是否存在对象的本机类型安全包装和拆箱？

Question

My inquiry is on wrapping, using the Object class as my wrapper.

My goal is to take the primitive int (or a wrapper of int) and unbox it from a generic Object.

The only problem is that I will be working with mixed types at runtime, so I am unable to generically type the return type of the method. T var1 != (T) var2

What I can do right now has excessive overhead because I cannot assume the Object wraps a value of type int, so I parse it.

Object int_wrapped_in_object = 1;

Integer.valueOf(String.valueOf(int_wrapped_in_object));

//I could cast from Object to int with:
//(int) int_wrapped_in_object
//but this will always anticipate a classcastexception in case the value was wrapped with another type, like String

This works, but I would ideally like to skip a step of parsing and just unwrap box for the integer value.

Integer wrapper class does not support valueOf(Object o) , likely because Object does not implement the relative cast. It (Object) supports toString() . But not toInt(). It is very peculiar because Object indeed can wrap a primitive int. Why that is the case is above my level of knowledge, so I can only work with what I have. For all I know there may even be native support for unwrapping an Object as an int.

Please help me find a solution that involves using minimal parsing and introducing as few exceptions to get the primitive int value.

Answer 1

Misconception on your end:

Object int_wrapped_in_object = 1;

The type of int_wrapped_in_object is Integer , not Object! The compiler already boxes for you!

In other words: there is no way that the java compiler would "box" an int into something that is "only" Object .

Thus a simple int_wrapped_in_object instanceof Integer is all you need to figure if that Object is actually an Integer. And then you can simply cast !

Answer 2

Your value gets wrapped to an Integer and then the view to it is reduced by casting it to Object .

You can later safely cast it back if you check the type with instanceof . Then you can unwrap it to int with the Integer#intValue method.

// Indirect conversion int -> Integer and then reduced view to Object
Object int_wrapped_in_object = 5;

if (int_wrapped_in_object instanceof Integer) {
    // Checked cast as the object indeed is an Integer but with reduced view
    Integer intAsInteger = (Integer) int_wrapped_in_object;

    // Retrieve the int value from the Integer
    // (you could also do an implicit unwrapping here) like
    // int value = intAsInteger;
    int value = intAsInteger.intValue();

    System.out.println("The value is: " + value);
} else {
    throw new IllegalStateException("Value is no Integer but it should.");
}

Note that the line

Object int_wrapped_in_object = 5;

gets indirectly transformed to

Object int_wrapped_in_object = (Object) Integer.valueOf(5);

If you need to do such conversions often then just create an utility method which does that for you.

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Note that the object itself stays Integer and gets not converted to an Object , that is not how casting works, only its view is reduced. You can check that with

System.out.println(int_wrapped_in_object.getClass())

It will print class java.lang.Integer and not class java.lang.Object . So the type of an object always stays the same, even after casting. You only reduce the view to it which has advantages like higher modularity.