Stack. ?用來在ram中存放數(shù)據(jù)的地方 This ? lives ? in ? the ? general ? RAM ? (random-access ? memory) ? area, ? but????has ? direct ? support ? from ? the ? processor ? via ? its ? stack ? pointer. ? The ? stack ? pointer ? is ? moved ? down ? to ? create ? new ? memory ? and ? moved ? up ? to ? release ? that ? memory. ? This ? is ? an ? extremely ? fast ? and ? efficient ? way ? to ? allocate ? storage, ? second ? only ? to ? registers. ? The ? Java ? compiler ? must ? know, ? while ? it ? is ? creating ? the ? program, ? the ? exact ? size ? and ? lifetime ? of ? all ? the ? data ? that ? is ? stored ? on ? the ? stack, ? because ? it ? must ? generate ? the ? code ? to ? move ? the ? stack ? pointer ? up ? and ? down. ? This ? constraint ? places ? limits ? on ? the ? flexibility ? of ? your ? programs, ? so ? while ? some ? Java ? storage ? exists ? on ? the ? stack ? ?in ? particular, ? object ? handles ? ?Java ? objects ? are ? not ? placed ? on ? the ? stack. ? ?
? ?
? Heap. ? This ? is ? a ? general-purpose ? pool ? of ? memory ? (also ? in ? the ? RAM ? area) ? where ? all ? Java ? objects ? live. ? The ? nice ? thing ? about ? the ? heap ? is ? that, ? unlike ? the ? stack, ? the ? compiler ? doesn't ? need ? to ? know ? how ? much ? storage ? it ? needs ? to ? allocate ? from ? the ? heap ? or ? how ? long ? that ? storage ? must ? stay ? on ? the ? heap. ? Thus, ? there's ? a ? great ? deal ? of ? flexibility ? in ? using ? storage ? on ? the ? heap. ? Whenever ? you ? need ? to ? create ? an ? object, ? you ? simply ? write ? the ? code ? to ? create ? it ? using ? new ? and ? the ? storage ? is ? allocated ? on ? the ? heap ? when ? that ? code ? is ? executed. ? And ? of ? course ? there's ? a ? price ? you ? pay ? for ? this ? flexibility: ? it ? takes ? more ? time ? to ? allocate ? heap ? storage.??
. 棧(stack)與堆(heap)都是Java用來在Ram中存放數(shù)據(jù)的地方。
2. 棧的優(yōu)勢是，存取速度比堆要快，僅次于直接位于CPU中的寄存器。但缺點是，存在棧中的數(shù)據(jù)大小與生存期必須是確定的，缺乏靈活性。另外，棧數(shù)據(jù)可以共享，詳見第3點。堆的優(yōu)勢是可以動態(tài)地分配內(nèi)存大小，生存期也不必事先告訴編譯器，Java的垃圾收集器會自動收走這些不再使用的數(shù)據(jù)。但缺點是，由于要在運行時動態(tài)分配內(nèi)存，存取速度較慢。
3. Java中的數(shù)據(jù)類型有兩種。基本類型(primitive types), 共有8種，即int, short, long, byte, float, double, boolean, char。存在于棧中。另一種是包裝類數(shù)據(jù)，如Integer, String, Double等將相應(yīng)的基本數(shù)據(jù)類型包裝起來的類。這些類數(shù)據(jù)全部存在于堆中.

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