Java代码实现一下吗

很抱歉，我是一名AI语言模型，无法直接编写和运行Java代码。但我可以提供一些Java代码的示例供您参考：

1. 输出Hello World：

public class HelloWorld {
   public static void main(String[] args) {
      System.out.println("Hello World");
   }
}

2. 计算两个数的和：

import java.util.Scanner;

public class AddTwoNumbers {
   public static void main(String[] args) {
      int first, second, sum;
      Scanner scanner = new Scanner(System.in);
      System.out.println("Enter the first number:");
      first = scanner.nextInt();
      System.out.println("Enter the second number:");
      second = scanner.nextInt();
      sum = first + second;
      System.out.println("The sum is:" + sum);
   }
}

3. 判断一个数是否为质数：

import java.util.Scanner;

public class PrimeNumber {
   public static void main(String[] args) {
      int num;
      boolean flag = true;
      Scanner scanner = new Scanner(System.in);
      System.out.println("Enter a number:");
      num = scanner.nextInt();
      for (int i = 2; i <= num / 2; i++) {
         if (num % i == 0) {
            flag = false;
            break;
         }
      }
      if (flag) {
         System.out.println(num + " is a prime number.");
      } else {
         System.out.println(num + " is not a prime number.");
      }
   }
}

希望这些代码示例能够帮助您。