Java Class Object Dynamic Memory Allocation

Question

Hey Guys i am new to java programming.I tried to experiment with java classes and what i have done is that i have created an instance variable x and then y which copies that values of x. Then I define a constructor which takes the value or x as an argument. Now when i try to print the value of y it gives the value as 0 while or x it gives 5. Why the problem is happening ? When we use new keyword and the Constructor then only all the instance fields are created so i feel like after we use

/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package test;
/**
 *
 * @author Mridul
 */
public class Test {
int x;
int y=x;
Test(int a)
{
    x=a;

}
void print()
{
    System.out.println(x); 
}
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Test ob=new Test(5);
        ob.print();
        System.out.println(ob.y);


        // TODO code application logic here
    }

}

Output
5
0

When we use new keyword and the Constructor then only all the instance fields are created so i feel like after we use

Test ob=new Test(5);

Then only all the codes in the class(x,y=x) should run and it shouldn't have created the problem. Please Help

for `int` `x=a` is a kind of *copy*, any change to `a` is not reflected into `x` after the `=` (and `int x;` is equivalent to `int x = 0`) — , Feb 21 '17 at 11:52
... because instance field initialization runs _before_ constructor statements. — Seelenvirtuose, Feb 21 '17 at 11:54
see also https://stackoverflow.com/questions/24109439/what-exaclty-happens-when-a-you-call-a-constructornew-class-do-instance-initia?rq=1 — , Feb 21 '17 at 11:56
What output are you expecting? Do you expect `y` to be `10`? — MC Emperor, Feb 21 '17 at 11:57

score 0 · Answer 1 · answered Feb 21 '17 at 11:55

0

When you create a new instance of Test, this is happening (in order):

int x; // x = 0
int y=x; // y = 0
Test(int a)
{
    x=a; // x = your a value
}

=> community wiki, feel free to....

answered Feb 21 '17 at 11:55

score 0 · Answer 2 · edited May 23 '17 at 12:00

This here:

int y=x;

happens once, before your constructor is executed.

Thus the ctor changes the value of x; but y stays at its previous value. (x gets initially the default value of 0; that goes into y; and then the ctor is executed and changes x to the value of a).

And please note: this has nothing to do with "dynamic allocation". new causes three things:

memory is allocated
fields of the new object are init'ed
the specified constructor is invoked

(see here for more details). If you want y to have the same value as x; just do

public class Test final int x, y;

Test(int incoming) {
  x = incoming;
  y = incoming;
}

MC Emperor · Answer 3 · 2017-03-08T20:56:46.843

When you call a constructor using new, then Java begins to construct the newly allocated object. The values of the instance fields (in your case of field x and field y) are determined by a set of rules.

First, the instance fields are initialized. x has the default value (0); y gets the value of x, which is 0.
Then, in the constructor, x is set to the value of a.

y is still 0, because it gets its value before x is set to the value of a.

score 0 · Answer 4 · answered Feb 21 '17 at 12:23

0

new operator is for automatic memory allocation, to allocate the needed memory for an object, you must use the new operator.

answered Feb 21 '17 at 12:23

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Java Class Object Dynamic Memory Allocation

4 Answers4