我不明白的Java问题

Question

i am sorry that i might seem dumb but im new java as i am studying it in high school:

public class One
{
    public void display()
    {
        System.out.print ("One");
    }
}

public class Two extends One
{
    public void display ()
    {
        System.out.print("Two");
    }
}

What is printed after the following code segment is executed?

 One object1= new Two(); object1.display();

1. OneTwo

2. TwoOne

3. Two

4. One

5. No output

Can anyone please help me with answering the question above, i have read a lot about objects and classes and how they operate and how subclasses work but i cannot figure this question, plx help.

Edit: This is an AP computer Science A question i dont really have a tutor/teacher to reach out to most of java was self-studied for me, so i would consider myself as a beginner to java but i got a lot covered, thanks for the help!!

Answer 1

The good answer is 3. "Two".

To understand why, you have to understand polymorphism and overriding: In your case you have two classes:

+------------------+
|        Two       |
+------------------+
          |
        extend
          |
          v
+------------------+
|        One       |
+------------------+

All public and protected field from One are visible and overridable by Two. In your case, One.display() is overrided by Two.display() .

If the code followed the Java conventions, there should be the annotation @Override on top of the Two.display() method to explicit this behavior to other developers and to the Compiler who's check if the method already exists in the parent object.

I hope I have been helpful.

Answer 2

This problem actually common problem around high school studies. But you need to just simple think.

The Object class is superclass all of the classes in java. You think something class withoud any extend operation. And again imagine that, this something class has a method called method1. (void or not void. Not important.) When you create an instance in main method, and, when you try to call method1 from this instance, if you use any IDE for java, you will see something methods called notify(), notifyAll(), toString() and etc. This methods are methods of Object classes.

Beside, when you write a code like this
Object instance = new YourClass();
and after then, when you write your ide like instance. you can see Object methods.

This is Polymorphism