add a private constructor to hide the implicit public one（Utility classes should not have public constructors）

sonarlint提示add a private constructor to hide the implicit public one

Utility classes should not have public constructors

Utility classes, which are collections of static members, are not meant to be instantiated. Even abstract utility classes, which can be extended, should not have public constructors.
Java adds an implicit public constructor to every class which does not define at least one explicitly. Hence, at least one non-public constructor should be defined.
Noncompliant Code Example
class StringUtils { // Noncompliant

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}
Compliant Solution
class StringUtils { // Compliant

  private StringUtils() {
    throw new IllegalStateException("Utility class");
  }

  public static String concatenate(String s1, String s2) {
    return s1 + s2;
  }

}
Exceptions
When class contains public static void main(String[] args) method it is not considered as utility class and will be ignored by this rule.

意思是util类里面都是静态方法，不会去初始化这个类，所以不应该暴露一个public构造函数

解决方案：

     定义一个private构造函数