Java集合分组

public class Data {

	private Long id ;
	private Long courseId ;
	private String content ;

	public Long getId() {
		return id;
	}

	public Data setId(Long id) {
		this.id = id;
		return this ;
	}

	public Long getCourseId() {
		return courseId;
	}

	public Data setCourseId(Long courseId) {
		this.courseId = courseId;
		return this ;
	}

	public String getContent() {
		return content;
	}

	public Data setContent(String content) {
		this.content = content;
		return this ;
	}

}

public class GroupTest {

	/**
	 * 分組依據接口，用于集合分組時，獲取分組依據
	 * @author	ZhangLiKun
	 * @title	GroupBy
	 * @date	2013-4-23
	 */
	public interface GroupBy<T> {
		T groupby(Object obj) ;
	}	

	/**
	 *
	 * @param colls
	 * @param gb
	 * @return
	 */
	public static final <T extends Comparable<T> ,D> Map<T ,List<D>> group(Collection<D> colls ,GroupBy<T> gb){
		if(colls == null || colls.isEmpty()) {
			System.out.println("分組集合不能為空!");
			return null ;
		}
		if(gb == null) {
			System.out.println("分組依據接口不能為Null!");
			return null ;
		}
		Iterator<D> iter = colls.iterator() ;
		Map<T ,List<D>> map = new HashMap<T, List<D>>() ;
		while(iter.hasNext()) {
			D d = iter.next() ;
			T t = gb.groupby(d) ;
			if(map.containsKey(t)) {
				map.get(t).add(d) ;
			} else {
				List<D> list = new ArrayList<D>() ;
				list.add(d) ;
				map.put(t, list) ;
			}
		}
		return map ;
	}	

	@Test
	public void test() {
		// 准备一个集合
		final int loop = 1000 * 1000 ;
		List<Data> list = new ArrayList<Data> () ;	// 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")) ;
		}

		// 进行分组
		Map<Long ,List<Data>> map = group(list, new GroupBy<Long>() {
			@Override
			public Long groupby(Object obj) {
				Data d = (Data)obj ;
				return d.getCourseId() ;	// 分组依据为课程ID
			}
		}) ;

		Assert.assertEquals(3, map.size()) ;
		Assert.assertEquals(4*loop, map.get(200010L).size()) ;
		Assert.assertEquals(3*loop, map.get(200011L).size()) ;
		Assert.assertEquals(1*loop, map.get(200012L).size()) ;
		Assert.assertEquals("HHH", map.get(200012L).get(0).getContent()) ;

		// 长度为8 * 1000 * 1000的集合测试用时：6481毫秒!

	}

	private long time ;

	@Before
	public void setup() {
		time = System.currentTimeMillis() ;
	}

	@After
	public void teardown() {
		System.out.println(String.format("程序执行：%d毫秒!",System.currentTimeMillis() - time));
	}

}

转自：https://my.oschina.net/zhanglikun/blog/124645