Why the result of this java program is '44'?

Question

I thought the result could be '43' because the type of q was 'poly 1'. However, the result was '44'. I couldn't understand that. please give me the answer.

class poly1 {
int a;
public poly1(){
    a = 3;
}
public void print_a(){
    System.out.print(a);
}
}
public class poly2 extends poly1{
public poly2(){
    a = 4;
}

public void print_a(){
    System.out.print(a);
}

public static void main(String[] args){
    poly2 p = new poly2();
    p.print_a();

    poly1 q = new poly2();
    q.print_a();
}

}

Answer 1

When you invoke a class' constructor, the class' super type constructor is invoke first (until there is no super types).

When you invoke

new poly2();

The poly1 constructor is invoked first (because poly1 is a super type of poly2), setting a to 3 and then the poly2 constructor is invoked, setting a to 4 which is what you see.

---

the type of q was 'poly 1'

What seems to confuse you is that in the following code

poly1 q = new poly2();

the variable q is declared as type poly1. That makes no difference in this case. What actually matters is the run time type of the object. That's determined by the new statement. In this case, the object is of dynamic type poly2.

Answer 2

When you run this program,at

    poly2 p = new poly2();

a's value is 4 as it initialised by the constructor of poly2, again at

     poly1 q = new poly2();

a's value is initalised by the constructor of poly2 to 4,so it will print 4 again giving a output of "44". Everytime u instatiate "p" and "q" ,the value of "a" is first initialised by the constructor of poly1 and again the calue of "a" gets initialised by constructor of class poly2 as poly2 extends poly1 class.

Try debugging the code and you will get to know the exact flow of the program.

According to the language specs, instance variables aren't even initialized until a super() call has been made.

These are the steps performed during the constructor step of class instance creation/

• Assign the arguments for the constructor to newly created parameter variables for this constructor invocation.

• If this constructor begins with an explicit constructor invocation (§8.8.7.1) of another constructor in the same class (using this), then evaluate the arguments and process that constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason; otherwise, continue with step 5.

• This constructor does not begin with an explicit constructor invocation of another constructor in the same class (using this). If this constructor is for a class other than Object, then this constructor will begin with an explicit or implicit invocation of a superclass constructor (using super). Evaluate the arguments and process that superclass constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, continue with step 4.

• Execute the instance initializers and instance variable initializers for this class, assigning the values of instance variable initializers to the corresponding instance variables, in the left-to-right order in which they appear textually in the source code for the class. If execution of any of these initializers results in an exception, then no further initializers are processed and this procedure completes abruptly with that same exception. Otherwise, continue with step 5.

• Execute the rest of the body of this constructor. If that execution completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, this procedure completes normally.