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/*
 * Copyright (C) 2009 The Guava Authors
 *
 * 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.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.common.primitives.Booleans;
import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;
import java.util.Comparator;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A utility for performing a chained comparison statement. Note: Java 8+ users should
 * generally prefer the methods in {@link Comparator}; see below.
 *
 * 

Example usage of {@code ComparisonChain}: * *

{@code
 * public int compareTo(Foo that) {
 *   return ComparisonChain.start()
 *       .compare(this.aString, that.aString)
 *       .compare(this.anInt, that.anInt)
 *       .compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())
 *       .result();
 * }
 * }
* *

The value of this expression will have the same sign as the first nonzero comparison * result in the chain, or will be zero if every comparison result was zero. * *

Note: {@code ComparisonChain} instances are immutable. For this utility to work * correctly, calls must be chained as illustrated above. * *

Performance note: Even though the {@code ComparisonChain} caller always invokes its {@code * compare} methods unconditionally, the {@code ComparisonChain} implementation stops calling its * inputs' {@link Comparable#compareTo compareTo} and {@link Comparator#compare compare} methods as * soon as one of them returns a nonzero result. This optimization is typically important only in * the presence of expensive {@code compareTo} and {@code compare} implementations. * *

See the Guava User Guide article on {@code * ComparisonChain}. * *

Java 8+ equivalents

* * If you are using Java version 8 or greater, you should generally use the static methods in {@link * Comparator} instead of {@code ComparisonChain}. The example above can be implemented like this: * *
{@code
 * import static java.util.Comparator.comparing;
 * import static java.util.Comparator.nullsLast;
 * import static java.util.Comparator.naturalOrder;
 *
 * ...
 *   private static final Comparator COMPARATOR =
 *       comparing((Foo foo) -> foo.aString)
 *           .thenComparing(foo -> foo.anInt)
 *           .thenComparing(foo -> foo.anEnum, nullsLast(naturalOrder()));}
 *
 *   {@code @Override}{@code
 *   public int compareTo(Foo that) {
 *     return COMPARATOR.compare(this, that);
 *   }
 * }
* *

With method references it is more succinct: {@code comparing(Foo::aString)} for example. * *

Using {@link Comparator} avoids certain types of bugs, for example when you meant to write * {@code .compare(a.foo, b.foo)} but you actually wrote {@code .compare(a.foo, a.foo)} or {@code * .compare(a.foo, b.bar)}. {@code ComparisonChain} also has a potential performance problem that * {@code Comparator} doesn't: it evaluates all the parameters of all the {@code .compare} calls, * even when the result of the comparison is already known from previous {@code .compare} calls. * That can be expensive. * * @author Mark Davis * @author Kevin Bourrillion * @since 2.0 */ @GwtCompatible @ElementTypesAreNonnullByDefault public abstract class ComparisonChain { private ComparisonChain() {} /** Begins a new chained comparison statement. See example in the class documentation. */ public static ComparisonChain start() { return ACTIVE; } private static final ComparisonChain ACTIVE = new ComparisonChain() { @SuppressWarnings("unchecked") // unsafe; see discussion on supertype @Override public ComparisonChain compare(Comparable left, Comparable right) { return classify(((Comparable) left).compareTo(right)); } @Override public ComparisonChain compare( @ParametricNullness T left, @ParametricNullness T right, Comparator comparator) { return classify(comparator.compare(left, right)); } @Override public ComparisonChain compare(int left, int right) { return classify(Ints.compare(left, right)); } @Override public ComparisonChain compare(long left, long right) { return classify(Longs.compare(left, right)); } @Override public ComparisonChain compare(float left, float right) { return classify(Float.compare(left, right)); } @Override public ComparisonChain compare(double left, double right) { return classify(Double.compare(left, right)); } @Override public ComparisonChain compareTrueFirst(boolean left, boolean right) { return classify(Booleans.compare(right, left)); // reversed } @Override public ComparisonChain compareFalseFirst(boolean left, boolean right) { return classify(Booleans.compare(left, right)); } ComparisonChain classify(int result) { return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE; } @Override public int result() { return 0; } }; private static final ComparisonChain LESS = new InactiveComparisonChain(-1); private static final ComparisonChain GREATER = new InactiveComparisonChain(1); private static final class InactiveComparisonChain extends ComparisonChain { final int result; InactiveComparisonChain(int result) { this.result = result; } @Override public ComparisonChain compare(Comparable left, Comparable right) { return this; } @Override public ComparisonChain compare( @ParametricNullness T left, @ParametricNullness T right, Comparator comparator) { return this; } @Override public ComparisonChain compare(int left, int right) { return this; } @Override public ComparisonChain compare(long left, long right) { return this; } @Override public ComparisonChain compare(float left, float right) { return this; } @Override public ComparisonChain compare(double left, double right) { return this; } @Override public ComparisonChain compareTrueFirst(boolean left, boolean right) { return this; } @Override public ComparisonChain compareFalseFirst(boolean left, boolean right) { return this; } @Override public int result() { return result; } } /** * Compares two comparable objects as specified by {@link Comparable#compareTo}, if the * result of this comparison chain has not already been determined. * *

