Java: newInstance of class that has no default constructor

java reflection constructor new-operator instance
GermanK picture GermanK · Sep 8, 2010 · Viewed 49.9k times · Source

I'm trying to build an automatic testing framework (based on jUnit, but that's no important) for my students' homework. They will have to create constructors for some classes and also add some methods to them. Later, with the testing functions I provide, they will check if they went alright.

What I want to do is, by reflection, create a new instance of some class I want to test. The problem is that, sometimes, there is no default constructor. I don't care about that, I want to create an instance and initialize the instance variables myself. Is there any way of doing this? I'm sorry if this has been asked before, but just I couldn't find any answer.

Thanks in advance.

Answer

Jon Skeet picture Jon Skeet · Sep 8, 2010

Call Class.getConstructor() and then Constructor.newInstance() passing in the appropriate arguments. Sample code:

import java.lang.reflect.*;

public class Test {

    public Test(int x) {
        System.out.println("Constuctor called! x = " + x);
    }

    // Don't just declare "throws Exception" in real code!
    public static void main(String[] args) throws Exception {
        Class<Test> clazz = Test.class;
        Constructor<Test> ctor = clazz.getConstructor(int.class);
        Test instance = ctor.newInstance(5);           
    }
}

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