Question

The output produced from this code is not making sense, perhaps due to my lack of understanding. Correct me if I am wrong

import java.util.*;
class DemoA
{
  public DemoA(){
    System.out.println("DemoA object created");
  }
  public String methodA() {
    return "methodA in DemoA";
  }
}
class DemoB extends DemoA
{
  public DemoB(){
    super();
    System.out.println("DemoB object created");
  }
  public String methodA() {
   return "methodA in subclass (DemoB)";
  }
}
public class ExamQ1b
{
  public static void main(String[] args) {
    ArrayList<DemoA> aList = new ArrayList<DemoA>();
    aList.add(new DemoA());
    aList.add(new DemoB());
    for (DemoA obj: aList)
      System.out.println(obj.methodA());
  }
}

The output is

DemoA object created
DemoA object created
DemoB object created
methodA in DemoA
methodA in subclass (DemoB)

At first I didn’t understand how the output came about, but then i used the debugging feature and found why it is behaving such a way and discovered something rather confusing (not amazing).

Why is that these lines of code are producing the output?

aList.add(new DemoA());
    aList.add(new DemoB());

the output from the above lines are these, but in my thinking these should just add to the list not produce any output, what am I missing here?

DemoA object created
DemoA object created
DemoB object created

Answer 1

In DemoBs constructor,

  public DemoB(){
    super();
    System.out.println("DemoB object created");
  }

you call super() which calls the constructor for DemoA. Since both constructors have println statements, these two lines will get printed when creating a DemoB object.

DemoA object created

DemoB object created

These lines are printed when you create the object because System.out.println("DemoB object created"); is inside the constructor method that is called when you create the object.