Java小例子——穷举质数，求平方和，求质因子。

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;
}

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);
}
}
}

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;
}
}