Java - static initialization
-
21-04-2021 - |
문제
I have written a piece of code :
public class Child{
int y ;
private static final int z = getZ();
static {
System.out.println("The value of z is "+z);
}
public int getX(){
System.out.println("get x");
return 10;
}
public int getY(){
Child ch = new Child();
System.out.println("get y");
ch.y = getX();
return y;
}
public static int getZ(){
System.out.println("get z");
return new Child().getY();
}
public Child(){
System.out.println("Child constructor");
}
public static void main(String...args){
Child ch = new Child();
System.out.println("the value of z in main is "+z);
}
}
And the output is :
get z
Child constructor
Child constructor
get y
get x
The value of z is 0
Child constructor
the value of z in main is 0
Can anyone please explain me why the value of z is 0 and not 10 ?
EDIT:- Thanks everyone , I got the answer to my first question . I still have a doubt , as far as I know the static blocks are executed after the class is loaded and before the first object of the class is instantiated . Well then the SOP("The value of z is "+z) should have been executed before SOP("Child constructor") ! Ain't it ?
해결책
Look at getY():
public int getY(){
Child ch = new Child();
System.out.println("get y");
ch.y = getX();
return y;
}
The first three lines are irrelevant - they don't change the value of y
in this instance, which is what gets returned.
You're creating an awful lot of pointless objects in frankly spaghetti code, called while initializing the same class that you're constructing instances of. I suggest you try to keep your code a lot simpler than this. Static initializers should be avoided where possible to start with, let alone ones that go all round the houses to do no useful work.
다른 팁
because getY()
sets ch.y
to 10, but returns the value of this.y
.
In GetY you return this.y which never gets set.
On getY() you returned the uninitialized y variable instead of the Child ch instance.
Its because inside getY() method you are constructing a new CHild object and you are assigning 10 to that instance of variable y and when you return y that will be the current instance variable y. I hope it explains .
To your second point, static fields and static blocks are initialized / executed in textual order.
Eg:
public class StaticTest()
{
static
{
System.out.println("s1 in block 1 :" + s1);
}
private static String s1 = "s1";
static
{
System.out.println("s1 in block 2 : " + s1);
}
public static void main(String args[])
{
new StaticTest();
}
}
The output of this will be -
s1 in block 1 :
s1 in block 2 : s1