JAVA编程--阶乘运算

import java.math.BigInteger;
import java.util.ArrayList;

public class doFactorial {

    public static void main(String[] args) {
          int number=5;
          System.out.println("方法一算得"+number+"的阶乘为："+Wayone(number));
          System.out.println("方法二算得"+number+"的阶乘为："+Waytwo(number));
          System.out.println("方法三算得"+number+"的阶乘为："+Waythree(number));
          System.out.println("方法四算得"+number+"的阶乘为："+Wayfour(number));//方法四可以传入一个较大得number值
     }

    /**
      * 普通的循环方法求阶乘
      * @param num
      * @return
      */
     public static int Wayone(int num) {
          int sum=1;
          if(num<0) {
               throw new IllegalArgumentException("需要计算的参数必须为正数！");//抛出不合理参数异常
          }
          for(int i=1;i<=num;i++) {
               sum*=i;
          }
          return sum;
     } 

    /**
     * 运用递归算法求阶乘
      * @param num
      * @return
     */
     public static int Waytwo(int num) {
          int sum=1;
          if(num<0) {
               throw new IllegalArgumentException("需要计算的参数必须为正数！");//抛出不合理参数异常
          }
          if(num==1) {
               return 1;//跳出循环
          }else {
               sum=num*Waytwo(num-1);//递归
               return sum;
          }
     }

    /**
      * 数组添加计算阶乘
      * @param num
      * @return
      */
     public static long Waythree(int num) {
          long []arr=new long[21];
          arr[0]=1;
          int last=0;
          if(num>=arr.length) {
               throw new IllegalArgumentException("传入的值太大了！");//抛出不合理参数异常
          }
          if(num<0) {
              throw new IllegalArgumentException("需要计算的参数必须为正数！");//抛出不合理参数异常
         }
         while(last<num) {
               arr[last+1]=arr[last]*(last+1);
               last++;
          }
          return arr[num];
     }

    /**
      * 用BigInteger类计算阶乘
      * @param num
      * @return
      */
     public static synchronized BigInteger Wayfour(int num) {
          ArrayList list=new ArrayList();
          list.add(BigInteger.valueOf(1));
          for(int i=list.size();i<=num;i++) {
               BigInteger lastfact=(BigInteger) list.get(i-1);
               BigInteger nextfact=lastfact.multiply(BigInteger.valueOf(i));
               list.add(nextfact);
          }
          return (BigInteger) list.get(num);
     }
}