工具类：Collections集合工具集

/** * Copyright (c) 2005-2012 springside.org.cn */package com.thinkgem.jeesite.common.utils;import java.util.ArrayList;import java.util.Collection;import java.util.HashMap;import java.util.Iterator;import java.util.List;import java.util.Map;import org.apache.commons.beanutils.PropertyUtils;import org.apache.commons.lang3.StringUtils;/** * Collections工具集. * 在JDK的Collections和Guava的Collections2后, 命名为Collections3. * @author calvin * @version 2013-01-15 */@SuppressWarnings("rawtypes")public class Collections3 {/** * 提取集合中的对象的两个属性(通过Getter函数), 组合成Map. *  * @param collection 来源集合. * @param keyPropertyName 要提取为Map中的Key值的属性名. * @param valuePropertyName 要提取为Map中的Value值的属性名. */@SuppressWarnings("unchecked")public static Map extractToMap(final Collection collection, final String keyPropertyName,final String valuePropertyName) {Map map = new HashMap(collection.size());try {for (Object obj : collection) {map.put(PropertyUtils.getProperty(obj, keyPropertyName),PropertyUtils.getProperty(obj, valuePropertyName));}} catch (Exception e) {throw Reflections.convertReflectionExceptionToUnchecked(e);}return map;}/** * 提取集合中的对象的一个属性(通过Getter函数), 组合成List. *  * @param collection 来源集合. * @param propertyName 要提取的属性名. */@SuppressWarnings("unchecked")public static List extractToList(final Collection collection, final String propertyName) {List list = new ArrayList(collection.size());try {for (Object obj : collection) {list.add(PropertyUtils.getProperty(obj, propertyName));}} catch (Exception e) {throw Reflections.convertReflectionExceptionToUnchecked(e);}return list;}/** * 提取集合中的对象的一个属性(通过Getter函数), 组合成由分割符分隔的字符串. *  * @param collection 来源集合. * @param propertyName 要提取的属性名. * @param separator 分隔符. */public static String extractToString(final Collection collection, final String propertyName, final String separator) {List list = extractToList(collection, propertyName);return StringUtils.join(list, separator);}/** * 转换Collection所有元素(通过toString())为String, 中间以 separator分隔。 */public static String convertToString(final Collection collection, final String separator) {return StringUtils.join(collection, separator);}/** * 转换Collection所有元素(通过toString())为String, 每个元素的前面加入prefix，后面加入postfix，如<div>mymessage</div>。 */public static String convertToString(final Collection collection, final String prefix, final String postfix) {StringBuilder builder = new StringBuilder();for (Object o : collection) {builder.append(prefix).append(o).append(postfix);}return builder.toString();}/** * 判断是否为空. */public static boolean isEmpty(Collection collection) {return (collection == null || collection.isEmpty());}/** * 取得Collection的第一个元素，如果collection为空返回null. */public static <T> T getFirst(Collection<T> collection) {if (isEmpty(collection)) {return null;}return collection.iterator().next();}/** * 获取Collection的最后一个元素 ，如果collection为空返回null. */public static <T> T getLast(Collection<T> collection) {if (isEmpty(collection)) {return null;}//当类型为List时，直接取得最后一个元素 。if (collection instanceof List) {List<T> list = (List<T>) collection;return list.get(list.size() - 1);}//其他类型通过iterator滚动到最后一个元素.Iterator<T> iterator = collection.iterator();while (true) {T current = iterator.next();if (!iterator.hasNext()) {return current;}}}/** * 返回a+b的新List. */public static <T> List<T> union(final Collection<T> a, final Collection<T> b) {List<T> result = new ArrayList<T>(a);result.addAll(b);return result;}/** * 返回a-b的新List. */public static <T> List<T> subtract(final Collection<T> a, final Collection<T> b) {List<T> list = new ArrayList<T>(a);for (T element : b) {list.remove(element);}return list;}/** * 返回a与b的交集的新List. */public static <T> List<T> intersection(Collection<T> a, Collection<T> b) {List<T> list = new ArrayList<T>();for (T element : a) {if (b.contains(element)) {list.add(element);}}return list;}}