How does (this) overriding tostring method for LinkedList class work?

user11921851 :

I have created a class Test storing class Pair objects in linkedlist data structure defined in the library . To print the Pair class objects I have overridden method in pair class and it works as shown below -

import java.util.LinkedList;

class Test{
    static LinkedList<Pair> list=new LinkedList<Pair>();
    public static void main(String[] args){
        list.add(new Pair(31,78));
        list.add(new Pair(89,67));
        list.add(new Pair(90,43));

        System.out.println(list);
    }
}

class Pair{
    int x;
    int y;
    Pair(int m, int n){
        x=m;
        y=n;
    }
    @Override
    public String toString(){
        return "{ "+ this.x + ", " +this.y+" }";
    }
}

The output -

[{ 2, 4 }, { 6, 3 }, { 12, 6 }]

My doubt is regarding the toString() method-

• What happens internally when I print a Linked List ?
• If I would have stored some integers in a linked list, would Integer class toString() method would have been called?

Jon Skeet :

The implementation of LinkedList<E>.toString() is inherited from AbstractCollection<E>; it's documented like this:

Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

So String.valueOf is called on each element - which in turn will call Object.toString() for any non-null element. In your case, you've overridden that in Pair, so that's what's called.

If you add some logging in Pair.toString, like this:

public String toString(){
    String ret = "{ "+ this.x + ", " +this.y+" }";
    System.out.println("toString called: returning " + ret;
    return ret;
}

... you can see it being called (or you could use a debugger). And yes, if the list contained integers, the list's string representation would be "[1, 2, 3, 4, 5]" or whatever. (It's possible that String.valueOf is optimized to handle numeric types directly, but logically it would call the toString() override in Integer.)