素数筛选法

public class PrimeFilter2{  public static void main(String args[]){     int a[]={2,3,5,7};     print(a);     int b[] = filter(a,100);     print(b);       int c[] = filter(b,10000);     print(c);          int d[] = filter(c,100000000);     print(d);       }  static int[] filter(int a[],int n){  //a[]={2,3,5,7}  int[] b = new int[n];//0,2,3,0,5,0,7...100  b[1]=2;  for(int i=2;i<n;i+=2){  b[i] = i+1;  }  //筛选  for(int i=1;i<a.length;i++){  int x = n/a[i];  for(int j=2;j<=x;j++){  b[ a[i]*j-1 ] = 0;    }  }    //提取非0的数即素数  int num=1;  for(int i=2;i<b.length;i+=2){  if(b[i]!=0){  num++;  }  }  //把数组放到num个元素的数组当中  int[] c= new int[num];  c[0]=2;  int k=1;  for(int i=2;i<b.length;i+=2){  if(b[i]!=0){  c[k++] = b[i];  }  }  return c;    }    static void print(int[] a){  System.out.println("开始打印"+a.length+"个素数...");  for(int x: a){  System.out.print(x+" ");  }  System.out.println();  }}