Add this to your class:
public WeightIn(){
}
Please understand that default no-argument constructor is provided only if no other constructor is written
If you write any constructor, then compiler does not provided default no-arg constructor. You have to specify one.
In this you can’t do WeightIn weight1 = new WeightIn(); since default constructor is not defined.
So you can add
public WeightIn(){
}
Or you can do this
WeightIn weight1 = new WeightIn(3.65,1.7) // constructor accept two double values