Question

I am testing some small practicals with JAVA generics concept.

I try to return a List from a function that is generic but compile does not allow me to do so.

Check the code below :

package com.test.generic.method;

import java.util.ArrayList;
import java.util.List;


public class Sample_3<T> {

    public <T> List<T> testReturn(T t) {
        List<T> list = new ArrayList<T> ();
        list.add(t);
        return list;
    }

    public static void main(String a[]) {
        String s = "Gunjan";

        Sample_3 sample = new Sample_3<String>();
        List<String> list =(List<String>) sample.testReturn(sample);


        for(String ab : list){
            System.out.println(ab);
        }

    }
}

It gives ClassCastException.

How can I return the list from the generic function ?
And why JAVA has added such compile time feature ?

Thanks,
Gunjan.

Answer 1

The purpose of the generics is to allow you freedom to specify. So you can do something like:

public List<T> testReturn(T t){
        List<T> list = new ArrayList<T>();
        list.add(t); 
        return list; 
    }

which would then allow you to do e.g.

List<String> a = testReturn("A");
List<Integer> b = testReturn(1);

The issue with your code is that the intent of your method isn’t really generic – the method is written to return a List<String>. The <T> in your code doesn’t actually affect anything.

In response to your further question –
You’ve written the following:

public class Sample_3 {
   public <T> List<T> testReturn(T t) {
    List<T> list = new ArrayList<T> ();
    list.add(t);
    return list;
   }

   public static void main(String a[]) {
    String s = "Gunjan";
    // this is wrong - the Sample_3 class is not generic. It does not get a generic type parameter.
    Sample_3 sample = new Sample_3<String>();
    // this is also wrong. Since the method is generic the "T" you get is the same as the "T" that 
    // goes into the method. Here your String output type does not match the input type of the method,
    // which you've set here as a Sample_3 object
    List<String> list =(List<String>) sample.testReturn(sample);

}

Your generic parameter T sets the type that can vary. YOu can do it onthe method level, as above, or on the class level as well:

public class Sample_3<T> {
   public List<T> getList(){ ... }
}

e.g.

 List<Integer> l = new Sample_3<Integer>().getList();
 List<String> s = new Sample_3<String>().getList();
 List<Calendar> c = new Sample_3<Calendar>().getList();