在Java中实例化对象时，究竟发生了什么？

Question

I know that when creating an object of a class the constructor builds that object. Say I had these two class:

class Vehicle {
    public int a = func();

    public int func() { 
        System.out.println("9"); 
        return 9;
    }
}

class Car extends Vehicle  {

    public static void main(String[] args) {
        Car c = new Car();
    }
} 

The output of this project is "9". But why does that happen? What exactly happens when the Car constructor is called? I know that there is some type of default constructor but I am not sure how it exactly works.

Can anyone explain me the object construction with the above example?

Answer 1

The compiler provides a default constructor for Car which is effectively:

Car() {
    super();
}

And likewise Vehicle as a default constructor of:

Vehicle() {
    super();
}

Fields are initialized as part of initialization, after the call to the superclass constructor. So it's as if the Vehicle class was actually written like this:

class Vehicle {
    public int a;

    Vehicle() {
        super();
        a = func();
    }

    public int func() { 
        System.out.println("9"); 
        return 9;
    }
}

Now does that makes sense?

See section 15.9.4 of the Java language specification for a much more detailed description.

Answer 2

When the Car constructor is called a default call to it's super constroctor is made by the compiler which then intializes all the Super class's instance fields . during the initialization of your a field as it invokes func() method which has a sysout thus it prints 9.

public Car() {
super();// this invokes parent class's default cons
}

public Vehical() {
super();// invokes Object constructor then initializes all the instance variables of class vehical
}

Answer 3

1. The JVM is informed that a Car must be constructed.
2. The JVM determines that it is a Vehicle and therefore triggers a Vehicle construction.
3. The JVM understands that the construction of a Vehicle requires the initialisation of the int a instance variable.
4. The int a variable must be initialised to be the result of the call to func so func is called.
5. func has been called so the code prints "9" .

Answer 4

I think you are misunderstanding the meaning of Car c = new Car(); .

That statement creates an object reference, and that object's reference is c . The statement new Car(); creates an object and the reference of that object is sent stored in c .

Answer 5

When an object is created it initiates all instance variables present in that object.(to its variable's default value if no assignment is given).

If a default constructor of a particular class( Car ) is called, a call to the default constructor of its super class( Vehicle ) is made first. And while creating Vehicle object its instant variable "a" is assigned to function func() and it ll be executed, Hence printing 9.