import java.math.BigInteger;

public class MathProblemSolver {

  //開啟兩個線程
  public static void main(String arg[]){
    //第一個線程計算 100!
    FactorialCalculatingThread thread1 = new FactorialCalculatingThread(100);
    //第二個線程計算 100000!
    FactorialCalculatingThread thread2 = new FactorialCalculatingThread(100000);

    thread1.setName("Thread 1");
    thread2.setName("Thread 2");

    thread1.start();
    thread2.start();

    try {
      thread1.join(); //線程Jion，以使主線程在“線程1”和“線程2”都返回結果之前不會進一步執行
      thread2.join();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }

    BigInteger result = thread1.getResult().add(thread2.getResult());
    System.out.println("將兩個線程的計算結果相加等于：" + result);
  }

  //用于階乘計算的線程類
  private static class FactorialCalculatingThread extends Thread {
    private BigInteger result = BigInteger.ONE;
    private long num;

    public FactorialCalculatingThread(long num) {
      this.num = num;
    }

    @Override
    public void run() {
      System.out.println(Thread.currentThread().getName() + " 開始階乘的計算：" + num);
      factorialCalc(num);
      System.out.println(Thread.currentThread().getName() + "執行完成");
    }

    //數的階乘計算方法
    public void factorialCalc(long num) {
      BigInteger f = new BigInteger("1");
      for (int i = 2; i <= num; i++)
        f = f.multiply(BigInteger.valueOf(i));
      result = f;
    }

    public BigInteger getResult() { return result; }
  }
}