java參數(shù)傳遞時(shí)到底是值傳遞還是引用傳遞

1、java參數(shù)傳遞時(shí)到底是值傳遞還是弓I用傳遞（baidu搜集）最近比較長(zhǎng)一段時(shí)間以來(lái)，網(wǎng)上的IT同行里面比較流行“JAVA面試32問(wèn)”，很 多人的BLOG里都引用這些面試題，最近因?yàn)楣ぷ鲀?nèi)容比較枯燥，也來(lái)看看這 些試題以調(diào)節(jié)一下口味，其中有一道題讓我很費(fèi)解。原題是：當(dāng)一個(gè)對(duì)象被當(dāng)作參數(shù)傳遞到一個(gè)方法后，此方法可改變這個(gè)對(duì)象的屬性，并可返回變化后的結(jié)果，那么這里到底是值傳遞還是引用傳遞？用google查詢(xún)結(jié)果，得到答案基本上是：值傳遞。當(dāng)時(shí)覺(jué)得挺納悶兒，為什么 連參數(shù)的內(nèi)容都被修改了，怎么還能說(shuō)是“值傳遞”呢？因?yàn)樵趥鹘y(tǒng)的印象里（尤其是從C+過(guò)來(lái)以后），值傳遞都應(yīng)該是不改變?cè)瓍?shù)的。問(wèn)問(wèn)周?chē)耐?/p>

2、事，也大都這么講，但是也都講不清這種理論的根源是什么。我這 個(gè)人有個(gè)毛病，有事情想不通時(shí)就會(huì)憋得難受，后來(lái)在Thinking in Java的 一段內(nèi)容（注解1）里找到了自己的結(jié)論，我認(rèn)為（Thinking in Java的作 者也這么認(rèn)為）：可以說(shuō)是值傳遞，也可以說(shuō)是引用傳遞。一，認(rèn)為是值傳遞。得出這種結(jié)論的前提必須是“參數(shù)的值就是對(duì)該對(duì)象的引用， 而不是對(duì)象的內(nèi)容”，這句話可能有些費(fèi)解，舉個(gè)例子加以說(shuō)明。public class Paier public static void main（S tringU args） Paier paier = new PaierO； paier. tes

3、tO；public void testO Test Class paral = new Test ClassO； para1.se tTes t( new Int eger(1O)；Test Class resu lt1 = test1 (para1)；Sys tem.ou t.prin tln(para1 = + para1.ge tTes tO)；Sys tem.ou t.pri nt ln(resul t1 = + resul t1.ge tTes tO)；Test Class para2 = new Test ClassO； para2.se tTes t( new Int eger(

4、1O)；Tes tClass resu lt2 = tes t2(para2)；Sys tem.ou t.prin tln(para2 = + para2.ge tTes tO)；Sys tem.ou t.pri nt ln(resul t2 = + resul t2.ge tTes tO)；public Tes tClass tes t1 (Tes tClass t) t = new Test ClassO；t .se tTes t(new Int eger(20)； return t；public Tes tClass tes t2(Tes tClass t) t .se tTes t(n

5、ew Int eger(20)； return t；class Test Class Int eger test = null；public void set Tes t(In teger i) test = i；public Int eger get Tes t() ret urn test；執(zhí)行后的結(jié)果是：paral = 10resu lt1 = 20para2 = 20resu lt2 = 20為什么會(huì)這樣呢？因?yàn)閠estl想通過(guò)修改參數(shù)的引用來(lái)修改返回值,但是在JAVA中， 參數(shù)的引用是不可修改的，所以paral和result1分別指向不同的空間，結(jié)果也 不一樣。而在test2中，re

6、sult2和para2始終指向同一塊區(qū)域，test2方法修改的 是參數(shù)內(nèi)容，而不是參數(shù)的引用。從上面看來(lái)，因?yàn)閰?shù)的引用不可改變，如果理解為“參數(shù)的值就是對(duì)該對(duì)象的 引用”，那么java自然只有值傳遞。二,認(rèn)為是引用傳遞。還是上面的例子，如果在參數(shù)傳遞時(shí)理解為“參數(shù)的值就 是該對(duì)象的內(nèi)容”，那么顯然不是值傳遞，因?yàn)閷?duì)象的內(nèi)容已經(jīng)改變了。認(rèn)為是引用傳遞還有一個(gè)理由，就是java有一個(gè)保留字byvalue，現(xiàn)在的JDK 版本中還沒(méi)有實(shí)現(xiàn)這個(gè)保留字，似乎是在暗示對(duì)這種觀點(diǎn)的支持。(Thereappears to be some support for this view within Sun, si

7、nce one of thereserved but not implemented” keywords is byvalue.)所以說(shuō)，對(duì)于原題的結(jié)論，是值傳遞還是引用傳遞并不重要，重要的是要理解“對(duì) 象的內(nèi)容可以在被調(diào)用的方法中改變，但對(duì)象的引用是永遠(yuǎn)不會(huì)改變的?！弊⒔?：下面是在Thinking in Java中的原文：This brings up the terminology issue, which always seems good for an argument.The term is “pass by value, ” and the meaning depends on h

8、ow you perceive the opera tion of the program. The general meaning is that you get a local copy of whatever you re passing, but the real question is how you think about what youre passing. When it comes to the meaning of “pass by value,” t here are two fairly dis tinct camps:Java passes ever yt hing

9、 by value. When youre passing primi ti ves into a met hod, you get a distinct copy of the primi ti ve. When you re passing a handle into a met hod, you get a copy of the handle. Ergo, ever yt hing is pass by value. Of course, the assump tion is that youre always t hinking (and caring) that handles a

10、re being passed, but it seems like the Java design has gone a long way to ward allowing you to ignore (mos t of the ti me) that youre working with a handle. Tha t is, it seems to allow you to t hink of the handle as “the object,” since it implicitly dereferences it whenever you make a met hod call.J

11、ava passes primi ti ves by value (no argumen t t here), but objec ts are passed by reference. This is the world view that the handle is an alias for the object, so you don11hink about passing handles, but instead say “Im passing the objec t.” Since you dont get a local copy of the objec t when you p

12、ass it into a met hod, objec ts are clearly not passed by value. There appears to be some suppor t for t his view wi thin Sun, since one of the “reserved but not implemented” keywords is byvalue. (Theres no knowing, however, whet her that keyword will ever see the ligh t of day.)Having given both camps a good airing and after saying “It depends on how you think of a handle, ” I will attempt to sidest