LinkedList类的（此）重写tostring方法如何工作？

Question

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?

Answer 1

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 .)

Answer 2

Here is the source code for LinkedList:

http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/LinkedList.java

Which (after a few steps) ends up implementing AbstractCollection

http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/AbstractCollection.java

And as you can see the source code for toString is here:

    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";

        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }

Now we need to find out what StringBuilder does when adding e using .append()

Which is found in the StringBuilder source

http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/StringBuilder.java

    @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

And now the question is, what does String.valueOf do?

Here is the source:

http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/String.java

    public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

So the answer to your question is: Yes, it would have called Integer 's toString method.

Answer 3

Does object class uses the end class's toString() method to print

No, the println method calls the list objects toString() method.

The LinkedList has a toString() implementation that calls the toString() method of all the objects in the list to build the [item1, item2, ...] string.

Which of course means that your toString() method is called by LinkedList , not by Object class.

If I would have stored some integers in a linked list, would Integer class toString() method would have been called?

Yes.