Java - definition - "principle of substitution"

Question

I read a summary which always use the definition "principle of substitution" when it talk about inheritance. But It doesn't explain the meaning of this principle.

What is this principle?

Thank you.

Answer 1

It means plainly: a subclass must honor the contract set by the super class.

If you extend a super class, you should read its document and implement as it dictates. That's all.

If I hear one more time "Liskov substitution" I'm going to kill a kitten.

Answer 2

You probably know it, just not by this name.

http://en.wikipedia.org/wiki/Liskov_substitution_principle

It just says "If S is a T, then references to T can be changed to references to S."

Answer 3

Basically says that a class that implements a determined interface can be replaced by any other implementing the same inteface. You can found more information about this in Liskov's Substitution Principle quoting the definition:

"If a program module is using the reference of a Base class, then it should be able to replace the Base class with a Derived class without affecting the functioning of the program module. "

Answer 4

Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it

it is also known as the liskov substitution

Answer 5

The principle of substitution or Liskov Substitution Principle (LSP) basically states that an object should only be in a hierarchy if in every scenario that object can be used as its parent. For a concrete example:

public class Rectangle    {  
   //Stuff about rectangles    }  

public class Square extends Rectangle    {  
   //VIOLATES LSP!!  
} 

explanation here

public abstract class Shape  
{  
   //stuff about shapes  
}  

public class Square extends Shape  
{  
   //Square  
}  

public class Rectangle extends Shape  
{  
    //Rectangle  
}