为什么我的ArrayList打印第一个对象3次？

Question

I am relatively new to Java. I was practicing an exercise where we explore the polymorphic behavior of subclass and superclass instances. This exercise builds on the concepts of abstract classes and interfaces. ArrayLists are introduced in this exercise. One of the questions requires us to create an ArrayList of WrittenWork objects called products, and add the appropriate objects to the ArrayList.

My problem is that the ArrayList keeps printing the first item 3 times and then prints the rest of the items.

Could someone explain why this keeps happening?

EDIT: I realized the mistake I was making thanks to you guys! I changed the for loop from

   for(WrittenWork w: products)
       System.out.println(w1.toString());
       System.out.println(w2.toString());
       System.out.println(w3.toString());

To

for(WrittenWork w: products)
    System.out.println(w.toString());

This Image shows how all the classes are connected

Here is the code:

import java.util.*;

public class BookStore
{
  public static void main(String[] args)
  {
   Author a1 = new Author("Malcom Gladwell");
   Author a2 = new Author("Steven Johnson");
   Author a3 = new Author("Mathias Johansson");
   Author a4 = new Author("Evan Ackerman");
   Author a5 = new Author("Erico Guizzo");
   Author a6 = new Author("Fan Shi");

   WrittenWork w1 = new Novel(a1, "What the Dog Saw and other adventures", 503);
   WrittenWork w2 = new Novel(a2, "How We Got to Now: Six Innovations That Made the Modern World", 320);
   WrittenWork w3 = new Novel(a2, "Everything Bad Is Good For you: How Today's Popular Culture is Actually Making us Smarter", 254);


   ArrayList<WrittenWork>products = new ArrayList<>();
   products.add(w1);
   products.add(w2);
   products.add(w3);

   for(WrittenWork w: products)
       System.out.println(w1.toString());
       System.out.println(w2.toString());
       System.out.println(w3.toString());

   }
 }

This is the output

Answer 1

Other than not having used brackets to correctly identify the for block, you have a logic flaw in your small piece of code.

You wrote

the ArrayList keeps printing the first item 3 times and then prints the rest of the items

This lead me to think your code doesn't really represents what you actually want to accomplish.
This is totally normal in the beginning, don't worry. It seems you'd like to print all the WrittenWork .

For that you used an ( enhanced ) for loop

for (WrittenWork w : products)

but you never taken into consideration the w local variable.

What happens in the background is that an Iterator<WrittenWork> ( JavaDoc ) is created for you, and each iteration the next element in the products list is given to you via w .

You just need to use that w variable to print all the elements of products .
The Iterator will exhaust itself and the loop will stop.

---

For your knowledge, this for-loop style

for (WrittenWork w : products) { ... }

is equivalent to

for (Iterator<WrittenWork> iterator = products.iterator(); iterator.hasNext(); ) {
    final WrittenWork w = iterator.next();
    ...
}

You can see the checked condition is iterator.hasNext(); .

Answer 2

for(WrittenWork w: products)
       System.out.println(w1.toString());
       System.out.println(w2.toString());
       System.out.println(w3.toString());

   }

In a program exection fashion it may make you feel right and it runs but the logical issue incurs that if you opt for printing all objects with segregate System.out.println methods, then why did you use for(each)-loop ?

Besides, you don't need to call toString() method called implicitly and automatically if an object reference is passed to System.out.println method

Answer 3

It should be:

  for(WrittenWork w: products)
       System.out.println(w.toString());

  }

You're already going over all the WrittenWork objects, using w1, w2, w3 nullifies the use of using for-loop in the first place.

Answer 4

You are iterating over the ArrayList but printing out w1 , w2 and w3 on every iteration. As others pointed out print w in every iteration which points to the element corresponding to the iteration.

for (WrittenWork w: products){
   System.out.println(w.toString()); 
}

Answer 5

for (WrittenWork w: products){
   System.out.println(w.toString()); 
}

Answer 6

Try this:

for (WrittenWork w: products){
   System.out.println(w.toString()); 
}