Java OCA OCP Practice Question 1146

Question

Given the following class definition: 

class MyClass { 
  protected int i; 
  MyClass (int i)  {    this.i = i;     } 
  
} 
// 1  : Insert code here

Which of the following would be a valid class that can be inserted at // 1 ?

Select 2 options

  • A. class MySubClass {}
  • B. class MySubClass extends MyClass {}
  • C. class MySubClass extends MyClass { MySubClass () { System.out.println ("i = " + i); } }
  • D. class MySubClass { MySubClass () {} } 


Correct Options are  : A D

Note

Since class MySubClass does not have any constructor, the compiler will try to insert the default constructor, which will look like this: 

MySubClass (){  
    super();  //Notice that it is trying to call the no args constructor of the super class, MyClass . 
}

Since MyClass doesn't have any no-args constructor, the above code will fail to compile.

Notice that class MyClass does not define a no-argument constructor.

Also note that the class MySubClass does not define a constructor.

Thus, class MySubClass relies on the default constructor MySubClass().

Class MySubClass's default constructor looks like this: public MySubClass() {}

However, Constructors implicitly (if an explicit call to the superclass's constructor is not present) call their superclass's constructor super().

So, class MySubClass's default constructor actually looks like this: 

public MySubClass (){ 
  super (); 
}

Now, since class MyClass does not define a no-argument constructor the above code will not compile.

However, class MySubClass would be correct if changed to: 

class MySubClass extends MyClass { 
  MySubClass (){ 
    super (1); // pass it any integer 
   } 
  // or 
  MySubClass (int number){ 
    super (number); 
   } 
}

You could also add a no-argument constructor to class MyClass and leave class MySubClass as is.




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