Creating class instance based on class type?

Question

I am using JAVA7. I have below class.

public class SomeClass<B extends Base, L extends Last>{
   Class<L> last;
   Class<B> base;

   public SomeClass(Class<L> last, Class<B> base){
       this.last=last;
       this.base=base;
   }
}

here i need to create a method which will accept class type and create its instance and return. how can i do that?

public returnclassinstance createInstance(Class class)

I would call this method by passing either last/base. This method has to create the instance of the passed class type and return the same. Is it possible?

Thanks!

Answer 1

To create new instance based on java Class type, you should use something like this:

public <T> T createInstance(Class<T> clazz) throws InstantiationException, IllegalAccessException {
        return clazz.newInstance();
    }

Note that T should have public default constructor. If you want to use other that default construtors to create new instance of Class, then you should look into reflection (e.g. clazz.getDeclaredConstructors()).

EDIT: using constructor with arguments

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Test {
    public Test(String arg) {
        System.err.println("I'm in consturctor with one String arg: " + arg);
    }

    public static <T extends Test> T createInstance(Class<T> clazz) throws InstantiationException,
            IllegalAccessException, IllegalArgumentException, InvocationTargetException {
        Constructor<T>[] constructors = (Constructor<T>[]) clazz.getDeclaredConstructors();
        Constructor<T> constructor = constructors[0];
        return constructor.newInstance("ARG");
    }

    public static void main(String[] args) throws Exception {
        createInstance(Test.class);
    }
}

After running this code you should get on output:

I'm in consturctor with one String arg: ARG

But note how ugly this code is - using reflections in Java for such object creation is in my opinion the last option (if we don't have any other solutions).