Suppose I have one POJO class User with a constuctor public User(int id, String name){...}.
But when I instantiate the User object like User u=new User() with no parameter Eclipse gives error like The constructor User() is undefined. But it works fine when I have no parameterized Constructor. Can someone please explain why It requires to define default constructor?
The default (no-parameter) constructor is ONLY provided if you have provided no others. If you define even a single constructor for your class, you MUST use one of the explicitly defined (ie, in your code) constructors to instantiate the object. You can, of course, define your own zero-parameter, empty constructor if that works for what you're trying to do.
Edit:
Answer of why?
The compiler provides a default constructor so that the Object can be Instantiated when there are no constructors defined. But if you have defined a parametric constructor, it means that when you create a new instance of that class, its variables should initialized with the parameters you have passed(or do something similar). Without those initializations, the object might not behave in an expected way. Hence the compiler prevents such things from happening by not defining a default constructor(when you have defined one).