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Is SubClassName x = new SubClassName() a legitimate declaration in Java?

For example, lets say the UnderGrad and GradStudent classes are subclasses of the Student class.
Student u = new UnderGrad(); works because an UnderGrad is-a Student.
UnderGrad u = new Student(); does not labor is because a Student is not necessarily an UnderGrad (it could be a GradStudent.) So since technically(idk if this is legitimate in java) an UnderGrad is-a UnderGrad, is UnderGrad u = new Undergrad(); a legitimate declaration?