passing List<String> to method expecting List<Object>

Question

In Page 112 of CHAPTER 5 GENERICS in the book - Effective Java , these sentences appears

Just what is the difference between the raw type List and the parameterized type List<Object> ...While you can pass a List<String> to a parameter of type List, you can’t pass it to a parameter of type List<Object>

I tried this

public static void getMeListOfObjs(List<Object> al){
    System.out.println(al.get(0));
}
public static void main(String[] args) {

    List<Object> al = new ArrayList<Object>();

    String mys1 = "jon";

    al.add(mys1);

    getMeListOfObjs(al);


}

It runs without any error... Was that an error in book content? I am quoting from the second edition

Solution 1

Try this:

public static void getMeListOfObjs(List<? extends Object> al) {
    System.out.println(al.get(0));
}

public static void main(String[] args) {

    List<String> al = new ArrayList<String>();

    String mys1 = "jon";

    al.add(mys1);

    getMeListOfObjs(al);


}

This wont compile, because List<String> isn't match List<Object>

As @darijan pointed out the wildcard ? extends the type chack with all class that is descendant of it.

I recommend to read more about generics and wildcards

Solution 2

The code you provided works. I guess you want your al list to be List<String>. Then, you would have to make getMeListOfObjs like this:

public static void getMeListOfObjs(List<? extends Object> al) {
    System.out.println(al.get(0));
}

public static void main(String[] args) {
    List<String> al = new ArrayList<String>();
    String mys1 = "jon";
    al.add(mys1);
    getMeListOfObjs(al);
}

Notice the differnce?

public static void getMeListOfObjs(List<? extends Object> al) {

The wildcard ? changes any type that extends Object, i.e. any object.

Solution 3

The example you have created does not create a List<String>. You also use a List<Object> you just add Strings to the Object list. So you essentially pass Object list, that's why it works. You can add any object the the object list including string. It works since the String extends Object.