What is Liskov Substitution principle in Java?

Answer 1

Liskov Substitution principle (LSP) states that "Methods that use references to the base classes must be able to use the objects of the derived classes without knowing it." or in other words,
a.if a class B is a subclass of a class A,
b.if A has a method f(),
c.if b is an instance of B and a an instance of A,
d.then in all the parts of the code using “a.f()” should be able to use” b.f()” without modifying the behavior of the code.

Read this article from queryhome where LSP is explained with an example:
http://tech.queryhome.com/121052/liskov-substitution-principle-concept-object-oriented-design

Answer 2

According to Liskov Substitution Principle, Subtypes must be substitutable for super type i.e. methods or functions which uses super class type must be able to work with object of sub class without any issue". LSP is closely related to Single responsibility principle and Interface Segregation Principle. If a class has more functionality than subclass might not support some of the functionality ,and does violated LSP. In order to follow LSP SOLID design principle, derived class or sub class must enhance functionality, but not reduce them. LSP represent "L" on SOLID acronym.