随即轮询

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.junit.Test;

/** 随即获取N个数据，并把这些数据拍到数据最前面（假设所有数据都不为空） */
public class RandomRank {
	public static final int k = 56666;
	public static final int num = 56665;

	@Test
	public void test1() {
		String[] bookIDs = new String[k];
		for (int i = 0; i < k; i++) {
			bookIDs[i] = "" + (i + 1000);
		}
		long curr1 = System.currentTimeMillis();
		randomRank(bookIDs, num);
		System.out
				.println("最新的方法执行时间 ：" + (System.currentTimeMillis() - curr1));
	}

	@Test
	public void test2() {
		String[] bookIDs = new String[k];
		for (int i = 0; i < k; i++) {
			bookIDs[i] = "" + (i + 1000);
		}
		long curr2 = System.currentTimeMillis();
		randomRank1(bookIDs, num);
		System.out
				.println("先前的方法执行时间 ：" + (System.currentTimeMillis() - curr2));
	}

	@Test
	public void test3() {
		String[] bookIDs = new String[k];
		for (int i = 0; i < k; i++) {
			bookIDs[i] = "" + (i + 1000);
		}
		long curr2 = System.currentTimeMillis();
		randomRank2(bookIDs, num);
		System.out
				.println("第三个方法执行时间 ：" + (System.currentTimeMillis() - curr2));
	}

	@Test
	public void test4() {
		String[] bookIDs = new String[k];
		for (int i = 0; i < k; i++) {
			bookIDs[i] = "" + (i + 1000);
		}
		long curr2 = System.currentTimeMillis();
		randomRank3(bookIDs, num);
		System.out
				.println("先前的方法执行时间 ：" + (System.currentTimeMillis() - curr2));
	}

	/** 第一个方法 */
	public static String[] randomRank(String[] bookIDs, int num) {
		if (null == bookIDs || 0 == bookIDs.length) {
			return null;
		}

		// 如果轮转个数大于数组长度，则设置为数组长度
		if (num >= bookIDs.length) {
			num = bookIDs.length;
		}

		List<String> list = new ArrayList<String>(Arrays.asList(bookIDs));
		List<String> reList = new ArrayList<String>(bookIDs.length);

		int index;
		java.util.Random r = new java.util.Random();
		for (int i = 0; i < num; i++) {
			index = r.nextInt(list.size());
			String s = list.remove(index);
			reList.add(s);
		}
		// 把其他元素保存到新数组中
		reList.addAll(list);
		String[] ids = new String[reList.size()];
		return (String[]) reList.toArray(ids);
	}

	/** 第二个方法 */
	public static String[] randomRank1(String[] bookIDs, int num) {
		if (null == bookIDs || 0 == bookIDs.length) {
			return null;
		}
		// 如果轮转个数大于数组长度，则设置为1
		if (num >= bookIDs.length) {
			num = 1;
		}

		List<String> list = new ArrayList<String>(bookIDs.length);
		for (int i = 0; i < bookIDs.length; i++) {
			list.add(bookIDs[i]);
		}
		int[] ran = randon(num, bookIDs.length);// 随即取数组的下标
		List<String> reList = new ArrayList<String>(bookIDs.length);
		// 取出随机数
		for (int i = 0; i < ran.length; i++) // 按照下标获取数据
		{
			String o = (String) list.get(ran[i]);
			reList.add(o);
		}
		// 在原有数组中删除随机数
		for (int i = 0; i < reList.size(); i++) {
			list.remove(reList.get(i));
		}
		// 把其他元素保存到新数组中
		reList.addAll(list);
		String[] ids = new String[reList.size()];
		return (String[]) reList.toArray(ids);
	}

	public static String[] randomRank2(String[] bookIDs, int num) {
		if (null == bookIDs || 0 == bookIDs.length) {
			return null;
		}

		// 如果轮转个数大于数组长度，则设置为数组长度
		if (num >= bookIDs.length) {
			num = bookIDs.length;
		}

		List<String> list = new ArrayList<String>(Arrays.asList(bookIDs));
		String[] ids = new String[bookIDs.length];
		int index;
		java.util.Random r = new java.util.Random();
		for (int i = 0; i < num; i++) {
			index = r.nextInt(list.size());
			String s = list.remove(index);// 这方面花费时间会比randomRank3多,牵扯到数组的复制
			ids[i] = s;
		}

		for (int i = 0; i < list.size(); i++) {
			ids[num + i] = list.get(i);
		}
		list = null;
		return ids;
	}

	public static String[] randomRank3(String[] bookIDs, int num) {
		if (null == bookIDs || 0 == bookIDs.length) {
			return null;
		}

		if (num >= bookIDs.length) {
			num = bookIDs.length;
		}
		int[] tem = randon(num, bookIDs.length);// 获取所有随即数的数组下标
		String[] ids = new String[bookIDs.length];
		for (int i = 0; i < num; i++) {
			ids[i] = bookIDs[tem[i]];
			bookIDs[tem[i]] = null;
		}
		int k = num;
		for (int i = 0; i < bookIDs.length; i++) {
			if (bookIDs[i] != null) {
				ids[k++] = bookIDs[i];
			}
		}
		return ids;
	}

	private static int[] randon(int num, int length) {
		boolean[] cards = new boolean[length];// 构造一个比对数组，默认都为false
		for (int index = 0; index < length; index++) {
			cards[index] = false;
		}
		java.util.Random r = new java.util.Random();
		int x;

		// 随机返回下标数组
		int[] result = new int[num];
		for (int index = 0; index < num; index++) {
			do {
				x = getRandom(length, r);// 当card[x]元素为true时，一直循环，这浪费时间
			} while (cards[x]);
			cards[x] = true;
			result[index] = x;
		}
		return result;
	}

	private static int getRandom(int length, java.util.Random r) {
		int x;
		x = r.nextInt(length);
		return x;
	}
}