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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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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.functors;

import java.util.Collection;

import org.apache.commons.collections4.Predicate;

/**
 * Predicate implementation that returns true if only one of the
 * predicates return true.
 * If the array of predicates is empty, then this predicate returns false.
 * 
 * NOTE: In versions prior to 3.2 an array size of zero or one
 * threw an exception.
 *
 * @since 3.0
 */
public final class OnePredicate extends AbstractQuantifierPredicate {

    /** Serial version UID */
    private static final long serialVersionUID = -8125389089924745785L;

    /**
     * Factory to create the predicate.
     * 
     * If the array is size zero, the predicate always returns false.
     * If the array is size one, then that predicate is returned.
     *
     * @param  the type that the predicate queries
     * @param predicates  the predicates to check, cloned, not null
     * @return the any predicate
     * @throws NullPointerException if the predicates array is null
     * @throws NullPointerException if any predicate in the array is null
     */
    @SuppressWarnings("unchecked")
    public static  Predicate onePredicate(final Predicate... predicates) {
        FunctorUtils.validate(predicates);
        if (predicates.length == 0) {
            return FalsePredicate.falsePredicate();
        }
        if (predicates.length == 1) {
            return (Predicate) predicates[0];
        }
        return new OnePredicate<>(FunctorUtils.copy(predicates));
    }

    /**
     * Factory to create the predicate.
     *
     * @param  the type that the predicate queries
     * @param predicates  the predicates to check, cloned, not null
     * @return the one predicate
     * @throws NullPointerException if the predicates array is null
     * @throws NullPointerException if any predicate in the array is null
     */
    public static  Predicate onePredicate(final Collection> predicates) {
        final Predicate[] preds = FunctorUtils.validate(predicates);
        return new OnePredicate<>(preds);
    }

    /**
     * Constructor that performs no validation.
     * Use onePredicate if you want that.
     *
     * @param predicates  the predicates to check, not cloned, not null
     */
    public OnePredicate(final Predicate... predicates) {
        super(predicates);
    }

    /**
     * Evaluates the predicate returning true if only one decorated predicate
     * returns true.
     *
     * @param object  the input object
     * @return true if only one decorated predicate returns true
     */
    @Override
    public boolean evaluate(final T object) {
        boolean match = false;
        for (final Predicate iPredicate : iPredicates) {
            if (iPredicate.evaluate(object)) {
                if (match) {
                    return false;
                }
                match = true;
            }
        }
        return match;
    }

}