ArratList 重写toString 如何实现

List list = new ArrayList();
list.add(12);
System.out.println(list);//输出 [12]

如上，我们都知道，List、Set 继承了Collection集合，输出[12],是因为重写了toString，

但是toStringn你知道是具体在那实现的吗？

让我们先来看一下他们的继承结构：

原来是在AbstractCollection类中重写了toString方法，源码如下：

/**
     * 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 {@link String#valueOf(Object)}.
     *
     * @return a string representation of this collection
     */   
 
   public String toString() {
        Iterator 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(' ');
        }
    }