多态对象字符串

Question

I didn't get to understand the polymorphism.

taking this example :

Object o = new String ("foo");

I can't do

o.substring (1,2)

can anyone explain this problem to me ?

Answer 1

You are confusing dynamic and static typing with polymorphism .

Java is a statically typed language so the compiler recognizes that Object doesn't have a substring() method and throws an error. Polymorphism happens after the compilation, when the code is actually executed.

Answer 2

First thing It's not polymorphism.

In Simple way, In your case o object will only call methods which are defined in Object class. If String class has overridden any method of Object class then object o will execute that method of String class otherwise Object o will call Its own method.

For example :

substring() method is only defined in String class So In this case your code will throw exception, however if you call equals() , hashcode() , toString() methods (as these are defined in Object class ) then definition inside String class will get executed because String Class has overridden these methods. And if any of the methods from object Class has not been overridden in its child class then definition inside Object class will get executed.

Answer 3

substring is from the String class, not the Object class. So the below code will work:

String o = new String ("foo");
o.substring(1,2);

This code will not work:

Object o = new String ("foo");
o.substring(1,2);

Since String inherits from Object , String can call Object 's methods, such as toString . However, Object does not inherit from String , so an Object cannot call String 's methods, such as substring .

Answer 4

This is a consequence of the Liskov Substitution Principle , which states (summarized):

If S and T are objects, and T is a subtype of S, then T may be used where S is expected.

String is-a subtype of Object , so if your assignment operation expects Object , then it will happily accept Object or any of its subtypes.

(Note: Object is not a String . All String s are Object s, but not all Object s are String s.)

This doesn't mean you get access to any of the subtype's methods. Given the inheritance hierarchy, an Object has no clue about any of its children's specific methods, nor can it - there is no way to inform an ancestor class of its descendant's capabilities. Because Object has no substring method associated with it, your code correctly results in a compilation failure.

(And it should, given that Object is the ancestor of all classes. There's no guarantee that any given Object is a String .)

The standing advice is to not use an overly inspecific object type (as you go up the hierarchy chain, the capabilities become less specific - you lose functionality as you go up to Object ) to accomplish something specific to a more specific type.