This method is declared to accept any 2 {@code Comparable} objects, even if they are not mutually * comparable. If you pass objects that are not mutually comparable, this method may throw an * exception. (The reason for this decision is lost to time, but the reason might be that * we wanted to support legacy classes that implement the raw type {@code Comparable} (instead of * implementing {@code Comparable}) without producing warnings. If so, we would prefer today * to produce warnings in that case, and we may change this method to do so in the future. Support * for raw {@code Comparable} types in Guava in general is tracked as #989.) * * @throws ClassCastException if the parameters are not mutually comparable */ public abstract ComparisonChain compare(Comparable left, Comparable right); /** * Compares two objects using a comparator, if the result of this comparison chain has not * already been determined. */ public abstract ComparisonChain compare( @ParametricNullness T left, @ParametricNullness T right, Comparator comparator); /** * Compares two {@code int} values as specified by {@link Ints#compare}, if the result of * this comparison chain has not already been determined. */ public abstract ComparisonChain compare(int left, int right); /** * Compares two {@code long} values as specified by {@link Longs#compare}, if the result of * this comparison chain has not already been determined. */ public abstract ComparisonChain compare(long left, long right); /** * Compares two {@code float} values as specified by {@link Float#compare}, if the result * of this comparison chain has not already been determined. */ public abstract ComparisonChain compare(float left, float right); /** * Compares two {@code double} values as specified by {@link Double#compare}, if the result * of this comparison chain has not already been determined. */ public abstract ComparisonChain compare(double left, double right); /** * Discouraged synonym for {@link #compareFalseFirst}. * * @deprecated Use {@link #compareFalseFirst}; or, if the parameters passed are being either * negated or reversed, undo the negation or reversal and use {@link #compareTrueFirst}. * @since 19.0 */ @Deprecated public final ComparisonChain compare(Boolean left, Boolean right) { return compareFalseFirst(left, right); } /** * Compares two {@code boolean} values, considering {@code true} to be less than {@code false}, * if the result of this comparison chain has not already been determined. * *

Java 8+ users: you can get the equivalent from {@link Booleans#trueFirst()}. For example: * *

   * Comparator.comparing(Foo::isBar, {@link Booleans#trueFirst()})
   * 
* * @since 12.0 */ public abstract ComparisonChain compareTrueFirst(boolean left, boolean right); /** * Compares two {@code boolean} values, considering {@code false} to be less than {@code true}, * if the result of this comparison chain has not already been determined. * *

Java 8+ users: you can get the equivalent from {@link Booleans#falseFirst()}. For example: * *

   * Comparator.comparing(Foo::isBar, {@link Booleans#falseFirst()})
   * 
* * @since 12.0 (present as {@code compare} since 2.0) */ public abstract ComparisonChain compareFalseFirst(boolean left, boolean right); /** * Ends this comparison chain and returns its result: a value having the same sign as the first * nonzero comparison result in the chain, or zero if every result was zero. */ public abstract int result(); }