Parameterless constructor validation. Java

November 11, 2021

How can I check if there is a parameterless constructor in a class?

I tried:

        if (classToCheck.getConstructor() != null) {
            //Yes, there is one.
        } else {
            //No, there isn't one.
        }

But classToCheck.getConstructor() always returns true.

What am I doing wrong?

Edit.

    private boolean hasNoArgConstructor(Class classToCheck) {
    try {

        if (classToCheck.getConstructor() != null) {
            return true;
        }
    } catch (NoSuchMethodException e) {
        System.out.println("Error");
    }

    return false;
}

So, the main idea of a method is to get true if there is a constructor in a class that takes no arguments.

>Solution :

Just remember that if you dont write any explicit constructor , the compiler will give you a no args constructor implicitly .

So : class without any constructor ==> become a class with one no argument constructor after compilation.

you can use this code :

if(Arrays.stream(classToCheck.class.getConstructors()).filter(constructor -> constructor.getParameterCount() == 0).count() == 1 )
    System.out.println("exists");
else 
    System.out.println("not exists");