Instance initialization block and subclasses

Question

I'm getting confused about when the instance initialization block should run. According to Kathy Sierra's book:

Instance init blocks run every time a class instance is created

So, consider having two classes: a parent and a child, according to this question and java's documentation:

instantiating a subclass object creates only 1 object of the subclass type, but invokes the constructors of all of its superclasses.

According to the above: why does the instance initialization block located in superclasses gets called every time an object of the subclass is instantiated? it isn't like that a new object of the superclass is instantiated.

score 5 · Accepted Answer · edited Aug 14 '15 at 19:46

5

After compilation instance init blocks become part of constructors. javac simply adds the init block to each constructor, that is this:

public class Test1 {
    int x;
    int y;

    {
        x = 1;
    }

    Test1() {
        y = 1;
    }
}

Is equivalent to this:

public class Test1 {
    int x;
    int y;

    Test1() {
        x = 1;
        y = 1;
    }
}

So the init block runs when constructor runs.

edited Aug 14 '15 at 19:46

Anonymous

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answered Apr 21 '13 at 05:02

Evgeniy Dorofeev

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Actually, there is a little more to it, the instance initializer blocks get added to the constructor after any existing call to a super constructor. See http://java-blog.kbsbng.com/2016/01/instance-initializer-blocks-in-java.html for reference – kbsbng Jan 10 '16 at 09:28

score 1 · Answer 2 · answered Apr 21 '13 at 04:52

1

it isn't like that a new object of the superclass is instantiated.

Actually, it is like that.

Every instance of a subclass implicitly contains an instance of its superclass.

A superclass constructor is always invoked as the first step in any constructor (and that in turn runs any instance initializer blocks for the superclass)

answered Apr 21 '13 at 04:52

Thilo

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I know that all of the superclass constructors will be invoked, but what I understand is that regardless of invoking these constructors: only one object is being instantiated/created there, that is for the subclass. – a.u.r Apr 21 '13 at 04:55
1

You can look at it that way. But you could also say that the same object is also an instance of the superclass. And to be sure, the instance contains all the fields defined by the superclass in addition to its own. – Thilo Apr 21 '13 at 05:43

score 0 · Answer 3 · answered Mar 20 '17 at 13:26

Though it was a old post I came across this concept thought worth sharing it

Since we are talking about instance block here is how instance code flow executes in parent child relation class // Child extends Parent If we create a object for Child

1) Identification of instance members of the class from parent to child

2) execution of instance variable assignments and instance blocks only on parent class

3)execution of parent constructor

4)execution of instance variable assignments and instance blocks only on Child class

5)Execution of child constructor

score -1 · Answer 4 · answered Apr 21 '13 at 04:52

-1

Because there is always an implicit super() call(if not done explicitly) to the parent's constructor in the constructor of the child.

answered Apr 21 '13 at 04:52

shiladitya

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Instance initialization block and subclasses

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