您不需要调试器就能知道发生了什么.

System.out是PrintStream类型.

PrintStream.println(Object)的javadoc说：

Prints an Object and then terminate the line. This method calls at first String.valueOf(x) to get the printed object’s string value, then behaves as though it invokes print(String) and then println().

if the argument is null, then a string equal to “null”; otherwise, the value of obj.toString() is returned.

Returns a string representation of the object. In general, the toString method returns a string that “textually represents” this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@’, and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

getClass().getName() + '@' + Integer.toHexString(hashCode())