集合工具类 - CollectionUtil.java

Posted 2020-06-14

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)

  1 import java.util.ArrayList;
  2 import java.util.Arrays;
  3 import java.util.Collection;
  4 import java.util.LinkedHashSet;
  5 import java.util.List;
  6 import java.util.Set;
  7 import org.apache.commons.collections4.CollectionUtils;
  8 import org.apache.commons.lang.ArrayUtils;
  9 
 10 /**
 11  * 集合工具类
 12  * 
 13  */
 14 public class CollectionUtil {
 15 
 16     private static transient final String DEFAULT_SPLIT_STR = ",";
 17 
 18     /**
 19      * 数组转列表
 20      *
 21      * @param arr
 22      *            an array of T objects.
 23      * @param <T>
 24      *            a T object.
 25      * @return a {@link java.util.List} object.
 26      */
 27     public static final <T> List<T> array2List(T[] arr) {
 28         if (ArrayUtils.isEmpty(arr)) {
 29             return null;
 30         }
 31         return Arrays.asList(arr);
 32     }
 33 
 34     /**
 35      * 数组转SET
 36      *
 37      * @param arr
 38      *            an array of T objects.
 39      * @param <T>
 40      *            a T object.
 41      * @return a {@link java.util.Set} object.
 42      */
 43     public static final <T> Set<T> array2Set(T[] arr) {
 44         if (ArrayUtils.isEmpty(arr)) {
 45             return null;
 46         }
 47         return new LinkedHashSet<T>(Arrays.asList(arr));
 48     }
 49 
 50     /**
 51      * 合并集合
 52      *
 53      * @param collections
 54      *            a {@link java.util.Collection} object.
 55      * @param <T>
 56      *            a T object.
 57      * @return a {@link java.util.List} object.
 58      */
 59     @SuppressWarnings("unchecked")
 60     public static final <T> List<T> collections2List(
 61             Collection<T>... collections) {
 62         if (ArrayUtils.isEmpty(collections)) {
 63             return null;
 64         }
 65 
 66         final List<T> li = new ArrayList<T>();
 67         for (Collection<T> foo : collections) {
 68             if (CollectionUtils.isNotEmpty(foo)) {
 69                 li.addAll(foo);
 70             }
 71         }
 72         return li;
 73     }
 74 
 75     /**
 76      * 合并集合
 77      *
 78      * @param collections
 79      *            a {@link java.util.Collection} object.
 80      * @param <T>
 81      *            a T object.
 82      * @return a {@link java.util.Set} object.
 83      */
 84     @SuppressWarnings("unchecked")
 85     public static final <T> Set<T> collections2Set(Collection<T>... collections) {
 86         if (ArrayUtils.isEmpty(collections)) {
 87             return null;
 88         }
 89         final Set<T> set = new LinkedHashSet<T>();
 90         for (Collection<T> foo : collections) {
 91             if (CollectionUtils.isNotEmpty(foo)) {
 92                 set.addAll(foo);
 93             }
 94         }
 95         return set;
 96     }
 97 
 98     /**
 99      * 拼接集合字符串
100      *
101      * @param collection
102      *            a {@link java.util.Collection} object.
103      * @param joinStr
104      *            a {@link java.lang.String} object.
105      * @return a {@link java.lang.String} object.
106      */
107     @SuppressWarnings({ "unchecked", "rawtypes" })
108     public static final String join(final Collection collection, final String joinStr) {
109         if (CollectionUtils.isEmpty(collection)) {
110             return "";
111         }
112         if (joinStr == null) {
113             throw new IllegalArgumentException("join string is null.");
114         }
115 
116         Object[] arr = ArrayUtil.toArray(collection, Object.class);
117         return ArrayUtil.join(arr, joinStr);
118     }
119 
120     /**
121      * 拼接集合字符串
122      *
123      * @param collection
124      *            a {@link java.util.Collection} object.
125      * @return a {@link java.lang.String} object.
126      */
127     @SuppressWarnings("rawtypes")
128     public static final String join(final Collection collection) {
129         return join(collection, DEFAULT_SPLIT_STR);
130     }
131 
132     /**
133      * 计算笛卡儿积
134      *
135      * @param crossArgs
136      *            a {@link java.util.List} object.
137      * @param <T>
138      *            a T object.
139      * @return a {@link java.util.List} object.
140      */
141     public static <T> List<List<T>> decartes(List<List<T>> crossArgs) {
142 
143         // 计算出笛卡尔积行数
144         int rows = crossArgs.size() > 0 ? 1 : 0;
145 
146         for (List<T> data : crossArgs) {
147             rows *= data.size();
148         }
149 
150         // 笛卡尔积索引记录
151         int[] record = new int[crossArgs.size()];
152 
153         List<List<T>> results = new ArrayList<List<T>>();
154 
155         // 产生笛卡尔积
156         for (int i = 0; i < rows; i++) {
157             List<T> row = new ArrayList<T>();
158 
159             // 生成笛卡尔积的每组数据
160             for (int index = 0; index < record.length; index++) {
161                 row.add(crossArgs.get(index).get(record[index]));
162             }
163 
164             results.add(row);
165             crossRecord(crossArgs, record, crossArgs.size() - 1);
166         }
167 
168         return results;
169     }
170 
171     /**
172      * @param sourceArgs
173      * @param record
174      * @param level
175      */
176     private static <T> void crossRecord(List<List<T>> sourceArgs, int[] record, int level) {
177         record[level] = record[level] + 1;
178 
179         if (record[level] >= sourceArgs.get(level).size() && level > 0) {
180             record[level] = 0;
181             crossRecord(sourceArgs, record, level - 1);
182         }
183     }
184 
185     /**
186      * 从list中取得某段数据
187      * 
188      * @param <T>
189      * @param datas
190      * @param page
191      * @param PAGESIZE
192      * @return
193      */
194     public static <T> List<T> getLimit(List<T> datas, int begin, int end) {
195         List<T> objects = new ArrayList<T>();
196         if (datas.size() <= begin) {
197             return objects;
198         }
199         int temp = (datas.size() > end) ? end : datas.size();
200         for (int i = begin; i < temp; i++) {
201             objects.add(datas.get(i));
202         }
203         return objects;
204     }
205 
206 }