Generic Vector in Java

Question

Hi,

Why doesn't the add() method here compile?

Vector<?> vectorList[] = new Vector<?>[10];
vectorList[0].add("elem1");

Thanks

Answer 1

There are several problems with your code. Firstly, the elements of the array are uninitialized, and so you are adding to a non-existent vector. And, secondly, adding to a generic container where the type is a wildcard doesn't make sense, because it could be restricted to a more limited type, and so adding would not be safe.

You can fix the first problem, by filling your array:

for (int i = 0; i < 10; i++ ){
    vectorList[i] = new Vector<Integer>(); // can replace Integer with
                                           // any binding for ? 
}

Now that you've seen that it can be filled with vectors that do not accept type String, you can see why the add statement would subsequently fail. You can fix that by binding to a specific type (e.g. String) earlier.

See Java Generics: Wildcards from the Java tutorial for more information.

EDIT
It seems I am doubted, and so I have the following complete, compilable example:

import java.util.*;

public class Main
{
    public static void main(String[] args){
        Vector<?> vectorList[] = new Vector<?>[10];
        for (int i = 0; i < 10; i++ ){
            vectorList[i] = new Vector<Integer>();
        }
        System.out.println("Hello world");
    }
}

Compiling with javac Main.java gives no compilation errors.

Answer 2

Because you have to instantiate each vector in the array, not just the array itself

vectorList[0] = new Vector<?>();

Answer 3

It doesn't compile because vectorList[0] is of type Vector<?>, and you can't call add(T) on a wildcard collection. You can't call add(T) because you can't ensure that the element you are adding is of the (unknown) type that vectorList[0] contains.

The lesson here is that generic types and arrays do not play well together. That is, you can't make vectorList of type Vector<String>[] without an unchecked cast, type Vector[] will give you a raw type to deal with, and you can't do what you want with a Vector<?>[] (the only one that doen't produce a warning). Instead use some List<Vector<String>>.