java - Generic type lost for member of raw type

Question

I have found a strange behavior when working with generics.

In this class Foo<T>, the strings member doesn't have anything to do with T:

package test;
import java.util.ArrayList;

public class Foo<T> {
    ArrayList<String> strings;

    T getSome() {
        return null;
    }
}

The class is used in main:

package test;

public class Main {

    public static void main() {
        Foo<Integer> intFoo = new Foo<>();
        Integer i = intFoo.getSome();
        String s1 = intFoo.strings.get(0);

        Foo rawFoo = new Foo();
        Object o = rawFoo.getSome();
        String s2 = rawFoo.strings.get(0); // Compilation error on this line
    }
}

The compilation error is "incompatible types. required: String found: Object".

It appears that Java forgets the String type argument to ArrayList when raw type of Foo is used.

My java version is 1.7.0_21

Answer 1

Simply put, because rawFoo is raw, its non-static members also become raw.

This is outlined in JLS §4.8:

More precisely, a raw type is defined to be one of:

• The reference type that is formed by taking the name of a generic type declaration without an accompanying type argument list.

• An array type whose element type is a raw type.

• A non-static member type of a raw type R that is not inherited from a superclass or superinterface of R.

Note the last bullet.