根据经验
interface A{ } class B implements A{ public String testB(){ return "testB() from class B"; } } public class Demo{ public static void main(String[] args){ A a = new B(); System.out.println(a.toString()); System.out.println(a.equals(null)); // System.out.println(a.testB()); //错误 System.out.println(((B)a).testB()); } }既然接口的引用可以调用Object的方法为什么没有继承Object呢？
以下答案来自https://bbs.csdn.net/topics/350246605好棒的回答
参考：java语言规范 9.2 Interface MembersIf 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 InterruptedExceptionpublic 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。
