Defining your own Generic Class

Apart from using Generics with classes in the Java framework, we can create our own generic classes.

Lets create a generic class named MyGenericClass as follows :

public class MyGenericClass<T> {
    T obj;
    MyGenericClass(T o) {
        obj = o;    }

    T getObj() {
        return obj;    }

    public void show() {
        System.out.println("The type of obj is : " + obj.getClass().getName());    }
}

Here whatever the object we receive in the constructor and the value of T used while declaring the object tells what type  is going to be used.

Lets use this class in our test class to see its usage :

MyGenericClass<String> myG1 = new MyGenericClass<String>("Sandeep");System.out.println(myG1.getObj());myG1.show();

MyGenericClass<Integer> myG2 = new MyGenericClass<Integer>(100);System.out.println(myG2.getObj());myG2.show();

The above declarations change the value of T to java.lang.String and java.lang.Integer at runtime and produce the output accordingly.

Try this variation :

List<String> strList = new ArrayList<String>();strList.add("Sandeep");strList.add("Ravi");
MyGenericClass<List<String>> myG3 = new MyGenericClass<ArrayList<String>>(strList);System.out.println(myG1.getObj());myG1.show();

The above code doesnt compile as the type-parameter is not the same while creating the object.
The reference type of the parent can be used to create child object, but the type-parameter needs to be the same. So in this case, if MyGenericClass had a parent class, we could have used that; but we need to use either List or ArrayList on both sides of the declaration.

The code for the above example can be found here.