Accessibility of default constructors

[Ria Mathur]

The statement is from Manish's notes -
"If the compiler gives you the default no-args constructor for the class, its accessibility is same as the class. "
How can we see accessibility of the constructor which is given by the compiler? I mean since it is not in the source file?
Thanks,
- Ria

[J Hreich]

Hey Ria,
think inner classes!!!

[Valentin Crettaz]

You can't see it in the source file but you can see it in the class file.
Compile this:

public class Test1 {
   
}

And then decompile it using javap:
javap -c -l -s Test1
You are going to get this:

Compiled from Test.java
public class Test1 extends java.lang.Object {
    public Test1();
        /*   ()V   */
}
Method Test1()
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 return
Line numbers for method Test1()
   line 1: 0

As you can see the default no-arg constructor is public.
Now do the same with the following code:

class Test2{
}

Decompile it and you'll see this:

Compiled from Test2.java
class Test2 extends java.lang.Object {
    Test2();
        /*   ()V   */
}
Method Test2()
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 return
Line numbers for method Test2()
   line 1: 0

The constructor has a default accessibility just as the class Test2.
This confirms Manish's notes.
Learn how to use javap, you'd be amazed what you could discover with it.
[ March 26, 2002: Message edited by: Valentin Crettaz ]

[Corey McGlone]

My first approach was to use reflection, but reflection will only find you the public constrcutors. You can, do this:

import java.lang.reflect.*;
public class TestExecutable 
{
   public static void main(String[] args) throws Exception
   {
      Class thisClass = new TestExecutable().getClass();
      try
      {
         Constructor defaultConstructor = thisClass.getConstructor(null);
         int modifiers = defaultConstructor.getModifiers();

         if ( Modifier.isPublic(modifiers) )
         {
            System.out.println("Default constructor is public.");
         }
      }
      catch (NoSuchMethodException e)
      {
         System.out.println("This class does not have a public default constructor.");
      }
}

This solution isn't quite what you'd like, I'm sure. If you really want to see the inserted constructor, you'll have to look at the generated byte-codes. Look at Val's example above.
Corey

[Manish Hatwalne]

Thanks Valentin, I was going to suggest javap only
It really gives you an interesting insight into java classes.
HTH,
- Manish

[Arun Pai]

Val,Corey,Manish thanks I have learnt something new here by the question posted by Ria -> Thank U .
Valentine decompiling with javap-c-|-s Test1 . Should this command be from the DOS command prompt. Is it possible from textpad. I am using Textpad & dos prompt for compiling and running codes. Is there anything better out there for a beginner like me.

[Valentin Crettaz]

javap is part of the J2SDK suite tools. it is located in $JAVA_HOME/bin and is a normal utility just like java and javac.
Moreover, TextPad and a DOS console is the best you can hope for as a beginner. That's what I would recommend to any beginner: Your favorite text editor, the J2SE and a terminal. That way you really get a good idea of how Java works. When using IDEs all the dirty work is taken care of by the tool for you and you never know how that compilation/execution stuff works. Way to go

[Manish Hatwalne]

Besides, it is possible to configure your Textpad so that you can see the result of compiling, execution etc in the textpad window itself. That way you have all your learning, and you don't have to switch to dos prompt every now and then. I am sure that you would be able to configure textpad to run Javap as well.
HTH,
- Manish