Question

Why does following code not throws exception?

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

@SuppressWarnings("unchecked")
public class MainRunner {
    public static void main(String[] args) {
    List<String> s = new ArrayList<String>() {
        {
        add("a");
        add("1");
        add("1");
        }
    };
    // List<Integer> i = (List<Integer>) listConvertor(s, new Integer("1"));
    List<Integer> i = (List<Integer>) listConvertor(s, Integer.class);
    System.out.println(i);
    }

    @SuppressWarnings("unchecked")
    public static <T, P> List<?> listConvertor(List<T> inputList, P outputClass) {
    List<P> outputList = new ArrayList<P>(inputList.size());
    for (T t : inputList) {
        outputList.add((P) t); // shouldn't be classCastException here?
    }

    return outputList;

    }

}

I want to return List<P> instead of List<?> . But when I write List<P> , it means List<Class<P>> . i.e. in above case , it means List<Class<Integer>> , but I want List<Integer> as return.

I want below code: (so that i don’t have to cast again at when method returns)

List<Integer> i =   listConvertor(s, Integer.class);
        System.out.println(i);
        }

        @SuppressWarnings("unchecked")
        public static <T, P> List<P> listConvertor(List<T> inputList, P outputClass) {
        List<P> outputList = new ArrayList<P>(inputList.size());
        for (T t : inputList) {
            outputList.add((P) t); // shouldn't be classCastException here?
        }

        return outputList;

        }

    }

Answer 1

This should do the job with minimal fuss:

public static <T, P> List<P> listConvertor(List<T> inputList, Class<P> outputClass) {
    List<P> outputList = new ArrayList<P>(inputList.size());
    for (T t : inputList) {
        if( !outputClass.isInstance(t) )
            throw new ClassCastException("Faked CCException");
        @SuppressWarnings("unchecked")
        P p = (P) t;
        outputList.add(p);
    }

    return outputList;
}

• no cast on the caller side
• exception if inappropriate types are in the source list.