Append, filter and get the size of collections : Collection « Collections Data Structure « Java

Append, filter and get the size of collections

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 * 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
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * See the License for the specific language governing permissions and
 * limitations under the License.

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import org.w3c.dom.NodeList;

 * A number of helper methods for working with collections
 * @version $Revision: 747062 $
public final class CollectionHelper {

     * Utility classes should not have a public constructor.
    private CollectionHelper() {

     * Returns the size of the collection if it can be determined to be a collection
     * @param value the collection
     * @return the size, or <tt>null</tt> if not a collection
    public static Integer size(Object value) {
        if (value != null) {
            if (value instanceof Collection) {
                Collection collection = (Collection)value;
                return collection.size();
            } else if (value instanceof Map) {
                Map map = (Map)value;
                return map.size();
            } else if (value instanceof Object[]) {
                Object[] array = (Object[])value;
                return array.length;
            } else if (value.getClass().isArray()) {
                return Array.getLength(value);
            } else if (value instanceof NodeList) {
                NodeList nodeList = (NodeList)value;
                return nodeList.getLength();
        return null;

     * Sets the value of the entry in the map for the given key, though if the
     * map already contains a value for the given key then the value is appended
     * to a list of values.
     * @param map the map to add the entry to
     * @param key the key in the map
     * @param value the value to put in the map
    public static void appendValue(Map map, Object key, Object value) {
        Object oldValue = map.get(key);
        if (oldValue != null) {
            List list;
            if (oldValue instanceof List) {
                list = (List)oldValue;
            } else {
                list = new ArrayList();
        } else {
            map.put(key, value);

     * Filters the given list to skip instanceof filter objects.
     * @param list  the list
     * @param filters  objects to skip
     * @return a new list without the filtered objects
    public static List filterList(List list, Object... filters) {
        List answer = new ArrayList();
        for (Object o : list) {
            for (Object filter : filters) {
                if (!o.getClass().isInstance(filter)) {
        return answer;

    public static String collectionAsCommaDelimitedString(String[] col) {
        if (col == null || col.length == 0) {
            return "";
        return collectionAsCommaDelimitedString(Arrays.asList(col));

    public static String collectionAsCommaDelimitedString(Collection col) {
        if (col == null || col.isEmpty()) {
            return "";

        StringBuilder sb = new StringBuilder();
        Iterator it = col.iterator();
        while (it.hasNext()) {
            if (it.hasNext()) {

        return sb.toString();


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