如何实现Brute Force算法并修复Nullpointerexception？

Question

我正在练习我的 java 技能，我几乎已经完成了程序，但我在代码方面遇到了一些问题。所以我遇到的代码问题是我遇到了 Nullpointexceptions 并且不知道它在哪里。

其次，我正在尝试在我的代码中实施一种强力方法，但我在有效实施它方面遇到了问题。对于该方法，我必须查看字符串是否为英文，我已经完成但基于使用暴力方法的其他代码。我可以通过多种方式使用该方法，但不知道如何在我的代码中有效地实现它。

所以我的程序执行以下操作：

实施对称密码系统。
生成一个 16 位值密钥
消息是随机生成的偶数个字符串字符。
加密和解密方法。
为此实施暴力破解攻击使用随机生成的字符串的密码系统。
而且我只能调用Main中的方法

错误：线程异常 "main" java.lang.NullPointerException 在 StudentSystem.bruteForce(StudentSystem.java:112) 在 StudentSystem.main(StudentSystem.java:38)

代码：

private static String msg;

                private static String msgE;

                private static String msgD;

                private static int key;
                //This is the String for the program. So Upper is the chracters in upper letters
                private static String upper="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
                //This is for the chracters in the lower case
                private static String lower=upper.toLowerCase();
                private static String space=" ";
                //Alphanum is upper and lower and space combined and this is what I'll be using for this project. 
                private static String alphanum=upper+lower+space;


                public static void main(String[] args){

                //TODO: You can only call methods in main method

                key = generateKey();

                msg = generateMsg();

                msgE = encryption(key,msg);

                bruteForce(msgE);

                }

                private static int generateKey() {

                //TODO: implement step a (randomly generate 16-bit key)
                    //So in Decimal a key would be 2^16-1 
                    //So i'm using random rand to generate a key which is going to be a int and in power of 2^16 -1
                    Random rand=new Random();
                    return rand.nextInt((int) (Math.pow(2, 16)-1));


                }

                private static String generateMsg() {

                //TODO: implement step b (randonly generate a string with an even number of characters)
                    //For this method i'm going to generate a string which is random 
                    //This string is going to use the above alphanum.
                StringBuilder builder =new StringBuilder();
                //The while loop states while the builder length is divisible by 2 then you can print the random string

                while(builder.length()%2!=0) {
                    //The chracter is using the random rand and alphanum length to generate the String
                    int character=(int)(Math.random()*alphanum.length());
                    //Builder append is shortering the string into something more simple. 
                    builder.append(alphanum);
                }

                return builder.toString() ;

                }

                private static String encryption(int key, String msg) {

                //TODO: implement step c (encrypt the message)
                    //To encrypt the string we're going to use the key we generated before
                    /*The String Key is going to take the key and put that into a string*/
                    //then the loop is going to go through and put the String Key to generate a key
String Key=Integer.toString(key);
for(int i=0;i<=msg.length()/2;i++) {
    Key+=Key;
}
// This will return a mesage and a key at the same time
                return (msg+key);

                }

                private static void decryption(int key, String msgE) {

                //TODO: implement step d (decryption)
                    //For decryption we're going to use the key we got before and go through the loop
                    //We're going to go through the loop and put the String into String Key
                    //Then we're going to return the String with the key.
                    String Key=Integer.toString(key);
                    for(int i=0;i<msgE.length()/2;i++) {
                        Key+=Key;
                    }
                    String msgD;
                    msgD=msgE+key;

                }

                private static void bruteForce(String msgE) {

                //TODO: implement bruteForce algorithm, you may need the above decryption(key,msgE) method
                    boolean isEnglish=msgE.matches("[a-zA-Z]+");
                    if(isEnglish) {
                    System.out.println("This string is English");
                    }else {
                        System.out.println("This String is not English at all");
                    }
                    decryption(key,msgE);
                    boolean isEnglish2=msgD.matches("[a-zA-Z]");
                    if(isEnglish2) {
                        System.out.println("Encrypted Message is English: "+msgD);
                    }
                    else {
                        System.out.println("The message is not english at all: "+msgD);
                    }
                }
        }

Answer 1

您将 class 字段与方法变量赋予相同的名称，从而混淆了它们。 NullPointerException 是因为您从未初始化 msgD，class 字段。

所以在这里，你声明一个字段msgD:

private static String msgD;

但随后在 decryption 方法中，您声明了一个具有相同名称的新局部变量：

String msgD;
msgD=msgE+key;

任何时候你把一个变量的 class 放在它的名字之前，你就是在声明一个新变量。我再说一遍：这是一个 new 局部变量，它与您之前声明的字段无关，即使它们具有相同的名称。

所以，在你运行解密方法之后，你期望 msgD 有一个值，但它没有，当你试图在这一行引用它时抛出一个 NPE :

boolean isEnglish2=msgD.matches("[a-zA-Z]");

如果删除该行

String msgD;

从 decryption 方法来看，NPE 应该消失了，但我怀疑可能还有其他潜伏的问题。

如何实现Brute Force算法并修复Nullpointerexception？

How to implement BruteForce algorithm and fix Nullpointexception?

java

null

pointers

cryptography

brute-force