java - getConstructor with no parameters

Question

I can't seem to use getConstructor for constructors with no parameters.

I keep getting the following exception:

java.lang.NoSuchMethodException: classname.<init>()

Here is the code:

interface InfoInterface {
    String getClassName();
    String getMethodName();
    String getArgument();
}

class asa implements InfoInterface {
    @Override
    public String getClassName() {
        return ("jeden");
    }
    @Override
    public String getMethodName() {
        return ("metoda");
    }
    @Override
    public String getArgument() {
        return ("krzyk");
    }
}

class Jeden {
    Jeden() {
        System.out.println("konstruktor");
    }

    public void Metoda(String s) {
        System.out.println(s);
    }
}

class Start {
    public static void main(String[] argv) {
        if (argv.length == 0) {
            System.err.println("Uzycie programu: java Start nazwa_klasy nazwa_klasy2...");
            return;
        }

        try {
            for (int x = 0; x < argv.length; x++) {
                Class<?> c = Class.forName(argv[x]);
                InfoInterface d = (InfoInterface) c.newInstance();
                String klasa = d.getClassName();
                String metoda = d.getMethodName();
                String argument = d.getArgument();

                Class<?> o = Class.forName(klasa);
                // o.newInstance();

                Constructor<?> oCon = o.getConstructor();
                System.out.println("ASD");
                Class<?> p = (Class<?>) oCon.newInstance();
            }
        } catch (Exception e) {
            System.out.println(e);
        }

    }
}

o.newInstance(); prints "konstruktor" without problems.

Answer 1

The problem is clear when you read the javadoc of .getConstructor():

Returns a Constructor object that reflects the specified public constructor of the class represented by this Class object.

Emphasis mine.

In your code, the constructor is not public!

Example:

// Note: class is NOT public -- its default constructor won't be either
final class Test
{
    public static void main(final String... args)
        throws NoSuchMethodException
    {
        // throws NoSuchMethodException
        Test.class.getConstructor();
    }
}

Obligatory link to an SO answer which also gives the JLS reference. In particular, note that the default constructor has the same access modifier as the class.