Java打印素数(质数)

要求：打印 2 - 100000 当中的素数与非素数。（素数定义:在大于1的自然数中，除了1和它本身以外不再有其他因数）

  // sqrt 法   public static void printPrime1(int num) {     boolean[] isPrimes = new boolean[num + 1];     for (int i = 2; i < isPrimes.length; i++) {       isPrimes[i] = true;     }      for (int i = 3; i <= num; i++) {       for (int j = 2; j <= Math.sqrt(i); j++) {         if (i % j == 0) {           isPrimes[i] = false;           break;         }       }     }      System.out.print("质数有: ");     for (int i = 2; i < isPrimes.length; i++) {       if (isPrimes[i]) {         System.out.print(i + " ");       }     }     System.out.println("");     System.out.print("非质数有: ");     for (int i = 2; i < isPrimes.length; i++) {       if (!isPrimes[i]) {         System.out.print(i + " ");       }     }   }

　　注意这里用一个布尔类型的数组用于素数的判断与最终结果的打印，避免了使用两个容器去分别存放各自需要打印的两块内容。

  // 埃氏筛法   public static void printPrime2(int num) {     boolean[] isPrimes = new boolean[num + 1];     for (int i = 2; i < isPrimes.length; i++) {       isPrimes[i] = true;     }     for (int i = 2; i < num; i++) {       if (isPrimes[i] == true) {         for (int j = 2; i * j <= num; j++) {           isPrimes[i * j] = false;         }       }     }      System.out.print("质数有: ");     for (int i = 2; i < isPrimes.length; i++) {       if (isPrimes[i]) {         System.out.print(i + " ");       }     }     System.out.println("");     System.out.print("非质数有: ");     for (int i = 2; i < isPrimes.length; i++) {       if (!isPrimes[i]) {         System.out.print(i + " ");       }     }   }

　　最后提供一个main方法，完成对两个方法的调用与性能比较:

  public static void main(String[] args) {     long timePoint1 = System.currentTimeMillis();     printPrime1(100000);     long timePoint2 = System.currentTimeMillis();     System.out.println();     printPrime2(100000);     long timePoint3 = System.currentTimeMillis();      System.out.println();     System.out.println("sqrt法耗时: " + String.valueOf(timePoint2 - timePoint1)); // 390 ms     System.out.println("埃氏筛法耗时: " + String.valueOf(timePoint3 - timePoint2)); // 297 ms   }

原文：https://www.cnblogs.com/binye-typing/p/9261118.html