Java for循环Map集合优化实现解析

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class ForUpdate {

  public static void main(String[] args) {

//    for (int i = 0; i < 10000; i += 10) {
//      loopGivenNum(i);
//    }
    for (int i = 10000; i < 100000; i += 10000) {
      loopGivenNum(i);
    }
    System.out.println("----- done -----");

  }

  private static void loopGivenNum(int i) {
    List<String> smallLoop = getLoopList(i);
    List<String> bigLoop = getLoopList(2 * i);
    long doByForTimes = doByFor(bigLoop, smallLoop);
    long doByMapTimes = doByMap(bigLoop, smallLoop);
    System.out.println("size " + i + ": " + doByForTimes + "," + doByMapTimes);
  }

  /**
   * 获取循环变量
   * @param size 循环变量元素个数
   */
  private static List<String> getLoopList(int size) {
    List<String> list = new ArrayList<>();
    for (int i = 0; i < size; i++) {
      list.add(String.valueOf(i));
    }
    return list;
  }

  private static long doByFor(List<String> bigLoop, List<String> smallLoop) {
    long startTime = System.currentTimeMillis();
    for (String str1 : smallLoop) {
      for (String str2 : bigLoop) {
        if (str1.equals(str2)) {
          continue;
        }
      }
    }
    return System.currentTimeMillis() - startTime;
  }

  /**
   * 使用 Map 优化
   * @param bigLoop
   * @param smallLoop
   */
  private static long doByMap(List<String> bigLoop, List<String> smallLoop) {
    long startTime = System.currentTimeMillis();
    // 转换成map
    Map<String, String> loopMap = bigLoop.stream().collect(Collectors.toMap(k -> k, Function.identity()));
    System.out.println(loopMap.size());
    for (String str1 : smallLoop) {
      if (loopMap.containsKey(str1)) {
        continue;
      }
    }
    return System.currentTimeMillis() - startTime;
  }
}

size 10000: 756,97
size 20000: 3091,8
size 30000: 4342,7
size 40000: 8848,7
size 50000: 16317,7
size 60000: 31652,7
size 70000: 37078,7