Java List集合实现MySQL Group By功能

例： 

List<String> list = new ArrayList<String>();
list.add("aaa");
list.add("bbb");
list.add("aaa");
list.add("aaa");
list.add("ccc");
list.add("bbb");

输出结果：

aaa 3

bbb 2

ccc 1

完整代码，包括写excel：

import java.io.FileOutputStream;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

import org.apache.poi.hssf.usermodel.HSSFCell;
import org.apache.poi.hssf.usermodel.HSSFCellStyle;
import org.apache.poi.hssf.usermodel.HSSFRow;
import org.apache.poi.hssf.usermodel.HSSFSheet;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;


/**
 * 
 *
 */
public class StatisticUser {

	
	public static void main(String[] args) {
		
		List<String> list = getUserLabelList(getUser());
		
		List<JavaBean> groupList = getListByGroup1(list);
		System.out.println("********************统计结果*********************");
		
		//按数量排序
		Collections.sort(groupList, new Comparator<JavaBean>(){
            public int compare(JavaBean o1, JavaBean o2) {
                //排序属性
                if(o1.getCount() < o2.getCount()){
                    return 1;
                }
                if(o1.getCount() == o2.getCount()){
                    return 0;
                }
                return -1;
            }
        });
		//遍历打印结果
		for (JavaBean bean : groupList) {
			//打印部分记录
			if(bean.getCount() > 5){
				System.out.print(bean.getGroup() + "		");
				System.out.println(bean.getCount());
			}
		}
		System.out.println("**************写Excel****************");
		long st = System.currentTimeMillis();
		
		writeExcel("d://user.xls", groupList);
		
		long ed = System.currentTimeMillis();
		System.out.println("耗时：" + (ed-st) );
	}
	
	
	/**
	 * 写Excel
	 * @param path
	 * @param groupList
	 */
	public static void writeExcel(String path,List<JavaBean> groupList) {
		FileOutputStream fos = null;

		HSSFCellStyle style = null;
		HSSFCellStyle style2 = null;
		
		String sheet1[] =new String[]{"用户","数量" };
		String sheetName[] = {"Sheet1"};
		try {
			HSSFWorkbook wb = new HSSFWorkbook();
			//style = createCellStyle(wb);//表头
			//style2 = createCellStyle2(wb);
			
			fos = new FileOutputStream(path);
			//System.out.println("----sheet----");
			HSSFSheet s = wb.createSheet(sheetName[0]);// sheet名称

			//写表格的头部
			HSSFRow row = s.createRow(0);
			//设置行高
			row.setHeight((short)(15.625*30)); 
			//设置行宽
			s.setColumnWidth(0, (int)55.7*100);
			HSSFCell cell = null;
			for (int i = 0; i < sheet1.length; i++) {
				cell = row.createCell(i);
				cell.setCellValue(sheet1[i]);
				//cell.setCellStyle(style);//设置样式
			}
			
			//遍历写单元格
			int flag = 1;
			for (JavaBean bean : groupList) {
				row = s.createRow(flag);
				//System.out.print(bean.getGroup() + "		");
				//System.out.println(bean.getMoney());
				cell = row.createCell(0);
				cell.setCellValue( bean.getGroup() );
				
				cell = row.createCell(1);
				cell.setCellValue( bean.getCount() );
				
				flag++;
			}
			
			wb.write(fos);
			fos.close();
		} catch (Exception e) {
			// TODO: handle exception
			System.out.println("写Excel异常：" + e);
		} finally {
			if (fos != null) {
				try {
					fos.close();
				} catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}
	}

	 
	
	/**
	 * List集合String类型
	 * @param list
	 * @return
	 */
	private static List<JavaBean> getListByGroup1(List<String> list) {
		List<JavaBean> result = new ArrayList<JavaBean>();
		Map<String, Integer> map = new HashMap<String, Integer>();

		for (Iterator iterator = list.iterator(); iterator.hasNext();) {
			String key = (String) iterator.next();
			//System.out.println("*****" + key  );
			if (map.containsKey( key )) {
				//System.out.println("*****" + key + "*" + map.get( key  ) );
				map.put( key , map.get(  key  ) + 1);
			} else {
				map.put( key , 1);
			}
		}
		for (Entry<String, Integer> entry : map.entrySet()) {
			result.add(new JavaBean(entry.getKey(), entry.getValue()));
		}
		return result;
	}

	
	/**
	 * 处理下list
	 * @param list
	 * @return
	 */
	private static List<String> getUserLabelList(List<String> list){
		List<String> list2 = new ArrayList<String>();
		
		for (Iterator iterator = list.iterator(); iterator.hasNext();) {
			String key = (String) iterator.next();
			//System.out.println("*****" + key  );
			String key_arr[] = key.split(",");
			for (int i = 0; i < key_arr.length; i++) {
				list2.add(key_arr[i]);
			}
		}
		return list2;
	}
	

	/**
	 * @return
	 */
	public static List<String> getUser() {
		
		List<String> list = new ArrayList<String>();
		
		list.add("美食,名人明星,文艺,时尚");
		list.add("名人明星");
		list.add("动漫");
        list.add("动漫,星座运势");
        list.add("动漫,八卦杂谈");
        list.add("动漫,宅女,看书");
        list.add("美图摄影,动漫,汽车");


		return list;
	}
	
}


class JavaBean {
	private String group;
	private int count;

	public JavaBean() {
	}

	public JavaBean(String group, int count) {
		this.group = group;
		this.count = count;
	}

	public String getGroup() {
		return group;
	}

	public void setGroup(String group) {
		this.group = group;
	}

	public int getCount() {
		return count;
	}

	public void setCount(int count) {
		this.count = count;
	}

}