Lisi list =new ArrayList();
.......
syso(list);大家都知道List接口中并没有定义toString()方法，应该是无法多态使用子类中的ArrayList中的toString()方法的，但是结果却是用到了，这是怎么回事呢？
参考：java语言规范 9.2 Interface Members
If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface. It is a compile-time error if the interface explicitly declares such a method m in the case where m is declared to be final in Object.
如果一个接口没有直接的父接口，那么，这个接口就隐含的声明了这样的一些成员方法m，这个方法是public abstract的，签名是s，返回类型是r，抛出t类型异常，与Object类中声明的public的，签名是s，返回类型是r，抛出t类型异常的方法相对应，除非，接口中明确定义了这样的方法。
如果接口明确的声明了这样的方法m，m在Object类中是final的，这将会产生编译时error。
Object的public方法：
public final native Class<?> getClass();
public final native void notify();
public final native void notifyAll();
public final void wait() throws InterruptedException
public boolean equals(Object obj)
public native int hashCode();
public String toString()
protected native Object clone() throws CloneNotSupportedException;
protected void finalize() throws Throwable { }
也就是说，接口里面默认有equals、hashcode、toString、clone和finalize这5个方法。
做了个实验，挺有意思：
在eclipse里面：
public interface MyInterfsace {
public void toString();//compile error! The return type is incompatible with Object.toString()
public void equals(Object obj);//compile error!The return type is incompatible with Object.equals(Object)
public String finalize() throws Throwable;//compile OK!but:The return type is incompatible with Object.finalize(), thus this interface cannot be implemented
public String hashCode();//compile error! The return type is incompatible with Object.hashCode()
public String clone() throws CloneNotSupportedException;//没啥特别的反应。
}
It follows that is a compile-time error if the interface declares a method with a signature that is override-equivalent (§8.4.2) to a public method of Object, but has a different return type or incompatible throws clause.
如果接口声明了一个方法，跟Object的方法构成override，但是却有不一样的返回类型或者是不兼容的异常类型，也是编译时error。
另外也有说法是接口继承了Object类
《深入java虚拟机》第二版第126页第六行
《The JavaTM Virtual Machine Specification》Second Edition CHAPTER 4 The class File Format
4.1 The ClassFile Structure中对于super_class标志的描述，我就复制一下内容吧：
super_class
For a class, the value of the super_class item either must be zero or must be a valid index into the constant_pool table. If the value of the super_class item is nonzero, the constant_pool entry at that index must be a CONSTANT_Class_info (§4.4.1) structure representing the direct superclass of the class defined by this class file. Neither the direct superclass nor any of its superclasses may be a final class.
If the value of the super_class item is zero, then this class file must represent the class Object, the only class or interface without a direct superclass.
For an interface, the value of the super_class item must always be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info structure representing the class Object.
两本都是很著名的书，都阐述一个道理，接口中Object中方法，所以List可以多态调用子类中的toString方法。