collectionutils.java

iBATIS似乎已远离众说纷纭的OR框架之列

/*
 * Copyright 1999-2004 The Apache Software Foundation
 *
 * 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 org.apache.commons.collections;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

import org.apache.commons.collections.iterators.ArrayIterator;
import org.apache.commons.collections.iterators.EnumerationIterator;
/**
 * A set of {@link Collection} related utility methods.
 *
 * @since 1.0
 * @author Rodney Waldhoff
 * @author Paul Jack
 * @author Stephen Colebourne
 * @author Steve Downey
 * @version $Revision: 1.18.2.2 $ $Date: 2004/05/22 12:14:02 $
 */
public class CollectionUtils {

    /**
     * The empty iterator (immutable).
     * @deprecated use IteratorUtils.EMPTY_ITERATOR
     */
    public static final Iterator EMPTY_ITERATOR = IteratorUtils.EMPTY_ITERATOR;

    /**
     * Please don't ever instantiate a <code>CollectionUtils</code>.
     */
    public CollectionUtils() {
    }

    /**
     * Returns a {@link Collection} containing the union
     * of the given {@link Collection}s.
     * <p>
     * The cardinality of each element in the returned {@link Collection}
     * will be equal to the maximum of the cardinality of that element
     * in the two given {@link Collection}s.
     *
     * @see Collection#addAll
     */
    public static Collection union(final Collection a, final Collection b) {
        ArrayList list = new ArrayList();
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);
        Set elts = new HashSet(a);
        elts.addAll(b);
        Iterator it = elts.iterator();
        while(it.hasNext()) {
            Object obj = it.next();
            for(int i=0,m=Math.max(getFreq(obj,mapa),getFreq(obj,mapb));i<m;i++) {
                list.add(obj);
            }
        }
        return list;
    }

    /**
     * Returns a {@link Collection} containing the intersection
     * of the given {@link Collection}s.
     * <p>
     * The cardinality of each element in the returned {@link Collection}
     * will be equal to the minimum of the cardinality of that element
     * in the two given {@link Collection}s.
     *
     * @see Collection#retainAll
     * @see #containsAny
     */
    public static Collection intersection(final Collection a, final Collection b) {
        ArrayList list = new ArrayList();
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);
        Set elts = new HashSet(a);
        elts.addAll(b);
        Iterator it = elts.iterator();
        while(it.hasNext()) {
            Object obj = it.next();
            for(int i=0,m=Math.min(getFreq(obj,mapa),getFreq(obj,mapb));i<m;i++) {
                list.add(obj);
            }
        }
        return list;
    }

    /**
     * Returns a {@link Collection} containing the exclusive disjunction
     * (symmetric difference) of the given {@link Collection}s.
     * <p>
     * The cardinality of each element <i>e</i> in the returned {@link Collection}
     * will be equal to
     * <tt>max(cardinality(<i>e</i>,<i>a</i>),cardinality(<i>e</i>,<i>b</i>)) - min(cardinality(<i>e</i>,<i>a</i>),cardinality(<i>e</i>,<i>b</i>))</tt>.
     * <p>
     * This is equivalent to
     * <tt>{@link #subtract subtract}({@link #union union(a,b)},{@link #intersection intersection(a,b)})</tt>
     * or
     * <tt>{@link #union union}({@link #subtract subtract(a,b)},{@link #subtract subtract(b,a)})</tt>.
     */
    public static Collection disjunction(final Collection a, final Collection b) {
        ArrayList list = new ArrayList();
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);
        Set elts = new HashSet(a);
        elts.addAll(b);
        Iterator it = elts.iterator();
        while(it.hasNext()) {
            Object obj = it.next();
            for(int i=0,m=((Math.max(getFreq(obj,mapa),getFreq(obj,mapb)))-(Math.min(getFreq(obj,mapa),getFreq(obj,mapb))));i<m;i++) {
                list.add(obj);
            }
        }
        return list;
    }

    /**
     * Returns a {@link Collection} containing <tt><i>a</i> - <i>b</i></tt>.
     * The cardinality of each element <i>e</i> in the returned {@link Collection}
     * will be the cardinality of <i>e</i> in <i>a</i> minus the cardinality
     * of <i>e</i> in <i>b</i>, or zero, whichever is greater.
     *
     * @see Collection#removeAll
     */
    public static Collection subtract(final Collection a, final Collection b) {
        ArrayList list = new ArrayList( a );
        Iterator it =  b.iterator();
        while(it.hasNext()) {
            list.remove(it.next());
        }
        return list;
    }

    /**
     * Returns <code>true</code> iff some element of <i>a</i>
     * is also an element of <i>b</i> (or, equivalently, if 
     * some element of <i>b</i> is also an element of <i>a</i>).
     * In other words, this method returns <code>true</code>
     * iff the {@link #intersection} of <i>a</i> and <i>b</i>
     * is not empty.
     * @since 2.1
     * @param a a non-<code>null</code> Collection
     * @param b a non-<code>null</code> Collection
     * @return <code>true</code> iff the intersection of <i>a</i> and <i>b</i> is non-empty
     * @see #intersection
     */
    public static boolean containsAny(final Collection a, final Collection b) {
        // TO DO: we may be able to optimize this by ensuring either a or b
        // is the larger of the two Collections, but I'm not sure which.
        for(Iterator iter = a.iterator(); iter.hasNext();) {
            if(b.contains(iter.next())) {
                return true;
            }
        }
        return false;
    }

