Confusing Java Class concept

Question

New to java and a concept is confusing me a lot.

As a c++ programmer when we declare a class we can not have a property having an object of same class like lets say we have a class name Foo as belows

class Foo {
int age;
Foo someName;
}

the above code will give error. While in java i can do it. Is there a special reason behind it? And how does it happen. Any good read will be helpful.

• Matteo: I am not a C++ expert but I'm pretty sure that you can have a class with a member of the same type in C++ too.
• asgs: Obviously you're a little confused with C++ concepts too.
• S. A. Malik: following error pops up in C++ error: field 'someName' has incomplete type compilation terminated due to -Wfatal-errors.

Answer 1

When you write Foo someName in Java, you're creating a reference to an object of type Foo. This is similar to writing Foo& someName in C++, which is allowed.

What is not allowed in C++ is for class Foo to have a member of type Foo (i.e. not Foo& or Foo*). If you think about it, this construct can't possibly make sense as it would require sizeof(Foo) to be infinitely large. This -- disallowed -- C++ construct has no direct Java equivalent.

Answer 2

In Java when you declare Foo someName, the someName is really a reference to an object of class Foo. So there is no problem to have a property referencing an object of the same type.

This is similar to how you can have Foo& someName in C++.

Answer 3

Java stores objects as references. C++ doesn't. Therein is the difference.

With Java it does not need to know how much space to reserve for the Foo object. However in C++ the compiler needs to. So the C++ has an impossible task.

• Don Roby: C++ just has to be told it's a reference. See the other answers.

Answer 4

Thats because of an important between C++ and Java : in C++ , Foo above would be an object ; in Java Foo above is just a reference - not an object. ( You will have to write Foo someref = new Foo() for creating the object.