集合工具类

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;
    }

}