public static void main(String[] args) {
        
        //範圍：2到多少
        int maxfanwei = 20;
        
        boolean[] bolist = new boolean[maxfanwei+1];
        
        for (int i = 2; i < bolist.length; i++) {
            boolean isnotsushu = bolist[i];//不是素數true，是素數false
            if(!isnotsushu){
                
 
int xiabiao = i;
                //這個數的倍數設定為true
                while (true) {
                    xiabiao+=i;
                    if (xiabiao>=(maxfanwei+1)) {
                        break;//超過範圍退出設定
                    }
                    bolist[xiabiao] = true;
                }
            }
            
        }
        
 
//列印質數
        for (int i = 2; i < bolist.length; i++) {
            if (!bolist[i]) {
                System.out.println(i+"    ");
            }
            
        }
        
        
    }