Java getConstructor() method

Question

import java.util.*;
import java.text.*;
import java.lang.reflect.*;

public class Test {
String name;
public Test()
{
System.out.println("In Construtor");
}
public Test(String name)
{
this.name=name;
System.out.println("In Construtor paramitarized-----"+name);
}

public int q() {
    System.out.println("working");
    return 1;

}
public static void main(String args[] ) throws Exception {
    Class c=Class.forName("Test");
    Test t=(Test)c.newInstance();
    t.q();
    Constructor cons[]=c.getConstructors();
    for(Constructor ci:cons)
    ci.newInstance();
    Constructor<?> pcon=c.getConstructor(String.class);
    pcon.newInstance();
    System.out.println(Test.class.getName());
    }
}

I was just trying grasp the concepts of Class and ClassLoader Classes. Now for this line Constructor<?> pcon=c.getConstructor(String.class); its showing that IllegalArgumentException: why?

Thank you Jon Skeet :) I changed my code with this and its working

Class c=Class.forName("Test");
        Test t=(Test)c.newInstance();
        t.q();


        Constructor<?> pcon=c.getConstructor(String.class);
        pcon.newInstance("arijit");
        System.out.println(Test.class.getName());

But When I am compiling with -Xlint ,its giving below mentioned warning: Uncheck call getConstructor(java.lang.Class....) as a member of raw type java.lang.Class

What does this mean?

Jon Skeet · Accepted Answer · 2013-01-04 20:04:34Z

8

You've misdiagnosed it. That's not the statement which is throwing an exception - this is:

ci.newInstance();

You're trying to call that for both constructors (due to the for loop) - so it'll be fine when it calls the parameterless constructor, but not when it calls the one with a String parameter. You'd have to change that call to pass in the right number of arguments depending on which constructor ci refers to at the time.

You have the same problem later:

pcon.newInstance();

This time you know there's a String parameter (you've just asked for a constructor with a string parameter) so you absolutely know that you should provide a String argument, e.g.

pcon.newInstance("foo");

Additionally, you should try to work out why you thought it was the getConstructor() call which was failing. The exception should have given you a stack trace with the right line on it... it's important to be able to diagnose problems accurately, so it's worth looking at what went wrong this time.

edited Jan 4, 2013 at 20:04

answered Jan 4, 2013 at 19:58

Jon Skeet

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7 Comments

Ted Hopp Over a year ago

The problem is that he's calling ci.newInstace() with every constructor.

Jon Skeet Over a year ago

@TedHopp: Indeed. Right statement, right reason - just missed the fact that it might already have worked once before the failure :)

Arijit Saha Over a year ago

Thank you very much. I changed my code like this and its working now. Class c=Class.forName("Test"); Test t=(Test)c.newInstance(); t.q(); Constructor<?> pcon=c.getConstructor(String.class); pcon.newInstance("arijit"); System.out.println(Test.class.getName());

Arijit Saha Over a year ago

@Jon Skeet But When I am compiling with -Xlint ,its giving below mentioned warning: Uncheck call getConstructor(java.lang.Class....) as a member of raw type java.lang.Class What does this mean?

Jon Skeet Over a year ago

Your c variable is just of type Class, which is a raw type. Use Class<?> and I suspect the error will go away.

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