Is an object created first and then it's constructor executed?

Question

My question is that is

A a=new A();

here is the object created first and then the constructor method is called? Or the object is created because the constructor method is called?

If the object is required to call the constructor, that means that the object should be created before the constructor is called and if I declare the constructor to be private (just to see if it works without the constructor), then I get an error.

Now if the constructor execution creates the object, I fail to understand how that logically works, I mean that without an object how can a method of the object can execute?

Can someone explain this?

Answer 1

Memory is allocated for the object before the constructor is called, yes.

You can think of it in terms of the following steps:

1. Memory is allocated for the object.
2. Various constructors are executed (always from the top of the hierarchy down to the most specific one, e.g., Object's constructor is executed first, then whatever other super-classes you have, then the actual class's).
3. So the object already has memory for it, and it's being initialized by the constructors, not created. The constructors are simply setting up the state of the object.
4. The object is unusable until the constructors are finished executing.

Point 4 actually isn't entirely true, because you can leak a reference to the object via passing this to another method within the constructor, but that's a bit of a fringe case. It's kind of a fascinating phenomenon, because you can access things like final variables before they're initialized and retrieve two different values from them, depending on where in the execution they are.

Addressing the comments below: the constructor itself doesn't return anything. It has a void return type. How the variable actually receives the object isn't as straightforward as you might think, and it's a really good question. You can read very detailed answers about this subject on the following Stack Overflow answers, which do a much better and thorough job than I could replicate here.

• Can we have a return type for a constructor in Java?
• Why do constructors in Java not have a return type?

As a side note, as Peter Lawrey mentions in the comments, you are able to use the Unsafe API to create instances of an object without executing a constructor. Based on some other Stack Overflow discussions, however, the general opinion seems to be that anything you do with Unsafe doesn't really qualify as normal Java behavior.

Answer 2

You tagged the question with more than one language, you should pick one.

But the convention is to first allocate memory for the object and its fields, then run the constructor and then, if applicable, assign a reference to this new object to the variable a.

Some nuances exist. C#, for example, does things a little differently for value types.

1. Allocate memory for value type instance
2. Run constructor
3. Copy object, bit-by-bit, to the location a is pointing to.

This makes sure a never points to a partially-constructed instance of a value-type. It also makes sure that, if the constructor throws an exception, the memory location a points to is not left corrupted.

Answer 3

Answering for C++:

A new-expression (let's say new A()) is compiled as follows:

1 Check that the default constructor of A is accessible within the context; otherwise, signal a compilation error.

2a If class A has an overloaded allocation function A::operator new(), call it to obtain space for the object.

2b Otherwise, call the allocation function ::operator new() to obtain space for the object.

3 Call the default constructor A::A() in the space obtained from the allocation function.