Java小例子——穷举质数，求平方和，求质因子。

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public static void main(String[] args) throws
IOException

    {

        int
n;

        String s;

        BufferedReader buf;

        buf=new
BufferedReader(new
InputStreamReader(System.in));

        System.out.print("请输入一个自然数：");

        s=buf.readLine();

        n=Integer.parseInt(s);

        System.out.print("f(n)=1");

        for(int
i=2;i<=n;i++)

            System.out.print("+"+i*i);

        System.out.print("="+Square(n));

    }

    public
static long Square(int
n)

    {

        long
f=0;

        if(n<0)

            System.out.println("n<0，输入错误！");

        else
if

            (n==1)f=1;

        else

            f=Square(n-1)+n*n;

        return
f;

    }

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public static void main(String args[])

    {

        int
i,j;

        for(j=2;;j++) 

        {

            for(i=2;i<=j/2;i++)

            {

                if(j%i==0)

                    break;

            }

            if(i>j/2) 

            {

                System.out.println(j);

            }

        }

    }

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public static void main(String[] args)

    {

        int
n = 0;

        String s = null;

        BufferedReader buf;

        buf=new
BufferedReader(new
InputStreamReader(System.in));

        System.out.print("请输入一个数：");

        try
{

            s=buf.readLine();

        } catch
(IOException e) {

            e.printStackTrace();

        }

        n=Integer.parseInt(s);

        int
temp = n;

        for
(int i = 2; i <= temp; i++)

        {

            if
(!isPrime(i))

            {

                continue;

            }

            while
(true)

            {

                if
(temp%i == 0)

                {

                    temp = temp/i;

                    System.out.println(i);

                }

                else
{

                    break;

                }

            }

        }

    }

    public
static boolean isPrime(int
n)

    {

        int
i;

        for
(i = 2; i <= n; i++)

        {

            if
(n%i == 0)

            {

                break;

            }

        }

        if
(i >= n)

        {

            return
true;

        }

        else
{

            return
false;

        }

    }