Boxing (computer programming): Difference between revisions

Revision as of 18:59, 10 January 2008

Revision as of 17:59, 19 February 2008

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

<source lang="java">

Integer i = new Integer(9);

Integer j = new Integer(13);

Integer k = i + j; // error!

</source>

As of J2SE 5.0, the <code>Integer</code>s <code>i</code> and <code>j</code> are unboxed into <code>int</code>s, the two are added, and then the sum is autoboxed into a new <code>Integer</code>. [http://java.sun.com/j2se/1.5.0/docs/guide/language/autoboxing.html] Originally, the compiler would not accept the last line. <code>Integer</code>s are objects, on the surface no different from <code>List</code>, <code>Object</code>, and so forth; mathematical operators such as <code>+</code> were not meaningfully defined for them. But the following code would of course be accepted without complaint:

<source lang="java">

int i = 9;

int j = 13;

int k = i + j;

</source>

Another example:

<source lang="java">

int x = 4;

int y = 5;

// Integer qBox = new Integer(x + y);

Integer qBox = x + y; // would have been an error, but okay now - equivalent to previous line

</source>

[[Category:Data types]]