Location of toString() Method of java.util.LinkedList

Question

I am studying LinkedLists and wanted to check Java's implementation.

However, I have some questions.

I saw that I can print the contents of java.util.LinkedList.

Like this:

LinkedList ll = new LinkedList<Integer>();

ll.add(1);
ll.add(2);

System.out.println(ll);

// prints "[1, 2]"

However, I can't find the toString() method of LinkedList.

How does Java do that? How can it trigger the toString() method of every data type it has?

Answer 1

The JavaDoc tells you which method is implement in whch parent class, for example the ones in AbstractCollection

Methods declared in class java.util.AbstractCollection
containsAll, isEmpty, removeAll, retainAll, toString

---

The way is LinkedList > AbstractSequentialList > AbstractList > AbstractCollection

public class LinkedList<E> extends AbstractSequentialList<E> 
                           implements List<E>, Deque<E>, Cloneable, java.io.Serializable

>> public abstract class AbstractSequentialList<E> extends AbstractList<E> {

>>> public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {

---

You ends up with AbstractCollection.toString that iterates over each item to call their toString and join them with a comma, the whole enclosed in brackets

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(' ');
    }
}

Answer 2

LinkedList inherits its implementation of toString() from java.util.AbstractCollection , which simply calls toString on the objects in the collection. (All objects have it from Object , so it can just call it.)