> over {@link ElementKind}s such that {@linkplain
022 * ElementKind#isClass() classes} precede {@linkplain ElementKind#isInterface() interfaces}.
023 *
024 * @author <a href="https://about.me/lairdnelson" target="_top">Laird Nelson</a>
025 *
026 * @see #compare(ElementKind, ElementKind)
027 */
028public final class ClassesThenInterfacesElementKindComparator implements Comparator<ElementKind> {
029
030  /**
031   * The sole instance of this class.
032   */
033  public static final ClassesThenInterfacesElementKindComparator INSTANCE = new ClassesThenInterfacesElementKindComparator();
034  
035  private ClassesThenInterfacesElementKindComparator() {
036    super();
037  }
038
039  /**
040   * Compares the two {@link ElementKind}s and returns the result.
041   *
042   * <p>If {@code k0 == k1}, returns {@code 0}.</p>
043   *
044   * <p>If {@code k0} is {@code null} and {@code k1} is not, returns a positive value.</p>
045   *
046   * <p>If {@code k0} {@linkplain ElementKind#isClass() is a class} and {@code k1} {@linkplain ElementKind#isInterface()
047   * is an interface}, returns a negative value.</p>
048   *
049   * <p>If {@code k0} {@linkplain ElementKind#isInterface() is an interface} and {@code k1} {@linkplain
050   * ElementKind#isClass() is a class}, returns a positive value.</p>
051   *
052   * <p>Returns {@code 0} in all other cases.
053   *
054   * @param k0 an {@link ElementKind}; may be {@code null}
055   *
056   * @param k1 an {@link ElementKind}; may be {@code null}
057   *
058   * @return a comparison result
059   *
060   * @see ElementKind#isClass()
061   *
062   * @see ElementKind#isInterface()
063   */
064  @Override // Comparator<ElementKind>
065  public final int compare(final ElementKind k0, final ElementKind k1) {
066    if (k0 == k1) {
067      return 0;
068    } else if (k0 == null) {
069      return 1;
070    } else if (k1 == null) {
071      return -1;
072    } else if (k0.isClass() && k1.isInterface()) {
073      return -1;
074    } else if (k0.isInterface() && k1.isClass()) {
075      return 1;
076    } else {
077      return 0;
078    }
079  }
080
081}