查看源码即可
1. 第一步
点击ArrayList查看源码,发现其中并没有toString()方法,查看他的父类
2. 第二步
在父类AbstractList中查找,依然没有,在查看上一层父类
3. 第三步
在AbstractList的父类AbstractCollection查找,发现重写
/**
* 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
* (<tt>"[]"</tt>). Adjacent elements are separated by the characters
* <tt>", "</tt> (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<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(' ');
}
}