小编Pau*_*aul的帖子

您好我正在尝试将日期和时间添加到JAVA中的文件名.我可以在文件中打印日期和时间,这也是我想要完成的,但是当我将toString放在FileWriter中时,我得到一个空指针.

package com.mkyong;
import java.util.*;
import java.io.*;
import java.*;
import java.util.Date;
import java.text.DateFormat;
import java.text.SimpleDateFormat;

    public class Simplex {

        private static PrintWriter outFile;

        //Main Method
        public static void main(String[] args) throws IOException {



            // Instantiate a Date object
             Date date = new Date();

             // display time and date using toString()
             outFile.println(date.toString());
             outFile.println();
            //creates the new file to be saved


            outFile = new PrintWriter(new FileWriter("simplex" + (date.toString()) + ".txt"));

我试图用一些数字预先形成一个简单的数学运算.完成后,我希望将int类型的数字更改为String.我能够获得编译前4个if语句的代码,但是第5个是在创建n5x时给我一个问题.我不允许在else语句中创建值吗？如果有的话有办法解决这个问题吗？谢谢.

                           if (n1==1){
                                int n1x = ((n1*10 +n2) + (n3*10+n4)+(n5));
                                n1s = Integer.toString(n1x);

                                outFile.println(n1s);}
                            else if (n2==2){
                                int n2x = (n2 + (n1*10 + n3)+(n4*10+ n5));
                                n2s = Integer.toString(n2x);
                                outFile.println(n2s);}
                            else if (n3==3){
                                int n3x = ((n3*10 +n2) + n1+ (n4*10+n5));
                                n3s = Integer.toString(n3x);
                                outFile.println(n3s);}
                            else if (n4 == 4){
                                int n4x =  (n4 + (n3 +n2*10)+(n5+n1*10));
                                n4s = Integer.toString(n4x);
                                outFile.println(n4s);}
                            else 
                                int n5x  = ((n5) + (n4+ n3*10) +  (n2+n1*10));
                                n5s = Integer.toString(n5x);
                                outFile.println(n5s);