BitMap介绍

 
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
 
public class IsNumberExist {
  private int[] bitmap;
  private int size;
  private int SHIFT = 5;// 2的5次方是32
 
  public boolean isNumberExist(int number) {
    int bit = number >> SHIFT;
    int index = number & ((1 << SHIFT) - 1);
    return ((1 << index) & bitmap[bit]) != 0;
  }
 
  public IsNumberExist(int size) {
    this.size = size;
    bitmap = new int[(size >> SHIFT) + 1];
  }
 
  public void insertDate(int number) {
    int bit = number >> SHIFT;
    int index = number & ((1 << SHIFT) - 1);
    bitmap[bit] = bitmap[bit] | (1 << index);
  }
 
  public void insertFromTxt(String filename) {
    try {
      BufferedReader br = new BufferedReader(new FileReader(filename));
      String str = null;
      while ((str = br.readLine()) != null) {
        insertDate(Integer.valueOf(str));
      }
      br.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
 
  public static void main(String[] args) {
    Runtime rt = Runtime.getRuntime();
    System.out.println("当前JVM所占内存：" + (rt.totalMemory() - rt.freeMemory())
        / 1024 / 1024 + "M");
    IsNumberExist tool = new IsNumberExist(1000000000);
    System.out.println("当前JVM所占内存：" + (rt.totalMemory() - rt.freeMemory())
        / 1024 / 1024 + "M");
    // Date.makeNumbers(100000000);//生成一亿个数到number.txt
    tool.insertFromTxt("numbers.txt");// 使用这个一亿个数初始化bitmap的状态
    System.out.println(tool.isNumberExist(88888888));// 判断88888888是否在这个文件中
    System.out.println(tool.isNumberExist(99999999));// 判断99999999是否在这个文件中
    System.out.println(tool.isNumberExist(91725151));// 判断91725151是否在这个文件中
 
  }
}

 
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Random;
 
public class Date {
  public static boolean makeNumbers(int size) {
    boolean flag = true;
    Random random = new Random();
    try {
      BufferedWriter bw = new BufferedWriter(
          new FileWriter("numbers.txt"));
      for (int i = 0; i < size; i++) {
        bw.write(String.valueOf(Math.abs(random.nextInt(size))));
        bw.newLine();
      }
      bw.close();
    } catch (IOException e) {
      flag = false;
      e.printStackTrace();
    }
    return flag;
  }
 
  public static void main(String[] args) {
    System.out.println(makeNumbers(100000000));
  }
}