Question

我哪里错了？无所谓我做什么它说String.replace（）上的空指针异常我想从文件中替换Charecter并将更改的内容写入其他文件..即使它很容易我也无法写它。请帮助！

package tinku;
/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.LineNumberReader;
import java.util.Scanner;

/**
 *
 * @author gajasurve
 */
public class Tinku {
    static int i;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws FileNotFoundException, IOException {
        // TODO code application logic here
        String str;

        final String Y;
        String Key;
        FileOutputStream fos = new FileOutputStream("/home/gajasurve/Desktop/done2.txt");
        DataOutputStream output = new DataOutputStream(fos);
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("\n Enter Complete File Path with Extension  (ex: /home/gajasurve/Desktop/info.txt)  : ");
        String Source;
        Source=r.readLine();
        System.out.print("Enter Key Element to Find In the File ");
        Key=r.readLine();   
        File f= new File(Source);   
        LineNumberReader lr= new LineNumberReader(new FileReader(f));

        while((str=lr.readLine())!=null)
        {
            if(str.contains(Key))
            {
                i=lr.getLineNumber();
                System.out.print("The Entered Key Element Can B Found In " + i + "   Line");
            } 
        }

        System.out.println("Do u wish to change the Key Element? Y|N");
        String Dec= r.readLine();
        switch (Dec) {
            case "Y" :     
                System.out.print("Enter New Key Element to Replace");
                String d2;
                d2 = r.readLine();
                str= str.replaceAll(Key, d2);  //NPE here
                System.out.println(str);
                break;
        }
    }
}

Answer 1

当您运行while循环时，退出条件是当str变为null时循环结束，因此当您下次尝试访问str时它为空并且所以给出一个NullPointerException。

您可以通过在while代码周围运行类似的replaceAll()循环来解决此问题，或者您可以移动所有内容，直到replaceAll()循环内的while方法调用。（注意：这将询问您是否要在每次出现时替换Key

可能的固定代码：

package tinku;

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

import java.io.*;

/**
 *
 * @author gajasurve
 */
public class Tinku {
    static int i;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws FileNotFoundException, IOException {
        // TODO code application logic here
        String str;

        final String Y;
        String Key;
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        System.out.println("\n Enter Complete File Path with Extension (ex: /home/gajasurve/Desktop/info.txt)  : ");
        String Source;
        Source = br.readLine();

        File f= new File(Source);
        LineNumberReader lr= new LineNumberReader(new FileReader(f));

        System.out.print("Enter Key Element to Find In the File : ");
        Key = br.readLine();

        while((str = lr.readLine()) != null) {
            if(str.contains(Key)) {
                i = lr.getLineNumber();
                System.out.println("The Entered Key Element Can B Found In " + i + " Line");
            } 
        }

        lr.close();

        System.out.println("Do u wish to change the Key Element? Y|N");
        String Dec= br.readLine();
        switch(Dec) {
            case "Y" : {
                System.out.print("Enter String to replace Key with : ");
                String d2;
                d2 = br.readLine();
                lr= new LineNumberReader(new FileReader(f));
                String outputFile = "/home/gajasurve/Desktop/done2.txt";
                PrintWriter pw = new PrintWriter(new FileWriter(outputFile));

                while((str = lr.readLine()) != null) {
                    if(str.contains(Key)) {
                        pw.println(str.replaceAll(Key, d2));
                    } else {
                        pw.println(str);
                    }
                }
                pw.close();
                break;
            }
        }
    }
}

String Replace（）给出空指针异常

1 个答案: