two constructors call each other..

[Murgan Sub]

What happens when two constructors call each other?
Can you get an infinite loop out of the process?

public class ConsRecurs {
public ConsRecurs(int i) {
this(i*1.0);
}
public ConsRecurs(double d) {
this((int)d);
}
public static void main(String[] a) {
System.out.println("About to construct...");
new ConsRecurs(Math.PI);
System.out.println("Hey, I'm still alive!");
}
}

[Brian Lugo]

You will get a stack overflow error!
This is a good example.
When you have multiple constructors and if there is a situation where each constructor calls another constructor through "this()" method call, then there needs to be atleast one constructor which does not have "this()" method call on its first line. It can have super or nothing.
Brian
[ February 15, 2002: Message edited by: Brian Lugo ]

[Dave Vick]

Murgan
From the JLS section 8.8.5:

It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this.

Both JBuilder and JDeveloper caught it when I tried it.
Hope that answers it for you.

[Brian Lugo]

I don't think you get a compile time error, for this example, with existing compilers. I believe this specification has not been implemented in the existing compilers. Feel free to correct me if I am wrong.
I wrote this program a while back:

class BaseConstChain {
BaseConstChain(int i) {System.out.println("In BaseConstChain int constructor i = " + i);}

// A default base class constructor needs to be put in the base class
// as the compiler will implicity put a call to it in the derived class constructor
// if no call to super(int) constructor is called
BaseConstChain() {System.out.println("In explicit default BaseConstChain constructor");}
}
public class TestConstChain extends BaseConstChain
{
TestConstChain(int i) {
// Not possible both super and this.
// In this case it complains that this must
// be the first line
//super(i);
this("" + i) ;
// Not possible - Either this or super not both
// in the same constructor
//super(i);
System.out.println("In TestConstChain int constructor i = " + i);
}

TestConstChain(String s){
// Ooohhh Don't even think about doing this
// This results in java.lang.StackOverFlowError
// It is absolutely prohibited
 this(Integer.parseInt(s));
// Compiler puts a implicit super call over here.
// This example also proves that atleast one 
// derived class constructor should not have
// this on the first line
 System.out.println("In TestConstChain string constructor, String = " + s);}

public static void main(String[] args) {
// Illegal to do this as there is no default
// constructor defined in the derived class explicitly.
// Compiler won't put the default constructor
//TestConstChain testDerived = new TestConstChain();
TestConstChain testDerived = new TestConstChain(5);
}
}

Compile it and run it . The IDE that I was using froze, so be careful. It might be worth executing it from command line.
Brian
[ February 15, 2002: Message edited by: Brian Lugo ]
[ February 15, 2002: Message edited by: Brian Lugo ]

[Roy Ben Ami]

actually you are right brian.
i use the jdk 1.4 and both exmamples compile but will get stuck at runtime.

[keerthidhar dongre]

When I tried to run in Jbuilder (version 3) it said "cyclic 'this' constructor calls involving constructor TestConstChain(int) "

[Valentin Crettaz]

What the JLS has to say about recursive constructor invocation:
15.9.6 Example: Evaluation Order and Out-of-Memory Detection

[Brian Lugo]

Originally posted by keerthi dongre:
When I tried to run in Jbuilder (version 3) it said "cyclic 'this' constructor calls involving constructor TestConstChain(int) "

My guess is JBuilder IDE has a check in it or it is not using SUN's java compiler.