//字符轉換成ASCII 碼數(shù)值
 char charA = 'a';
 int intA = charA; //char 強轉為int 即得到對應的ASCII 碼值，'a'的值為97

//ASCII 碼值轉成char
int intA = 97;//97 對應的ASCII 碼'a'
char charA = (char) intA; //int 值強轉為char 即得到對應的ASCII 字符，即'a'

 /**
   * 加密
   * @param input 數(shù)據(jù)源（需要加密的數(shù)據(jù)）
   * @param key 秘鑰，即偏移量
   * @return 返回加密后的數(shù)據(jù)
   */
  public static String encrypt(String input, int key) {
    //得到字符串里的每一個字符
    char[] array = input.toCharArray();

    for (int i = 0; i < array.length; ++i) {
      //字符轉換成ASCII 碼值
      int ascii = array[i];
      //字符偏移，例如a->b
      ascii = ascii + key;
      //ASCII 碼值轉換為char
      char newChar = (char) ascii;
      //替換原有字符
      array[i] = newChar;

      //以上4 行代碼可以簡寫為一行
      //array[i] = (char) (array[i] + key);
    }

    //字符數(shù)組轉換成String
    return new String(array);
  }

  /**
   * 解密
   * @param input 數(shù)據(jù)源（被加密后的數(shù)據(jù)）
   * @param key 秘鑰，即偏移量
   * @return 返回解密后的數(shù)據(jù)
   */
  public static String decrypt(String input, int key) {
    //得到字符串里的每一個字符
    char[] array = input.toCharArray();
    for (int i = 0; i < array.length; ++i) {
      //字符轉換成ASCII 碼值
      int ascii = array[i];
      //恢復字符偏移，例如b->a
      ascii = ascii - key;
      //ASCII 碼值轉換為char
      char newChar = (char) ascii;
      //替換原有字符
      array[i] = newChar;

      //以上4 行代碼可以簡寫為一行
      //array[i] = (char) (array[i] - key);
    }

    //字符數(shù)組轉換成String
    return new String(array);
  }

/**
 * 頻率分析法破解凱撒密碼
 */
public class FrequencyAnalysis {
  //英文里出現(xiàn)次數(shù)最多的字符
  private static final char MAGIC_CHAR = 'e';
  //破解生成的最大文件數(shù)
  private static final int DE_MAX_FILE = 4;

  public static void main(String[] args) throws Exception {
    //測試1，統(tǒng)計字符個數(shù)
    //printCharCount("article1_en.txt");

    //加密文件
    //int key = 3;
    //encryptFile("article1.txt", "article1_en.txt", key);

    //讀取加密后的文件
    String artile = file2String("article1_en.txt");
    //解密（會生成多個備選文件）
    decryptCaesarCode(artile, "article1_de.txt");
  }

  public static void printCharCount(String path) throws IOException{
    String data = file2String(path);
    List<Entry<Character, Integer>> mapList = getMaxCountChar(data);
    for (Entry<Character, Integer> entry : mapList) {
      //輸出前幾位的統(tǒng)計信息
      System.out.println("字符'" + entry.getKey() + "'出現(xiàn)" + entry.getValue() + "次");
    }
  }

  public static void encryptFile(String srcFile, String destFile, int key) throws IOException {
    String artile = file2String(srcFile);
    //加密文件
    String encryptData = MyEncrypt.encrypt(artile, key);
    //保存加密后的文件
    string2File(encryptData, destFile);
  }

  /**
   * 破解凱撒密碼
   * @param input 數(shù)據(jù)源
   * @return 返回解密后的數(shù)據(jù)
   */
  public static void decryptCaesarCode(String input, String destPath) {
    int deCount = 0;//當前解密生成的備選文件數(shù)
    //獲取出現(xiàn)頻率最高的字符信息（出現(xiàn)次數(shù)越多越靠前）
    List<Entry<Character, Integer>> mapList = getMaxCountChar(input);
    for (Entry<Character, Integer> entry : mapList) {
      //限制解密文件備選數(shù)
      if (deCount >= DE_MAX_FILE) {
        break;
      }

      //輸出前幾位的統(tǒng)計信息
      System.out.println("字符'" + entry.getKey() + "'出現(xiàn)" + entry.getValue() + "次");

      ++deCount;
      //出現(xiàn)次數(shù)最高的字符跟MAGIC_CHAR的偏移量即為秘鑰
      int key = entry.getKey() - MAGIC_CHAR;
      System.out.println("猜測key = " + key + "， 解密生成第" + deCount + "個備選文件" + "\n");
      String decrypt = MyEncrypt.decrypt(input, key);

      String fileName = "de_" + deCount + destPath;
      string2File(decrypt, fileName);
    }
  }

  //統(tǒng)計String里出現(xiàn)最多的字符
  public static List<Entry<Character, Integer>> getMaxCountChar(String data) {
    Map<Character, Integer> map = new HashMap<Character, Integer>();
    char[] array = data.toCharArray();
    for (char c : array) {
      if(!map.containsKey(c)) {
        map.put(c, 1);
      }else{
        Integer count = map.get(c);
        map.put(c, count + 1);
      }
    }

    //輸出統(tǒng)計信息
    /*for (Entry<Character, Integer> entry : map.entrySet()) {
      System.out.println(entry.getKey() + "出現(xiàn)" + entry.getValue() + "次");
    }*/

    //獲取獲取最大值
    int maxCount = 0;
    for (Entry<Character, Integer> entry : map.entrySet()) {
      //不統(tǒng)計空格
      if (/*entry.getKey() != ' ' && */entry.getValue() > maxCount) { 
        maxCount = entry.getValue();
      }
    }

    //map轉換成list便于排序
    List<Entry<Character, Integer>> mapList = new ArrayList<Map.Entry<Character,Integer>>(map.entrySet());
    //根據(jù)字符出現(xiàn)次數(shù)排序
    Collections.sort(mapList, new Comparator<Entry<Character, Integer>>(){
      @Override
      public int compare(Entry<Character, Integer> o1,
          Entry<Character, Integer> o2) {
        return o2.getValue().compareTo(o1.getValue());
      }
    });
    return mapList;
  }

  public static String file2String(String path) throws IOException {
    FileReader reader = new FileReader(new File(path));
    char[] buffer = new char[1024];
    int len = -1;
    StringBuffer sb = new StringBuffer();
    while ((len = reader.read(buffer)) != -1) {
      sb.append(buffer, 0, len);
    }
    return sb.toString();
  }

  public static void string2File(String data, String path){
    FileWriter writer = null;
    try {
      writer = new FileWriter(new File(path));
      writer.write(data);
    } catch (Exception e) {
      e.printStackTrace();
    }finally {
      if (writer != null) {
        try {
          writer.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }

  }
}

 int i = 97;
 String bit = Integer.toBinaryString(i);
 //輸出：97 對應的二進制數(shù)據(jù)為： 1100001
 System.out.println(i + "對應的二進制數(shù)據(jù)為： " + bit);

//byte 的取值范圍：-128 到127
System.out.println(Byte.MIN_VALUE + "到" + Byte.MAX_VALUE);

String data = "1234abcd";
byte[] bytes = data.getBytes();//內容為：49 50 51 52 97 98 99 100