java基础

/*
* java里不能使用人们熟悉的算术运算符处理大数值 而需要使用大数值类中的add和multiply方法
* BigInteger c = a.add(b)// c = a+b;
* BigInteger d = c.multiply(b.add(BigInteger.valueOf(2)))// d = c *(b+2)
* */

如计算n*(n-1)*...*(n-k+1) / （1*2*...k）;

package welcome;

import java.math.BigInteger;
import java.util.Scanner;


public class welcome {
    public static void main(String[] args)
    {
        
        Scanner in = new Scanner(System.in);
        int k = in.nextInt();
        int n = in.nextInt();
        BigInteger lotteryOdds = BigInteger.valueOf(1);
        for(int i=1;i<=k;i++)
        {
            lotteryOdds = lotteryOdds.multiply(BigInteger.valueOf(n-1-i)).divide(BigInteger.valueOf(i));
        }
        
        System.out.println("Your odds are 1 in "+ lotteryOdds +".Good luck");
    }

}

将一个数组的元素拷贝到另外一个数组中 调用这个方法的语法格式 System.arraycopy(from,fromIndex,to,toIndex,count)

实例：

package welcome1;
import java.util.Scanner;
public class welcome1 {
    public static void main(String[] args)
    {
    Scanner in = new Scanner(System.in);
    int[] smallPrimes = {2,3,5,7,11,13};
    int[] luckyNumbers = {1001,1002,1003,1004,1005,1006,1007};
    System.arraycopy(smallPrimes,2,luckyNumbers,3,4);
    for(int i=0;i<luckyNumbers.length;i++)
    {
        System.out.println(i+":"+luckyNumbers[i]);
    }
    }
}
输出：

0:1001
1:1002
2:1003
3:5
4:7
5:11
6:13

数组排序调用Arrays类中的sort // Array.sort(a);

实例：

package LotteryDrawing;
import java.util.Arrays;
import java.util.Scanner ;
public class LotteryDrawing {
    public static void main(String[] args)
    {
        Scanner in = new Scanner(System.in);
        System.out.print("How many numbers do you need to draw");
        int k = in.nextInt();
        System.out.print("what is the highest number you can draw");
        int n = in.nextInt();
        int[] numbers = new int[n];
        for(int i=0;i<numbers.length;i++)
        {

            numbers[i] = i +1;
        }
        int[] result = new int[k];
        for(int i=0;i<result.length;i++)
        {
            int r = (int)(Math.random()*n);
            result[i] = numbers[r];
            numbers[r] = numbers[n-1];
            n--;
        }
        Arrays.sort(result);
        System.out.println("Bet the following combination.It'll make you rich!");
        for(int r:result)
            System.out.println(r);
    }

}