for迴圈中的String 不能直接被判斷為 null 模式，因為 判斷為 null模式會導致程式報錯。判斷寫出空指標異常報錯。應該的方式是 加一個判斷。把程式碼判斷的語句直接寫到if語句中。

我的錯誤程式碼是：
package com.guang.DataFrom;

import java.io.*;
import java.util.Scanner;

public class DataFrom {
public static void main(String[] args) throws IOException{
File f = new File(“D:\data.txt”);
f.createNewFile();
Scanner sc = new Scanner(

    for(int a = 0 ;a < 3;a++){
        System.out.println("請輸入第"+ (a+1) +"個驗證碼：");
        String str = sc.next();
        strs[a] = str;
    }

    BufferedWriter bo = new BufferedWriter(new FileWriter(f));

    for(int a = 0; a < 3; a++){
        bo.write(strs[a]);
        bo.write(System.lineSeparator());
    }

    bo.flush();
    bo.close();

    BufferedReader br = new BufferedReader(new FileReader(f));

    System.out.println("請輸入登入驗證碼：");
    String yan = sc.next();
    String line;
    while( true ){
        if( br.readLine().equals(yan)){
        	line = br.readLine();
            System.out.println("驗證碼存在");
            break;
        }else if( line == null ){
            System.out.println("驗證碼不存在");
            break;
        }
        }
    br.close();

    }
}
 

我的正確程式碼是：

import java.io.*;
import java.util.Scanner;

public class DataFrom {
public static void main(String[] args) throws IOException{
File f = new File(“D:\data.txt”);
f.createNewFile();
Scanner sc = new Scanner(System.in);
String[] strs = new String[3];

    for(int a = 0 ;a < 3;a++){
        System.out.println("請輸入第"+ (a+1) +"個驗證碼：");
        String str = sc.next();
        strs[a] = str;
    }

    BufferedWriter bo = new BufferedWriter(new FileWriter(f));

    for(int a = 0; a < 3; a++){
        bo.write(strs[a]);
        bo.write(System.lineSeparator());
    }

    bo.flush();
    bo.close();

    BufferedReader br = new BufferedReader(new FileReader(f));

    System.out.println("請輸入登入驗證碼：");
    String yan = sc.next();

    while( true ){
        if( br.readLine().equals(yan)){
            System.out.println("驗證碼存在");
            break;
        }else if( br.readLine() != null ){
            System.out.println("驗證碼不存在");
            break;
        }
        }
    br.close();

    }
}
 

String語句不能作為null判斷 不然會空指標異常。