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code-style Hide Utility Class Constructor (Checkstyle)

Utility classes should not have a public or default constructor.

Make sure that utility classes (classes that contain only static methods) do not have a public constructor.

Rationale: Instantiating utility classes does not make sense. Hence the constructors should either be private or (if you want to allow subclassing) protected. A common mistake is forgetting to hide the default constructor.

If you make the constructor protected you may want to consider the following constructor implementation technique to disallow instantiating subclasses:

public class StringUtils // not final to allow subclassing
{
    protected StringUtils() {
        throw new UnsupportedOperationException(); // prevents calls from subclass
    }

    public static int count(char c, String s) {
        // ...
    }
}