BooleanComparator

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 * 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.commons.collections4.comparators;

import java.io.Serializable;
import java.util.Comparator;

/**
 * A {@link Comparator} for {@link Boolean} objects that can sort either
 * true or false first.
 *
 * @see #getTrueFirstComparator()
 * @see #getFalseFirstComparator()
 * @see #booleanComparator(boolean)
 *
 * @since 3.0
 */
public final class BooleanComparator implements Comparator<Boolean>, Serializable {

    /** Serialization version. */
    private static final long serialVersionUID = 1830042991606340609L;

    /** Constant "true first" reference. */
    private static final BooleanComparator TRUE_FIRST = new BooleanComparator(true);

    /** Constant "false first" reference. */
    private static final BooleanComparator FALSE_FIRST = new BooleanComparator(false);

    /**
     * Returns a BooleanComparator instance that sorts
     * {@code <i>trueFirst</i>} values before
     * {@code &#x21;<i>trueFirst</i>} values.
     * <p>
     * Clients are encouraged to use the value returned from
     * this method instead of constructing a new instance
     * to reduce allocation and garbage collection overhead when
     * multiple BooleanComparators may be used in the same
     * virtual machine.
     * </p>
     *
     * @param trueFirst when {@code true}, sort
     * {@code true} {@code Boolean}s before {@code false}
     * @return a singleton BooleanComparator instance
     * @since 4.0
     */
    public static BooleanComparator booleanComparator(final boolean trueFirst) {
        return trueFirst ? TRUE_FIRST : FALSE_FIRST;
    }

    /**
     * Returns a BooleanComparator instance that sorts
     * {@code false} values before {@code true} values.
     * <p>
     * Clients are encouraged to use the value returned from
     * this method instead of constructing a new instance
     * to reduce allocation and garbage collection overhead when
     * multiple BooleanComparators may be used in the same
     * virtual machine.
     * </p>
     *
     * @return the false first singleton BooleanComparator
     */
    public static BooleanComparator getFalseFirstComparator() {
        return FALSE_FIRST;
    }

    /**
     * Returns a BooleanComparator instance that sorts
     * {@code true} values before {@code false} values.
     * <p>
     * Clients are encouraged to use the value returned from
     * this method instead of constructing a new instance
     * to reduce allocation and garbage collection overhead when
     * multiple BooleanComparators may be used in the same
     * virtual machine.
     * </p>
     *
     * @return the true first singleton BooleanComparator
     */
    public static BooleanComparator getTrueFirstComparator() {
        return TRUE_FIRST;
    }

    /** {@code true} iff {@code true} values sort before {@code false} values. */
    private final boolean trueFirst;

    /**
     * Creates a {@code BooleanComparator} that sorts
     * {@code false} values before {@code true} values.
     * <p>
     * Equivalent to {@link #BooleanComparator(boolean) BooleanComparator(false)}.
     * <p>
     * Please use the static factory instead whenever possible.
     */
    public BooleanComparator() {
        this(false);
    }

    /**
     * Creates a {@code BooleanComparator} that sorts
     * {@code <i>trueFirst</i>} values before
     * {@code &#x21;<i>trueFirst</i>} values.
     * <p>
     * Please use the static factories instead whenever possible.
     *
     * @param trueFirst when {@code true}, sort
     *  {@code true} boolean values before {@code false}
     */
    public BooleanComparator(final boolean trueFirst) {
        this.trueFirst = trueFirst;
    }

    /**
     * Compares two non-{@code null} {@code Boolean} objects
     * according to the value of {@link #sortsTrueFirst()}.
     *
     * @param b1  the first boolean to compare
     * @param b2  the second boolean to compare
     * @return negative if obj1 is less, positive if greater, zero if equal
     * @throws NullPointerException when either argument {@code null}
     */
    @Override
    public int compare(final Boolean b1, final Boolean b2) {
        final boolean v1 = b1.booleanValue();
        final boolean v2 = b2.booleanValue();

        return v1 ^ v2 ? v1 ^ trueFirst ? 1 : -1 : 0;
    }

    /**
     * Returns {@code true} iff <i>that</i> Object is
     * a {@link Comparator} whose ordering is known to be
     * equivalent to mine.
     * <p>
     * This implementation returns {@code true}
     * iff {@code <i>that</i>} is a {@link BooleanComparator}
     * whose value of {@link #sortsTrueFirst()} is equal to mine.
     *
     * @param object  the object to compare to
     * @return true if equal
     */
    @Override
    public boolean equals(final Object object) {
        return this == object ||
               object instanceof BooleanComparator &&
                this.trueFirst == ((BooleanComparator) object).trueFirst;
    }

    /**
     * Implement a hash code for this comparator that is consistent with
     * {@link #equals(Object) equals}.
     *
     * @return a hash code for this comparator.
     */
    @Override
    public int hashCode() {
        final int hash = "BooleanComparator".hashCode();
        return trueFirst ? -1 * hash : hash;
    }

    /**
     * Returns {@code true} iff
     * I sort {@code true} values before
     * {@code false} values.  In other words,
     * returns {@code true} iff
     * {@link #compare(Boolean,Boolean) compare(Boolean.FALSE,Boolean.TRUE)}
     * returns a positive value.
     *
     * @return the trueFirst flag
     */
    public boolean sortsTrueFirst() {
        return trueFirst;
    }

}