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Boxing (computer programming)

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In <a href="Computer_science" title="Computer science">computer science</a>, boxing (a.k.a. wrapping) is the transformation of placing a primitive type within an <a href="Object_(computer_science)" title="Object (computer science)">object</a> so that the value can be used as a <a href="Reference_type" class="mw-redirect" title="Reference type">reference</a>. Unboxing is the reverse transformation of extracting the primitive value from its wrapper object. Autoboxing is the term for automatically applying boxing and/or unboxing transformations as needed.

<div class="mw-heading mw-heading2"><h2 id="Boxing">Boxing</h2></div>

Boxing's most prominent use is in <a href="Java_(programming_language)" title="Java (programming language)">Java</a> where there is a distinction between <a href="Reference_type" class="mw-redirect" title="Reference type">reference</a> and <a href="Value_type" class="mw-redirect" title="Value type">value types</a> for reasons such as runtime efficiency and syntax and semantic issues. In Java, a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList</code> can only store values of type <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Object</code>. One might desire to have a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList</code> of <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code>, but this is not directly possible. Instead Java defines <a href="Primitive_wrapper_class" class="mw-redirect" title="Primitive wrapper class">primitive wrapper classes</a> corresponding to each <a href="Primitive_data_type" title="Primitive data type">primitive type</a>: <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> and <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code>, <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Character</code> and <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">char</code>, <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Float</code> and <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">float</code>, etc. One can then define a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList</code> using the boxed type <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> and insert <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code> values into the list by boxing them as <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> objects. (Using <a href="Generic_programming" title="Generic programming">generic</a> parameterized types introduced in <a href="Java_Platform%2C_Standard_Edition" title="Java Platform, Standard Edition">J2SE</a> 5.0, this type is represented as <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList<Integer></code>.)

On the other hand, <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> has no primitive wrapper classes, but allows boxing of any value type, returning a generic <code class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr">Object</code> reference. In <a href="Objective-C" title="Objective-C">Objective-C</a>, any primitive value can be prefixed by a <code class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr">@</code> to make an <code class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr">NSNumber</code> out of it (e.g. <code class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr">@123</code> or <code class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr">@(123)</code>). This allows for adding them in any of the standard collections, such as an <code class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr">NSArray</code>.

<a href="Haskell" title="Haskell">Haskell</a> has little or no notion of <a href="Reference_type" class="mw-redirect" title="Reference type">reference type</a>, but still uses the term "boxed" for the runtime system's uniform pointer-to-<a href="Tagged_union" title="Tagged union">tagged union</a> representation.<a href="#cite_note-1">[1]</a>

The boxed object is always a copy of the value object, and is usually <a href="Immutable_object" title="Immutable object">immutable</a>. Unboxing the object also returns a copy of the stored value. Repeated boxing and unboxing of objects can have a severe performance impact, because boxing <a href="Dynamic_memory_allocation" class="mw-redirect" title="Dynamic memory allocation">dynamically allocates</a> new objects and unboxing (if the boxed value is no longer used) then makes them eligible for <a href="Garbage_collection_(computer_science)" title="Garbage collection (computer science)">garbage collection</a>. However, modern garbage collectors such as the default Java HotSpot garbage collector can more efficiently collect short-lived objects, so if the boxed objects are short-lived, the performance impact may not be severe.

In some languages, 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.

<div class="mw-heading mw-heading2"><h2 id="Autoboxing">Autoboxing</h2></div>

Autoboxing is the term for getting a reference type out of a value type just through <a href="Type_conversion" title="Type conversion">type conversion</a> (either implicit or explicit). The compiler automatically supplies the extra source code that creates the object.

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

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>Integer i = new Integer(9);

Integer i = 9; // error in versions prior to 5.0!

