集合分组方法

CollectionUtils.java

package com.chargerlink.app.utils;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * chargerlink_v3
 * com.chargerlink.app.utils
 * CollectionUtils.java
 *
 * by hayukleung
 * at 2017-02-08 14:36
 */

/**
 * Java集合通用方法
 */
public class CollectionUtils {

  /**
   * 依据K将List<V>分组
   *
   * @param list
   * @param <V>
   * @return a list
   */
  public static <V extends Groupable> List<List<V>> groupAsList(List<V> list) {
    List<List<V>> result = new ArrayList<>();
    Map<Integer, List<V>> group = groupAsMap(list);
    for (Integer key : group.keySet()) {
      result.add(group.get(key));
    }
    return result;
  }

  /**
   * 依据K将List<V>分组
   *
   * @param list
   * @param <V>
   * @return a map
   */
  public static <V extends Groupable> Map<Integer, List<V>> groupAsMap(List<V> list) {
    Map<Integer, List<V>> group = new HashMap<>();
    if (null == list || 0 == list.size()) {
      return group;
    }
    for (V v : list) {
      if (!group.containsKey(v.getGroupableKey())) {
        List<V> vList = new ArrayList<>();
        vList.add(v);
        group.put(v.getGroupableKey(), vList);
      } else {
        group.get(v.getGroupableKey()).add(v);
      }
    }
    return group;
  }

  public static interface Groupable {
    int getGroupableKey();
  }
}