java - Is autoboxing possible for the classes I create?

Question

Is there any way to use autoboxing for the classes I create? For example, I have this subclass of Number.

public class UnsignedInteger extends Number {
    int n;

    public UnsignedInteger(int n) {
        if(n >= 0)
            this.n = n;
        else
            throw new IllegalArgumentException("Only positive integers are supported");
    }
}

Now, UnsignedInteger i = new UnsignedInteger(88); works perfectly fine, but is there any way to make this compile : UnsignedInteger i = 88;? It won't for me. Thanks in advance!

Answer 1

In short, no. There's no way to get that to compile.

Java only defines a limited set of pre-defined boxing conversions.

From the JLS, section 5.1.7:

Boxing conversion converts expressions of primitive type to corresponding expressions of reference type. Specifically, the following nine conversions are called the boxing conversions:

• From type boolean to type Boolean

• From type byte to type Byte

• From type short to type Short

• From type char to type Character

• From type int to type Integer

• From type long to type Long

• From type float to type Float

• From type double to type Double

• From the null type to the null type

Additionally, one might think of overloading the = operator to perform this conversion, but operator overloading is not supported in Java, unlike in C++, where this would be possible.

So your conversion is not possible in Java.