</pre></div>

Compilers prior to 5.0 would not accept the last line. <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> are reference objects, on the surface no different from <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">List</code>, <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Object</code>, and so forth. To convert from an <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code> to an <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code>, one had to "manually" instantiate the Integer object. As of J2SE 5.0, the compiler will accept the last line, and automatically transform it so that an Integer object is created to store the value <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">9</code>.<a href="#cite_note-2">[2]</a> This means that, from J2SE 5.0 on, something like <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer c = a + b</code>, where <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">a</code> and <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">b</code> are <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> themselves, will compile now — a and b are unboxed, the integer values summed up, and the result is autoboxed into a new <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code>, which is finally stored inside variable <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">c</code>. The equality operators cannot be used this way, because the equality operators are already defined for reference types, for equality of the references; to test for equality of the value in a boxed type, one must still manually unbox them and compare the primitives, or use the <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Objects.equals</code> method.

Another example: J2SE 5.0 allows the programmer to treat a collection (such as a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList</code>) as if it contained <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code> values instead of <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">Integer</code> objects. This does not contradict what was said above: the collection still only contains references to dynamic objects, and it cannot list primitive types. It cannot be a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList<int></code>, but it must be a <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">LinkedList<Integer></code> instead. However, the compiler automatically transforms the code so that the list will "silently" receive objects, while the source code only mentions primitive values. For example, the programmer can now write <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">list.add(3)</code> and think as if the <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">int</code> <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">3</code> were added to the list; but, the compiler will have actually transformed the line into <code class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr">list.add(new Integer(3))</code>.

<div class="mw-heading mw-heading3"><h3 id="Automatic_unboxing">Automatic unboxing</h3></div>

With automatic unboxing the compiler automatically supplies the extra source code that retrieves the value out of that object, either by invoking some method on that object, or by other means.

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

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>Integer k = new Integer(4);

int l = k.intValue(); // always okay

int m = k; // would have been an error, but okay now

</pre></div>

C# doesn't support automatic unboxing in the same meaning as Java, because it doesn't have a separate set of primitive types and object types. All types that have both primitive and object version in Java, are automatically implemented by the C# compiler as either primitive (value) types or object (reference) types.

In both languages, automatic boxing does not downcast automatically, i.e. the following code won't compile:

C#:

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>int i = 42;

object o = i; // box

int j = o; // unbox (error)

Console.WriteLine(j); // unreachable line, author might have expected output "42"

</pre></div>

Java:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>int i = 42;

Object o = i; // box

System.out.println(j); // unreachable line, author might have expected output "42"

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Boxing_in_Rust">Boxing in Rust</h2></div>

<a href="Rust_(programming_language)" title="Rust (programming language)">Rust</a> has the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">Box</code> type, which represents a uniquely owned, heap-allocated value:<a href="#cite_note-3">[3]</a>

<div class="mw-highlight mw-highlight-lang-rust mw-content-ltr" dir="ltr"><pre>let number: Box<i32> = Box::new(42);

</pre></div>

If the value needs to have shared ownership (e.g. between threads), one can use <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">Arc</code>, which represents a reference-counted, heap-allocated value.<a href="#cite_note-4">[4]</a><a href="#cite_note-5">[5]</a>

<div class="mw-heading mw-heading2"><h2 id="Type_helpers">Type helpers</h2></div>

Modern <a href="Object_Pascal" title="Object Pascal">Object Pascal</a> has yet another way to perform operations on simple types, close to boxing, called type helpers in <a href="FreePascal" class="mw-redirect" title="FreePascal">FreePascal</a> or record helpers in <a href="Delphi_(programming_language)" class="mw-redirect" title="Delphi (programming language)">Delphi</a> and <a href="FreePascal" class="mw-redirect" title="FreePascal">FreePascal</a> in Delphi mode.

The dialects mentioned are Object Pascal compile-to-native languages, and so miss some of the features that C# and Java can implement. Notably run-time <a href="Type_inference" title="Type inference">type inference</a> on strongly typed variables.

But the feature is related to boxing.

It allows the programmer to use constructs like

<div class="mw-highlight mw-highlight-lang-pascal mw-content-ltr" dir="ltr"><pre>{$ifdef fpc}{$mode delphi}{$endif}

uses sysutils; // this unit contains wraps for the simple types

var

x:integer=100;

s:string;

begin

s:= x.ToString;

writeln(s);

end.

