Java Boxing, Unboxing and Missing Numbers

Understand the idea

Integer, Long, Double and the other wrappers are reference types. Autoboxing converts suitable primitive values into wrappers, and unboxing obtains a primitive value from a wrapper. Generic type parameters use reference types, so List<Integer> is valid while List<int> is not. A nullable Integer is a missing object reference, not a numeric zero. Decide how absence should behave before unboxing at a boundary.

Run it locally

Use a local JDK 21 or newer. Save the complete example as Main.java in a separate lab folder, then compile and run it with the commands below. Java examples do not run in Bhylu’s browser playground. The -ea option enables assertion checks used by some lessons.

JDK and terminal setup

javac --release 21 -encoding UTF-8 -d out Main.java
java -ea -cp out Main

A worked example

public class Main {
    public static void main(String[] args) {
        Integer entered = Integer.valueOf("18");
        int total = entered + 4;
        Integer missing = null;
        System.out.println(total);
        System.out.println(missing == null);
    }
}

Expected output

22
true

The addition unboxes entered into an int and produces twenty-two. The second check inspects a missing reference without attempting arithmetic on it. Numeric parsing has its own failure contract: malformed text or an out-of-range integer throws NumberFormatException rather than returning an arbitrary default.