    /**
     * Returns a {@link Map} mapping each unique element in
     * the given {@link Collection} to an {@link Integer}
     * representing the number of occurances of that element
     * in the {@link Collection}.
     * An entry that maps to <tt>null</tt> indicates that the
     * element does not appear in the given {@link Collection}.
     */
    public static Map getCardinalityMap(final Collection col) {
        HashMap count = new HashMap();
        Iterator it = col.iterator();
        while(it.hasNext()) {
            Object obj = it.next();
            Integer c = (Integer)(count.get(obj));
            if(null == c) {
                count.put(obj,new Integer(1));
            } else {
                count.put(obj,new Integer(c.intValue() + 1));
            }
        }
        return count;
    }

    /**
     * Returns <tt>true</tt> iff <i>a</i> is a sub-collection of <i>b</i>,
     * that is, iff the cardinality of <i>e</i> in <i>a</i> is less
     * than or equal to the cardinality of <i>e</i> in <i>b</i>,
     * for each element <i>e</i> in <i>a</i>.
     *
     * @see #isProperSubCollection
     * @see Collection#containsAll
     */
    public static boolean isSubCollection(final Collection a, final Collection b) {
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);
        Iterator it = a.iterator();
        while (it.hasNext()) {
            Object obj = it.next();
            if (getFreq(obj, mapa) > getFreq(obj, mapb)) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns <tt>true</tt> iff <i>a</i> is a <i>proper</i> sub-collection of <i>b</i>,
     * that is, iff the cardinality of <i>e</i> in <i>a</i> is less
     * than or equal to the cardinality of <i>e</i> in <i>b</i>,
     * for each element <i>e</i> in <i>a</i>, and there is at least one
     * element <i>f</i> such that the cardinality of <i>f</i> in <i>b</i>
     * is strictly greater than the cardinality of <i>f</i> in <i>a</i>.
     *
     * @see #isSubCollection
     * @see Collection#containsAll
     */
    public static boolean isProperSubCollection(final Collection a, final Collection b) {
        // XXX optimize me!
        return CollectionUtils.isSubCollection(a,b) && (!(CollectionUtils.isEqualCollection(a,b)));
    }

    /**
     * Returns <tt>true</tt> iff the given {@link Collection}s contain
     * exactly the same elements with exactly the same cardinality.
     * <p>
     * That is, iff the cardinality of <i>e</i> in <i>a</i> is
     * equal to the cardinality of <i>e</i> in <i>b</i>,
     * for each element <i>e</i> in <i>a</i> or <i>b</i>.
     */
    public static boolean isEqualCollection(final Collection a, final Collection b) {
        if(a.size() != b.size()) {
            return false;
        } else {
            Map mapa = getCardinalityMap(a);
            Map mapb = getCardinalityMap(b);
            if(mapa.size() != mapb.size()) {
                return false;
            } else {
                Iterator it = mapa.keySet().iterator();
                while(it.hasNext()) {
                    Object obj = it.next();
                    if(getFreq(obj,mapa) != getFreq(obj,mapb)) {
                        return false;
                    }
                }
                return true;
            }
        }
    }

    /**
     * Returns the number of occurrences of <i>obj</i>
     * in <i>col</i>.
     */
    public static int cardinality(Object obj, final Collection col) {
        int count = 0;
        Iterator it = col.iterator();
        while(it.hasNext()) {
            Object elt = it.next();
            if((null == obj && null == elt) || obj.equals(elt)) {
                count++;
            }
        }
        return count;
    }

    /** 
     * Finds the first element in the given collection which matches the given predicate.
     * <p>
     * If the input collection or predicate is null, null is returned.
     *
     * @param collection  the collection to search, may be null
     * @param predicate  the predicate to use, may be null
     * @return the first element of the collection which matches the predicate or null if none could be found
     */
    public static Object find(Collection collection, Predicate predicate) {
        if (collection != null && predicate != null) {
            for (Iterator iter = collection.iterator(); iter.hasNext();) {
                Object item = iter.next();
                if (predicate.evaluate(item)) {
                    return item;
                }
            }
        }
        return null;
    }
    
    /** 
     * Executes the given closure on each element in the collection.
     * <p>
     * If the input collection is null, there is no change made.
     * 
     * @param collection  the collection to get the input from, may be null
     * @param closure  the closure to perform, may not be null
     * @throws NullPointerException if the closure is null
     */
    public static void forAllDo(Collection collection, Closure closure) {
        if (collection != null) {
            for (Iterator iter = collection.iterator(); iter.hasNext();) {
                Object element = iter.next();
                closure.execute(element);
            }
        }
    }

    /** 
     * Filter the collection by applying a Predicate to each element. If the
     * predicate returns false, remove the element.
     * <p>
     * If the input collection or predicate is null, there is no change made.