Java Final arraylist

Question

My question is regarding declaring an arraylist as final. I know that once I write final ArrayList list = new ArrayList(); I can add, delete objects from this list,

Answer 1

Just to "bring a little water to your Mill" you will understand the interest of final when you'll want to make your list publically availiable but unmodifiable.

In java one can make a list unmodifiable with Collections.unmodifiableList(modifiableList).

Now have a look to the following code :

public class MyClass{
  public static List MY_PUBLIC_LIST;
  static{
    ArrayList tmp = new ArrayList();
    tmp.add("a");
    tmp.add("b");
    tmp.add("c");
    MY_PUBLIC_LIST = tmp;
  }
}

Well, in anyclass, anywhere in your code you can do something like this

MyClass.MY_PUBLIC_LIST = null;
MyClass.MY_PUBLIC_LIST = new ArrayList();
MyClass.MY_PUBLIC_LIST.clear();
MyClass.MY_PUBLIC_LIST.add("1");

When you add the final keyword to your variable, the first two won't be allowed

public static final List MY_PUBLIC_LIST;

But you'll still be able to modify the content of the list :

MyClass.MY_PUBLIC_LIST.clear();
MyClass.MY_PUBLIC_LIST.add("1");

By adding a Collections.unmodifiableList(modifiableList) at the end of the static block you'll prevent this too :

MY_PUBLIC_LIST = Collections.unmodifiableList(tmp);

Ok we are almost there. Just to be sure you get the whole picture lets keep the Collections.unmodifiableList(modifiableList) but let me remove the final modifier

public class MyClass{
  public static List MY_PUBLIC_LIST;
  static{
    ArrayList tmp = new ArrayList();
    tmp.add("a");
    tmp.add("b");
    tmp.add("c");
    MY_PUBLIC_LIST = Collections.unmodifiableList(tmp);
  }
}

What can you do in that case ?

...

...

Well you can do whatever you want like in the first case (given that you assign the new list first) :

MyClass.MY_PUBLIC_LIST = null;
MyClass.MY_PUBLIC_LIST = new ArrayList();
MyClass.MY_PUBLIC_LIST.clear();
MyClass.MY_PUBLIC_LIST.add("1");