JAVA集合Listグループ分けの2つの方法

http://blog.csdn.net/yangxiaojun9238/article/details/51500934
1.工具類

"code" class="java">import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import com.framework.util.ParamUtils;

public class CommonUtils {

    /**
     *       ，       ，      
     * 
     * @author ZhangLiKun
     * @title GroupBy
     * @date 2013-4-23
     */
    public interface GroupBy {
        T groupby(Object obj);
    }

    /**
     * 
     * @param colls
     * @param gb
     * @return
     */
    public static final extends Comparable, D> Map> group(Collection colls, GroupBy gb) {
        if (colls == null || colls.isEmpty()) {
            System.out.println("        !");
            return null;
        }
        if (gb == null) {
            System.out.println("         Null!");
            return null;
        }
        Iterator iter = colls.iterator();
        Map> map = new HashMap>();
        while (iter.hasNext()) {
            D d = iter.next();
            T t = gb.groupby(d);
            if (map.containsKey(t)) {
                map.get(t).add(d);
            } else {
                List list = new ArrayList();
                list.add(d);
                map.put(t, list);
            }
        }
        return map;
    }
    /**
     *  List  V methodName     （      K  ）  ，   Map> 

     *       method   V           ，         K  
     * 
     * @param list
     *                  
     * @param map
     *                  map
     * @param clazz
     *              V   
     * @param methodName
     *               
     */
    public static  void listGroup2Map(List list, Map> map, Class clazz, String methodName) {
        //        
        if (null == list || null == map || null == clazz || !ParamUtils.chkString(methodName)) {
            System.out.print("CommonUtils.listGroup2Map     ，list：" + list + " ；map：" + map + " ；clazz：" + clazz + " ；methodName：" + methodName);
            return;
        }

        //     
        Method method = getMethodByName(clazz, methodName);
        //     
        if (null == method) {
            return;
        }

        //     
        listGroup2Map(list, map, method);
    }
    /**
     *        ，      
     * 
     * @param clazz
     * @param methodName
     * @return
     */
    public static Method getMethodByName(Class> clazz, String methodName) {
        Method method = null;
        //       
        if (null == clazz || !ParamUtils.chkString(methodName)) {
            System.out.print("CommonUtils.getMethodByName     ，clazz：" + clazz + " ；methodName：" + methodName);
            return method;
        }

        try {
            method = clazz.getDeclaredMethod(methodName);
        } catch (Exception e) {
            System.out.print("       ！");
        }

        return method;
    }
    /**
     *  List  V        （      K  ）  ，   Map> 

     *       method   V           ，         K  
     * 
     * @param list
     *                  
     * @param map
     *                  map
     * @param method
     *              
     */
    @SuppressWarnings("unchecked")
    public static  void listGroup2Map(List list, Map> map, Method method) {
        //        
        if (null == list || null == map || null == method) {
            System.out.print("CommonUtils.listGroup2Map     ，list：" + list + " ；map：" + map + " ；method：" + method);
            return;
        }

        try {
            //     
            Object key;
            List listTmp;
            for (V val : list) {
                key = method.invoke(val);
                listTmp = map.get(key);
                if (null == listTmp) {
                    listTmp = new ArrayList();
                    map.put((K) key, listTmp);
                }
                listTmp.add(val);
            }
        } catch (Exception e) {
            System.out.print("    ！");
        }
    }

}
2.   
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

import com.framework.common.CommonUtils.GroupBy;

public class Test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        //       

        final int loop = 1000 * 1000;
        List list = new ArrayList(); // size=8 * loop
        for (int i = 0; i < loop; i++) {
            list.add(new Data().setId(1L).setCourseId(200010L).setContent("AAA"));
            list.add(new Data().setId(2L).setCourseId(200010L).setContent("BBB"));
            list.add(new Data().setId(3L).setCourseId(200011L).setContent("CCC"));
            list.add(new Data().setId(4L).setCourseId(200011L).setContent("DDD"));
            list.add(new Data().setId(5L).setCourseId(200010L).setContent("EEE"));
            list.add(new Data().setId(6L).setCourseId(200011L).setContent("FFF"));
            list.add(new Data().setId(7L).setCourseId(200010L).setContent("GGG"));
            list.add(new Data().setId(8L).setCourseId(200012L).setContent("HHH"));
        }
        //      1
        long time = System.currentTimeMillis();
        Map> map2 = new LinkedHashMap>();
        CommonUtils.listGroup2Map(list, map2, Data.class, "getId");//      

        long duration = System.currentTimeMillis() - time;

        System.out.println("     ：" + duration + "  !");

        //    
        time = System.currentTimeMillis();
        Map> map = CommonUtils.group(list, new GroupBy() {
            @Override
            public Long groupby(Object obj) {
                Data d = (Data) obj;
                return d.getCourseId(); //        ID
            }
        });

        duration = System.currentTimeMillis() - time;

        System.out.println("     ：" + duration + "  !");

    }

}