标题：程序代码——输出素数

public class Exec {
        public static void main(String args[]) {
                for (int i = 1; i <= 1000000; i += 2) {
                        boolean flag = true;

                        for (int j = 2; j < Math.sqrt(i); j++) {
                                if (i % j == 0) {
                                        flag = false;
                                        break;
                                }
                        }

                        if (flag)
                                System.out.println(i);
                }

        }
}

public class Esce {
 public static void main(String[] args) {
  int i = 1, j = 2;
  for (i = 1; i < 101; i++) {
   for (j = 2; j < i; j++)
    if (i % j == 0) {
     break;
    } else
     System.out.print(i + " ");

  }
 }

}
为什么把  else  换成   if（i==j） 才对？  

  if（i==j）在这起什么作用？？

你的错误来自于排版，请注意格式Ctrl+Shift+F

如果是else则与if语句一起作为for的内部子语句

public class Exec {
        public static void main(String[] args) {
                int i = 1, j = 2;
                for (i = 1; i < 101; i++) {
                        for (j = 2; j < i; j++)
                                if (i % j == 0) {
                                        break;
                                } else
                                        System.out.print(i + " ");
                }
        }
}

如果是if (i == j)则此语句与for语句同级，不是for的内部子语句

public class Exec {
        public static void main(String[] args) {
                int i = 1, j = 2;
                for (i = 1; i < 101; i++) {
                        for (j = 2; j < i; j++)
                                if (i % j == 0) {
                                        break;
                                }
                        if (i == j)
                                System.out.print(i + " ");

                }
        }
}