prime1 spoj生成两个数字之间的质数

Question

package primesieve1;

import java.io.InputStreamReader;
import java.util.Scanner;

public class Primesieve1 {

    public boolean[] sieveOfEratosthenes(int max){

    boolean[] primeno; //defaults to false
        primeno = new boolean[max];
    for(int i=2; i<max; i++ ){primeno[i]=true;}

    for(int i=2; i<Math.sqrt(max);i++){
        if(primeno[i] == true){
            //all multiples of i*i, except i, are not primeno
            for(int j = i + i; j<max; j=j+i){
                primeno[j]=false;
            }
        }

    }
    return primeno;
}

    public void printTrue(boolean[] arr){
    for(int i=0; i<arr.length; i++){
        if(arr[i]==true){
            System.out.print(i + ", ");
        }
    }
    }

    public static void main(String[] args) {

        System.out.println("enter limit");
        Scanner sc = new Scanner(new InputStreamReader(System.in));
     int a = sc.nextInt();
        boolean a1[];
        Primesieve1 obj = new Primesieve1();
        a1 = obj.sieveOfEratosthenes(a);

        obj.printTrue(a1);

    }
}

giving out this error didnt understand why java.lang.OutOfMemoryError: Java heap space

Answer 1

Although I'm not 100% sure I think a boolean[] still uses about 1 byte per entry. Max will probably get quite big so even increasing the memory for the JVM will probably not do the trick. One thing you can do however is not use a boolean[] but a BitSet instead, this way you'll only use 1 bit per number and thus you can probably cover until the max value of int.