小数点位数的解决

控制小数点位数(圆的面积为例)

import java.text.DecimalFormat;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner e = new Scanner(System.in);
		double r = e.nextDouble();
		double PI=3.14159265358979323;
		DecimalFormat df=new DecimalFormat("0.0000000");
		System.out.println(df.format((r*r)*PI));
	}
}

数字过大用

BigInteger bi = new BigInteger("12345678");