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://downloads.haskell.org/~ghc/6.12.1/docs/html/users_guide/primitives.html">"7.2. Unboxed types and primitive operations"</a>. downloads.haskell.org. Retrieved 10 August 2022.</cite>

</li>

<li id="cite_note-2"><a href="#cite_ref-2">^</a> <a rel="nofollow" class="external text" href="https://docs.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html">oracle.com Java language guide entry on autoboxing</a>

</li>

<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://doc.rust-lang.org/std/boxed/index.html">"std::boxed - Rust"</a>. doc.rust-lang.org. Retrieved 2 June 2025.</cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://doc.rust-lang.org/std/sync/struct.Arc.html">"Arc in std::sync - Rust"</a>. doc.rust-lang.org. Retrieved 18 January 2025.</cite>

</li>

<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://doc.rust-lang.org/rust-by-example/std/arc.html">"Arc - Rust By Example"</a>. doc.rust-lang.org. Retrieved 18 January 2025.</cite>

</li>

</ol></div></div>

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</style><div id="Data_types177" style="font-size:114%;margin:0 4em"><a href="Data_type" title="Data type">Data types</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Units_of_information" title="Units of information">Uninterpreted</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Ternary_numeral_system#Tryte" title="Ternary numeral system">Tryte</a></li>

<li><a href="Bit_array" title="Bit array">Bit array</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Numeric</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Arbitrary-precision_arithmetic" title="Arbitrary-precision arithmetic">Arbitrary-precision or bignum</a></li>

<li><a href="Complex_data_type" title="Complex data type">Complex</a></li>

<li><a href="Decimal_data_type" title="Decimal data type">Decimal</a></li>

<li><a href="Fixed-point_arithmetic" title="Fixed-point arithmetic">Fixed point</a></li>

<li><a href="Block_floating_point" title="Block floating point">Block floating point</a></li>

<li><a href="Floating-point_arithmetic" title="Floating-point arithmetic">Floating point</a>

<ul><li>Reduced precision

<ul><li><a href="Minifloat" title="Minifloat">Minifloat</a></li>

<li><a href="Half-precision_floating-point_format" title="Half-precision floating-point format">Half precision</a></li>

<li><a href="Bfloat16_floating-point_format" title="Bfloat16 floating-point format">bfloat16</a></li></ul></li>

<li><a href="Single-precision_floating-point_format" title="Single-precision floating-point format">Single precision</a></li>

<li><a href="Double-precision_floating-point_format" title="Double-precision floating-point format">Double precision</a></li>

<li><a href="Quadruple-precision_floating-point_format" title="Quadruple-precision floating-point format">Quadruple precision</a></li>

<li><a href="Octuple-precision_floating-point_format" title="Octuple-precision floating-point format">Octuple precision</a></li>

<li><a href="Extended_precision" title="Extended precision">Extended precision</a>

<ul><li><a href="Long_double" title="Long double">Long double</a></li></ul></li></ul></li>

<li><a href="Integer_(computer_science)" title="Integer (computer science)">Integer</a>

<ul><li><a href="Signedness" title="Signedness">signedness</a></li></ul></li>

<li><a href="Interval_arithmetic#Implementations" title="Interval arithmetic">Interval</a></li>

<li><a href="Rational_data_type" title="Rational data type">Rational</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Pointer_(computer_programming)" title="Pointer (computer programming)">Pointer</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Memory_address" title="Memory address">Address</a>

<ul><li><a href="Physical_address" title="Physical address">physical</a></li>

<li><a href="Virtual_address_space" title="Virtual address space">virtual</a></li></ul></li>

<li><a href="Reference_(computer_science)" title="Reference (computer science)">Reference</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Plain_text" title="Plain text">Text</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Character_(computing)" title="Character (computing)">Character</a></li>

