Question

Suppose I have a method like this

 public void readAndInitialize(StringReader reader, Class className)
  {
    // Here, I want to to something like this - 

   // obj = new (object of the specified className)

      obj.readAndInitialize(reader);
  }

How to do this in Java?

Answer 1

While everyone is quick to point out Class.forName("com.myorg.MyClass"); and the related newInstance() method, it is important to remember that it will only call a default constructor taking no parameters.

If you find that you need to call a particular constructor of the class, you need to use reflections to find the correct constructor, and then call it.

import java.lang.reflect.Constructor;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class ConstructorSift {
    public static void main(String... args) {
    try {
        Class<?> cArg = Class.forName(args[1]);

        Class<?> c = Class.forName(args[0]);
        Constructor[] allConstructors = c.getDeclaredConstructors();
        for (Constructor ctor : allConstructors) {
        Class<?>[] pType  = ctor.getParameterTypes();
        for (int i = 0; i < pType.length; i++) {
            if (pType[i].equals(cArg)) {
            out.format("%s%n", ctor.toGenericString());

            Type[] gpType = ctor.getGenericParameterTypes();
            for (int j = 0; j < gpType.length; j++) {
                char ch = (pType[j].equals(cArg) ? '*' : ' ');
                out.format("%7c%s[%d]: %s%n", ch,
                       "GenericParameterType", j, gpType[j]);
            }
            break;
            }
        }
        }

        // production code should handle this exception more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    }
    }
}

lists all constructors, and a tutorial is available here.

Once you have found the desired constructor, you can call it like so

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import static java.lang.System.out;

class EmailAliases {
    private Set<String> aliases;
    private EmailAliases(HashMap<String, String> h) {
    aliases = h.keySet();
    }

    public void printKeys() {
    out.format("Mail keys:%n");
    for (String k : aliases)
        out.format("  %s%n", k);
    }
}

public class RestoreAliases {

    private static Map<String, String> defaultAliases = new HashMap<String, String>();
    static {
    defaultAliases.put("Duke", "duke@i-love-java");
    defaultAliases.put("Fang", "fang@evil-jealous-twin");
    }

    public static void main(String... args) {
    try {
        Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class);
        ctor.setAccessible(true);
        EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases);
        email.printKeys();

        // production code should handle these exceptions more gracefully
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (InvocationTargetException x) {
        x.printStackTrace();
    } catch (NoSuchMethodException x) {
        x.printStackTrace();
    }
    }
}

which is again, from the tutorial about reflection.