Why Generic type???

1. It helps us to separate logic from data type, means no matter what type of data type we are passing to method, will be handled using the same function.
2. In other way, we can avoid polymorphism, means only one function can handle different data type rather than defining many function with different kind of data type.
3. Generic type invocation as being similar to an ordinary method invocation, but instead of passing an argument to a method, you're passing a type argument

So how to use it,
to use this we need to define the class data type at time of creating the object, here is an example for that suppose we have a class defince as follows :

public class Box<T> {

    // T stands for "Type"
    private T t;

    public void add(T t) {
        this.t = t;
    }

    public T get() {
        return t;
    }
}

So while creating the object of the Box class we do as follows:

Box<Integer> integerBox = new Box<Integer>();// which means the invoking is always knowns as parameterized type, means whenever we create and object we also pass the data type,

Check out these videos for more understanding :

Generic in c#

Generic in Java :