字符串替換（）給出空指針異常

Question

哪裏Iam出錯？無論我做什麼它說String.replace空指針異常（） 我想從文件替換一個Charecter並將改變的內容寫入其他文件..即使它很容易，我無法寫它。請幫助！

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

當您運行while循環，退出條件是當str成爲null的循環結束，因此，當您試圖訪問str在下一次爲空，因此給出了一個NullPointerException。

您可以通過運行圍繞replaceAll()代碼類似while循環修復它，或者你可以將一切，直到while循環內的replaceAll()方法調用。 （注：這將詢問您是否要替換Key的每次出現）

可能的固定碼：

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; 
      } 
     } 
    } 
}