Guava 提供了一系列用于字符串处理的工具：

1. nullToEmpty(@Nullable String string)：如果非空，则返回给定的字符串；否则返回空字符串

   java

   public static String nullToEmpty(@Nullable String string) {
       //如果string为null则返回空字符串，否则返回给定的string
       return string == null ? "" : string;
   }

2. .isNullOrEmpty(@Nullable String string)：如果字符串为空或长度为0返回true，否则返回false

   java

   public static boolean isNullOrEmpty(@Nullable String string) {
       return string == null || string.length() == 0;
   }

3. emptyToNull(@Nullable String string)：如果非空，则返回给定的字符串；否则返回null

   java

   public static String emptyToNull(@Nullable String string) {
       //调用isNullOrEmpty方法，如果返回true则return null，否则返回原字符串
       return isNullOrEmpty(string)?null:string;
   }

生成指定字符串的字符串副本

1. padStart(String string, int minLength, char padChar)：根据传入的minLength进行补充，如果minLength小于原来字符串的长度，则直接返回原来字符串，否则在字符串开头添加string.length() - minLength个padChar字符

   java

   public static String padStart(String string, int minLength, char padChar) {
       //使用Preconditions工具类进行字符串验空处理   
       Preconditions.checkNotNull(string);
       //如果原字符串长度大于传入的新长度则直接返回原字符串
       if(string.length() >= minLength) {
           return string;
       } else { //否则
           StringBuilder sb = new StringBuilder(minLength);
           //先在字符串前面添加string.length()-minLength个padChar字符
           for(int i = string.length(); i < minLength; ++i) {
               sb.append(padChar);
           }
           //最后将原始字符串添加到尾部
           sb.append(string);
           return sb.toString();
       }
   }

2. padEnd(String string, int minLength, char padChar)：根据传入的minLength进行补充，如果minLength小于原来字符串的长度，则直接返回原来字符串，否则在字符串结尾添加 string.length() - minLength 个padChar字符

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