類ArrayList的toString()方法的過載

List list = new ArrayList;
System.out.println(list.toString());

//ArrayList類
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

沒有toString()方法，那麼就找其父類

//AbstractList類  
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>
 

沒有toString()方法，那麼就找其父類
 

//AbstractCollection
public abstract class AbstractCollection<E> implements Collection<E>

有了toString()方法，實際上list.toString()呼叫的就是下面這個方法
  
注意：這個是在抽象類中定義的非抽象方法

  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(' ');
        }