What should the declaration of a helper class, with only static methods, be?

Question

I've got a class here, that only contains static methods, so I call it a helper class, an example:

public class FileUtils {
    public static String readFileEntry(final Path path, final String entryKey) throws IOException { }

    public static String readAndModifyFileEntry(final Path path, final String entryKey, final UnaryOperator<String> operator) throws IOException { }
}

How should I declare the class (abstract, final, static, etc.), such that it is not possible to instantiate the class? As you are not supposed to do so.
If that is not possible, then what is the best practice?

I'm using Java 8 if that is of any extra help.

@MohammadAdil This is Java, not C#, so you cannot necessarilly have the same modifier combinations. — skiwi, Mar 27 '14 at 08:04

score 3 · Accepted Answer · answered Mar 27 '14 at 08:01

3

You can declare the constructor as private and use the final keyword to prevent extensions :

public final class FileUtils {
    private FileUtils() {
    }

    public static String readFileEntry(final Path path, final String entryKey) throws IOException { }

    public static String readAndModifyFileEntry(final Path path, final String entryKey, final UnaryOperator<String> operator) throws IOException { }
}

answered Mar 27 '14 at 08:01

giorashc

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Is there any additional benefit to also declaring the methods `final`? – skiwi Mar 27 '14 at 08:02
in your case no because your class cannot be extended – giorashc Mar 27 '14 at 08:04
And - as static methods are bound at compile time - final does not do anything to static methods. – Seelenvirtuose Mar 27 '14 at 08:06
@Seelenvirtuose it prevents hiding them by subclasses but since his class is final there is no need for final – giorashc Mar 27 '14 at 08:07
1

Additionally - as I am a bit paranoid - I do a "throw new AssertionError(FileUtils.class)" in the constructor. This asserts that even with reflection, you cannot instantiate that class. – Seelenvirtuose Mar 27 '14 at 08:07
`final` is not necessary but, however, you can say the same about making the constructor `private` as creating instances is unintended but still doesn’t harm. So, making such a class `final` and adding a `private` constructor is a *convention*. And adhering to conventions has strong advantages. – Holger Mar 27 '14 at 08:42

score 3 · Answer 2 · answered Mar 27 '14 at 08:02

3

This is a common pattern:

public final class Helper {

    private Helper() {}

}

answered Mar 27 '14 at 08:02

PeterMmm

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score 3 · Answer 3 · answered Mar 27 '14 at 08:02

I'd make the class final just as a precaution, so no body extends by mistake. More importantly, I'd add a private constructor so it can't be instantiated:

public final class FileUtils {

    /** Empty private constructor, just to prohibit instantiation */
    private FileUtils() {}

    // Rest of the class...
}

score 2 · Answer 4 · answered Mar 27 '14 at 08:03

static modifier is relevant for inner classes only and does not prevent its instantiation. final modifier prevents extension of class but does not affect ability to create instances. abstract modifier indeed prevents creating instances of the class. It can be a good practice to mark pure utility classes as abstract. Other way to prevent instanitation of class is to create private constructor:

public class FileUtils {
    private FileUtils() {
        // empty constructor needed just to make it impossible to write new FileUtils()
    }
}

What should the declaration of a helper class, with only static methods, be?

4 Answers4