CODEWITHSUNDEEP
javaoop
hardik
hardik

Reputation: 9329

Explain execution flow for following java programe

"Can anyone explain me execution flow for the following java code ??" Sorry for my misleading statement... my question is ...

Main b = new Main();

control goes to the A class constructer, then control directly goes to the overriden method of class Main

public void PrintFields() {
    System.out.println("x = "+x+" y = "+y);
}

Why compiler doesn't give error because x and y is not created..!! i am confused in this only.

When will x and y created in memory and when its initialization takes place ?? is the x and y created when control reaches to following statements in Main class ?

int x = 1;
int y;

Code:

class A
{
    public A() {
        PrintFields();
    }
    public void PrintFields() {}
}
class Main extends A
{
    int x = 1;
    int y;
    public Main() {
        y = -1;
        PrintFields();
    }
    public void PrintFields() {
        System.out.println("x = "+x+" y = "+y);
    }
}
class Test
{
    public static void main(String[] args) {
        Main b = new Main();
    }
}

Output:

run:
x = 0 y = 0 //why 0 0 ?
x = 1 y = -1 // why 1 -1 ?
BUILD SUCCESSFUL (total time: 0 seconds)

Thank you.

Upvotes: 0

Views: 1499

Answers (3)

Java42
Java42

Reputation: 7716

If you change:

System.out.println("x = "+x+" y = "+y);

to:

new Exception().printStackTrace(System.out); // <-- CFPMD (poor man's debugger)
System.out.println("x = "+x+" y = "+y);

This might make it easier to understand what the other answerers are telling you.

Upvotes: 0

Ted Hopp
Ted Hopp

Reputation: 234857

The flow is as follows:

  • main calls new Main().
  • the Main() constructor implicitly calls A() as the first thing.
  • A() calls PrintFields(). However, because this method is overridden, what actually gets executed is Main.PrintFields(). This prints the first line x = 0 y = 0 because the x and y fields have default values.
  • After A() completes, the Main() constructor continues to execute. It initializes x and y. (Note that the assignment part of int x = 1; is not executed until after A() completes.) Main() then calls PrintFields() which executes again and prints the (now initialized) values of x and y.

The description of how all this works can be found in the Java Language Specification (§12.5).

Upvotes: 3

Chandra Sekhar
Chandra Sekhar

Reputation: 19500

Main b = new Main();

control goes to the A class constructer, then calls the overriden method of class Main

public void PrintFields() {
    System.out.println("x = "+x+" y = "+y);
}

At this time x and y have their default values

so x=0, y=0

Now control comes to

int x = 1;
int y;
public Main() {
    y = -1;
    PrintFields();
}

and then

public void PrintFields() {
    System.out.println("x = "+x+" y = "+y);
}

at this time x =1 and y=-1

so output is 

x=1, y=-1

Upvotes: 5

Related Questions