Is autoboxing possible for the classes I create?

问题

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!

回答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.

回答2:

No, unfortunately. Automatic boxing conversions (as per JLS §5.1.7) are only defined for the standard primitive wrapper classes.

回答3:

In short : No, it's not possible. For this to work, you need operator overloading, which is not available in Java. See link.

回答4:

If you use Groovy, you can set the boolean behavior by implementing the asBoolean method: http://groovy-lang.org/semantics.html#_customizing_the_truth_with_asboolean_methods

Example:

class Color {
    String name

    boolean asBoolean(){
        name == 'green' ? true : false
    }
}

assert new Color(name: 'green')
assert !new Color(name: 'red')

I know this is not plain Java but compiles to bytecode and runs on the JVM.

来源：https://stackoverflow.com/questions/17619724/is-autoboxing-possible-for-the-classes-i-create

标签

java

autoboxing