Object of the class as instance variable inside the class [duplicate]

Question

Possible Duplicate:
How can a class have a member of its own type, isnt this infinite recursion?

The Code:

public class Test2{
    private Test2 subject = new Test2(); //Create Test2 object in Test2
    private int num;
}

The Questions:

1. Why does Java permit the above code to be executed, but C++ doesn't?

2. Does the code above create infinite number of objects? Since Test2 itself contains a Test2 object which again contains a Test2 object which itself has a Test2 object and so on.

Answer 1

Re question 2 - if you run this code, you get a StackOverflowException => Yes it creates an inifinite number of objects (well it tries...)

public class Test2 {

    private Test2 subject = new Test2(); //Create Test2 object in Test2

    public static void main(String[] args) throws Exception {
        Test2 t = new Test2();
    }
}

Answer 2

Why does Java permit the above code to be executed but C++ doesn't?

Since 2011, C++ also allows class members to be initalised in their declarations.

However, it wouldn't allow this case: you can only instantiate complete types, and a class type is incomplete within the class definition, so it would have to be initialised in the constructor, or by a call to a function:

class Test;
Test * make_test();

class Test {
    // Test is incomplete, so "new Test" is not possible here
    Test * test = make_test();
};

// Test is complete, so we can instantiate it here
Test * make_test() {return new Test;}

Java doesn't have a concept of incomplete types, so the class can be instantiated anywhere you're allowed to instantiate any class.

Does the code above create infinite objects?

Yes, trying to instantiate such a class would throw your program into a recursive death spiral.