Object type

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In computer science, an object type (a.k.a. wrapping object) is a datatype that is used in object-oriented programming to wrap a non-object type to make it look like an object.

Some object-oriented programming languages make a distinction between objects and non-objects, often referred to as primitive types, for reasons such as runtime efficiency and syntax or semantic issues. For example, Java has primitive wrapper classes corresponding to each primitive type: Integer and int, Character and char, Float and float, etc. Languages like C++ make little or no distinction between objects and non-objects; thus, the use of object type is of little interest.

[edit] Boxing

Boxing is to place a primitive type within an object so that the primitive can be used as an object, in a language where there is a distinction between a primitive type and an object type. For example, lists may have certain methods which arrays might not, but the list might also require that all of its members be objects. In this case, the added functionality of the list might be unavailable to a simple list of numbers.

For a more concrete example, in Java, a LinkedList can change its size, but an array must have a fixed size. One might desire to have a LinkedList of ints, but the LinkedList class only lists objects—it cannot list primitive types.

To get around this, ints can be boxed into Integers, which are objects, and then added to a LinkedList of Integers. (Using generic parameterized types introduced in J2SE 5.0, this type is represented as LinkedList<Integer>.)

[edit] Autoboxing

Autoboxing is the term for treating a primitive type as an object type without any extra source code. The compiler automatically supplies the extra code needed to perfrom the type conversion.

For example, as of J2SE 5.0, Java will now allow the programmer to create a LinkedList of ints. This does not contradict what was said above: the LinkedList still only lists objects, and it cannot list primitive types. But now, when Java expects an object but receives a primitive type, it immediately converts that primitive type to an object.

This action is called autoboxing, because it is boxing that is done automatically and implicitly instead of requiring the programmer to do so manually.

[edit] Unboxing

Unboxing is the term for treating an object type as a primitive type without any extra code.

For example, in versions of Java prior to J2SE 5.0, the following code did not compile:

Integer i = new Integer(9);
Integer j = new Integer(13);
Integer k = i + j; // error!

Originally, the compiler would not accept the last line. Integers are objects, on the surface no different from List, Object, and so forth; mathematical operators such as + were not meaningfully defined for them. But the following code would of course be accepted without complaint:

int i = 9;
int j = 13;
int k = i + j;

int x = 4;
int y = 5;
// Integer qBox = new Integer(x + y);
Integer qBox = x + y; // would have been error, but okay now - equivalent to previous line

As of J2SE 5.0, the Integer is unboxed into an int, the two are added, and then the sum is autoboxed into a new Integer. [1]