Question

public static List<Long> abc = new ArrayList<Long>(){{ //Asks for SerialVersionUID
        abc.add(5L);
        abc.add(7L);
    }};

public static List<Long> abc = new ArrayList<Long>();//Does not need SerialVersionUID
    static{
        abc.add(5L);
        abc.add(7L);
    }

Answer 1

In the second example, you’re instantiating a class that already has a defined serialVersionUID (i.e. ArrayList).

In the first example, you’re defining an anonymous subclass of ArrayList, and your subclass needs to have its own serialVersionUID defined. It’s not always obvious that double-brace initialization actually defines an anonymous class.