What are the differences between List, List<?>, List<T>, List<E>, and List<Object>?

1. List

List: is a raw type, therefore not typesafe. It will only generate a runtime error when the casting is bad. We want a compile time error when the cast is bad. Not recommended to use.

2. List<?>

List<?> is an unbounded wildcard. But I’m not sure what it’s for? I can print a List<?> without issue:

public static void test(List<?> list){
    System.out.println(list);   // Works
}

Why can’t I add items to a List<?>?

public static void test(List<?> list){
    list.add(new Long(2));     // Error
    list.add("2");             // Error
    System.out.println(list);
}

3. List<T>

public static void test(List<T> list){   // T cannot be resolved
    System.out.println(list);
}

I don’t understand this syntax. I saw something like this, and it works:

public <T> T[] toArray(T[] a){
    return a;   
}

Sometimes, I see <T>, or <E>, or <U>, <T,E>. Are they all the same or do they represent something different?

4. List<Object>

This gives the error “The method test(List<Object>) is not applicable for the argument List<String>“:

public static void test(List<Object> list){
    System.out.println(list);
}

If I try this then I got “Cannot cast from List<String> to List<Object>“:

test((List<Object>) names);

I am confused. String is a subclass of Object, so why isn’t List<String> a subclass of List<Object>?