Java小例子——穷举质数，求平方和，求质因子。

求平方和

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;
	}

 穷举质数

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);
			}
		}
	}

 求质因子

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;
		}
	}