集合工具类 - CollectionUtil.java

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;
}

}