集合工具类 - CollectionUtil.java

集合工具类，提供数组转LIST、数组转SET、合并集合、计算笛卡儿积等方法。

源码如下：(点击下载 -  CollectionUtil.java、ArrayUtil.java、commons-lang-2.6.jar、commons-collections4-4.0.jar)

 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 import org.apache.commons.collections4.CollectionUtils;
 import org.apache.commons.lang.ArrayUtils;

 /**
  * 集合工具类
  *
  */
 public class CollectionUtil {

     private static transient final String DEFAULT_SPLIT_STR = ",";

     /**
      * 数组转列表
      *
      * @param arr
      *            an array of T objects.
      * @param <T>
      *            a T object.
      * @return a {@link java.util.List} object.
      */
     public static final <T> List<T> array2List(T[] arr) {
         if (ArrayUtils.isEmpty(arr)) {
             return null;
         }
         return Arrays.asList(arr);
     }

     /**
      * 数组转SET
      *
      * @param arr
      *            an array of T objects.
      * @param <T>
      *            a T object.
      * @return a {@link java.util.Set} object.
      */
     public static final <T> Set<T> array2Set(T[] arr) {
         if (ArrayUtils.isEmpty(arr)) {
             return null;
         }
         return new LinkedHashSet<T>(Arrays.asList(arr));
     }

     /**
      * 合并集合
      *
      * @param collections
      *            a {@link java.util.Collection} object.
      * @param <T>
      *            a T object.
      * @return a {@link java.util.List} object.
      */
     @SuppressWarnings("unchecked")
     public static final <T> List<T> collections2List(
             Collection<T>... collections) {
         if (ArrayUtils.isEmpty(collections)) {
             return null;
         }

         final List<T> li = new ArrayList<T>();
         for (Collection<T> foo : collections) {
             if (CollectionUtils.isNotEmpty(foo)) {
                 li.addAll(foo);
             }
         }
         return li;
     }

     /**
      * 合并集合
      *
      * @param collections
      *            a {@link java.util.Collection} object.
      * @param <T>
      *            a T object.
      * @return a {@link java.util.Set} object.
      */
     @SuppressWarnings("unchecked")
     public static final <T> Set<T> collections2Set(Collection<T>... collections) {
         if (ArrayUtils.isEmpty(collections)) {
             return null;
         }
         final Set<T> set = new LinkedHashSet<T>();
         for (Collection<T> foo : collections) {
             if (CollectionUtils.isNotEmpty(foo)) {
                 set.addAll(foo);
             }
         }
         return set;
     }

     /**
      * 拼接集合字符串
      *
      * @param collection
      *            a {@link java.util.Collection} object.
      * @param joinStr
      *            a {@link java.lang.String} object.
      * @return a {@link java.lang.String} object.
      */
     @SuppressWarnings({ "unchecked", "rawtypes" })
     public static final String join(final Collection collection, final String joinStr) {
         if (CollectionUtils.isEmpty(collection)) {
             return "";
         }
         if (joinStr == null) {
             throw new IllegalArgumentException("join string is null.");
         }

         Object[] arr = ArrayUtil.toArray(collection, Object.class);
         return ArrayUtil.join(arr, joinStr);
     }

     /**
      * 拼接集合字符串
      *
      * @param collection
      *            a {@link java.util.Collection} object.
      * @return a {@link java.lang.String} object.
      */
     @SuppressWarnings("rawtypes")
     public static final String join(final Collection collection) {
         return join(collection, DEFAULT_SPLIT_STR);
     }

     /**
      * 计算笛卡儿积
      *
      * @param crossArgs
      *            a {@link java.util.List} object.
      * @param <T>
      *            a T object.
      * @return a {@link java.util.List} object.
      */
     public static <T> List<List<T>> decartes(List<List<T>> crossArgs) {

         // 计算出笛卡尔积行数
         int rows = crossArgs.size() > 0 ? 1 : 0;

         for (List<T> data : crossArgs) {
             rows *= data.size();
         }

         // 笛卡尔积索引记录
         int[] record = new int[crossArgs.size()];

         List<List<T>> results = new ArrayList<List<T>>();

         // 产生笛卡尔积
         for (int i = 0; i < rows; i++) {
             List<T> row = new ArrayList<T>();

             // 生成笛卡尔积的每组数据
             for (int index = 0; index < record.length; index++) {
                 row.add(crossArgs.get(index).get(record[index]));
             }

             results.add(row);
             crossRecord(crossArgs, record, crossArgs.size() - 1);
         }

         return results;
     }

     /**
      * @param sourceArgs
      * @param record
      * @param level
      */
     private static <T> void crossRecord(List<List<T>> sourceArgs, int[] record, int level) {
         record[level] = record[level] + 1;

         if (record[level] >= sourceArgs.get(level).size() && level > 0) {
             record[level] = 0;
             crossRecord(sourceArgs, record, level - 1);
         }
     }

     /**
      * 从list中取得某段数据
      *
      * @param <T>
      * @param datas
      * @param page
      * @param PAGESIZE
      * @return
      */
     public static <T> List<T> getLimit(List<T> datas, int begin, int end) {
         List<T> objects = new ArrayList<T>();
         if (datas.size() <= begin) {
             return objects;
         }
         int temp = (datas.size() > end) ? end : datas.size();
         for (int i = begin; i < temp; i++) {
             objects.add(datas.get(i));
         }
         return objects;
     }

 }