<li><a href="String_(computer_science)" title="String (computer science)">String</a>

<ul><li><a href="Null-terminated_string" title="Null-terminated string">null-terminated</a></li></ul></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Composite_data_type" title="Composite data type">Composite</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Algebraic_data_type" title="Algebraic data type">Algebraic data type</a>

<ul><li><a href="Generalized_algebraic_data_type" title="Generalized algebraic data type">generalized</a></li></ul></li>

<li><a href="Array_(data_type)" title="Array (data type)">Array</a></li>

<li><a href="Associative_array" title="Associative array">Associative array</a></li>

<li><a href="Class_(computer_programming)" title="Class (computer programming)">Class</a></li>

<li><a href="Dependent_type" title="Dependent type">Dependent</a></li>

<li><a href="Intuitionistic_type_theory#Equality_type" title="Intuitionistic type theory">Equality</a></li>

<li><a href="Inductive_type" title="Inductive type">Inductive</a></li>

<li><a href="Intersection_type" title="Intersection type">Intersection</a></li>

<li><a href="Object_(computer_science)" title="Object (computer science)">Object</a>

<ul><li><a href="Metaobject" title="Metaobject">metaobject</a></li></ul></li>

<li><a href="Option_type" title="Option type">Option type</a></li>

<li><a href="Product_type" title="Product type">Product</a></li>

<li><a href="Record_(computer_science)" title="Record (computer science)">Record or Struct</a></li>

<li><a href="Refinement_type" title="Refinement type">Refinement</a></li>

<li><a href="Union_type" title="Union type">Union</a>

<ul><li><a href="Tagged_union" title="Tagged union">tagged</a></li></ul></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Boolean_data_type" title="Boolean data type">Boolean</a></li>

<li><a href="Bottom_type" title="Bottom type">Bottom type</a></li>

<li><a href="Container_(abstract_data_type)" title="Container (abstract data type)">Collection</a></li>

<li><a href="Enumerated_type" title="Enumerated type">Enumerated type</a></li>

<li><a href="Exception_handling" title="Exception handling">Exception</a></li>

<li><a href="Function_type" title="Function type">Function type</a></li>

<li><a href="Opaque_data_type" title="Opaque data type">Opaque data type</a></li>

<li><a href="Recursive_data_type" title="Recursive data type">Recursive data type</a></li>

<li><a href="Semaphore_(programming)" title="Semaphore (programming)">Semaphore</a></li>

<li><a href="Stream_(computing)" title="Stream (computing)">Stream</a></li>

<li><a href="Strongly_typed_identifier" title="Strongly typed identifier">Strongly typed identifier</a></li>

<li><a href="Type_class" title="Type class">Type class</a></li>

<li><a href="Empty_type" title="Empty type">Empty type</a></li>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related topics</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Abstract_data_type" title="Abstract data type">Abstract data type</a></li>

<li><a href="Data_structure" title="Data structure">Data structure</a></li>

<li><a href="Generic_programming" title="Generic programming">Generic</a></li>

<ul><li><a href="Metaclass" title="Metaclass">metaclass</a></li></ul></li>

<li><a href="Parametric_polymorphism" title="Parametric polymorphism">Parametric polymorphism</a></li>

<li><a href="Primitive_data_type" title="Primitive data type">Primitive data type</a></li>

<li><a href="Interface_(object-oriented_programming)" title="Interface (object-oriented programming)">Interface</a></li>

<li><a href="Subtyping" title="Subtyping">Subtyping</a></li>

<li><a href="Type_constructor" title="Type constructor">Type constructor</a></li>

<li><a href="Type_conversion" title="Type conversion">Type conversion</a></li>

<li><a href="Type_system" title="Type system">Type system</a></li>

<li><a href="Type_theory" title="Type theory">Type theory</a></li>

<li><a href="Variable_(computer_science)" title="Variable (computer science)">Variable</a></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-06-29" href="https://en.wikipedia.org/wiki/?title=Boxing_(computer_programming)&oldid=1298004437">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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