CODEWITHSUNDEEP
javareflection
Mike
Mike

Reputation: 477

Java reflection getTypeParameters().length returns 0

I have a simple class

public class Pet {
    private String petName;
    private int petAge;

    public Pet() {
    }

    public Pet(String petName, int petAge) {
        this.petName = petName;
        this.petAge = petAge;
    }
}

When I'm trying to find arguments, I get two zeros. I still can't find out why. Any suggestion?

        Constructor[] constructors = Pet.class.getDeclaredConstructors();
        for (Constructor c : constructors) {
            System.out.println(c.getTypeParameters().length);
        }

Upvotes: 1

Views: 400

Answers (2)

Vishal Gajera
Vishal Gajera

Reputation: 4207

c.getParameterCount()

will return 0 and 2 as per your current code for non-typed constructor.

Upvotes: 0

Eran
Eran

Reputation: 393781

You are using the wrong method.

To the get number of arguments of each constructor, use:

System.out.println("ctor len " + c.getParameterCount());

You'll get 0 and 2, as expected.

getTypeParameters().length returns the number of generic type parameters, and none of your constructors have generic type parameters.

If, for example, you change your second constructor to:

public <Y> Pet(String petName, int petAge) {
    ....
}

getTypeParameters().length will be 1 for that constructor.

See the Javadoc:

int java.lang.reflect.Constructor.getParameterCount()

Returns the number of formal parameters (whether explicitly declared or implicitly declared or neither) for the executable represented by this object.

TypeVariable[] java.lang.reflect.Constructor.getTypeParameters()

Returns an array of TypeVariable objects that represent the type variables declared by the generic declaration represented by this GenericDeclaration object, in declaration order. Returns an array of length 0 if the underlying generic declaration declares no type variables.

Upvotes: 6

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