Boxing
Boxing, otherwise known as wrapping, is the process of placing a primitive type within an object so that the primitive can be used as a reference object. For example, lists may have certain methods which arrays might not, but the list might also require that all of its members be dynamic objects. In this case, the added functionality of the list might be unavailable to a simple array 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 references to dynamic objects — it cannot list primitive types, which are value types.
To circumvent this, ints can be boxed into Integers, which are dynamic objects, and then added to a LinkedList of Integers. (Using generic parameterized types introduced in J2SE 5.0, this type is represented as LinkedList
The boxed object is always a copy of the value object, and is usually immutable. Unboxing the object also returns a copy of the stored value. Note that repeated boxing and unboxing of objects can have a severe performance impact, since it dynamically allocates new objects and then makes them eligible for Garbage collection.
There is a direct equivalence between an unboxed primitive type and a reference to an immutable, boxed object type. In fact, it is possible to substitute all the primitive types in a program with boxed object types. Whereas assignment from one primitive to another will copy its value, assignment from one reference to a boxed object to another will copy the reference value to refer to the same object as the first reference. However, this will not cause any problems, because the objects are immutable, so there is semantically no real difference between two references to the same object or to different objects (unless you look at physical equality). For all operations other than assignment, such as arithmetic, comparison, and logical operators, one can unbox the boxed type, perform the operation, and re-box the result as needed. Thus, it is possible to not store primitive types at all.
Read more about this topic: Object Type (object-oriented Programming)
Famous quotes containing the word boxing:
“I can entertain the proposition that life is a metaphor for boxing—for one of those bouts that go on and on, round following round, jabs, missed punches, clinches, nothing determined, again the bell and again and you and your opponent so evenly matched it’s impossible not to see that your opponent is you.... Life is like boxing in many unsettling respects. But boxing is only like boxing.”
—Joyce Carol Oates (b. 1938)
“... to paint with oil paints for the first time ... is like trying to make something exquisitely accurate and microscopically clear out of mud pies with boxing gloves on.”
—Brenda Ueland (1891–1985)