Reasons to use generic List over List of object

Question

I know two reason to use generic List over List of objects.

1. To restrict what type of objects can be inserted into the List

   List<Object> stringObjects = Arrays.asList("a",new Integer(5));
   List<String> genericStrings = Arrays.asList("a",new Integer(5));//compile error
   

2. To have access to the correct method in your IDE

   List<Object> stringObjects = Arrays.asList("a",new Integer(5));
   List<String> genericStrings = Arrays.asList("a");
   stringObjects.get(0).length();//compile error
   genericStrings.get(0).length();
   

When I gave this answer to an interview, but they didn't seem very happy with my answer. So are there any other reason to user Generic List over List of objects?

Answer 1

There are 2 main reasons:

1. Stronger type checks at compile time.

2. Elimination of casts.

source

Answer 2

In my opinion, what you had in mind was correct, but no well formated.

It's not about "having access to the correct method in your IDE", it is about polymorphism and avoiding explicit cast.

public interface DoAble {
    void doSomething();
}

public Class FirstImpl implements DoAble {
    public void doSomething() {
        // Do something here
    }
}

public Class SecondImpl implements DoAble {
    public void doSomething() {
        // Do something here
    }
}

Usage with a List of Object

List list = new Arrays.asList(new SecondImpl(), new FirstImple());
for (Object o : list) {
    ((DoAble) o).doSomething();
}

It works, but it's ugly.

• Since whatever can be inserted in the list, it can throw a java.lang.ClassCastException at any time.
• We need to do an explicit cast, which we should always try to avoid.

Usage with Generic List

List<DoAble> list = new Arrays.asList(new SecondImpl(), new FirstImple());
for (DoAble o : list) {
    o.doSomething();
}

Easier to use and to read.

• No explicit casts.
• We are 100% that our list only contains objects that implements DoAble.