在Java中，迭代字符串中所有字符的最快方法是：String str = "a really, really long string";for (int i = 0, n = str.length(); i < n; i++) { char c = str.charAt(i);}或這個：char[] chars = str.toCharArray();for (int i = 0, n = chars.length; i < n; i++) { char c = chars[i];}編輯：我想知道的是，charAt在長時間的迭代過程中重復調用該方法的開銷是否小于還是大于toCharArray在開始時執(zhí)行一次單次調用然后在迭代過程中直接訪問數(shù)組的開銷。如果有人可以針對不同的字符串長度提供可靠的基準測試，那將是一個很好的選擇，不僅要考慮到JIT的預熱時間，JVM的啟動時間等，而且還不只是兩次調用的區(qū)別System.currentTimeMillis()。
3 回答
守著一只汪
TA貢獻1872條經(jīng)驗 獲得超4個贊
這只是您不應該擔心的微觀優(yōu)化。
char[] chars = str.toCharArray();
返回一個str字符數(shù)組的副本（在JDK中，它通過調用返回一個字符的副本System.arrayCopy）。
除此之外，str.charAt()僅檢查索引是否確實在范圍內(nèi)，并在數(shù)組索引內(nèi)返回一個字符。
第一個不會在JVM中創(chuàng)建額外的內(nèi)存。
蠱毒傳說
TA貢獻1895條經(jīng)驗 獲得超3個贊
如果需要處理大量數(shù)據(jù)，則不應在客戶端模式下使用JVM?？蛻舳四Ｊ讲⒎怯糜趦?yōu)化。
讓我們在客戶端模式和服務器模式下使用JVM比較@Saint Hill基準測試的結果。
Core2Quad Q6600 G0 @ 2.4GHz
JavaSE 1.7.0_40
另請參見：“ java -server”和“ java -client”之間的真正區(qū)別？
客戶模式：
len = 2: 111k charAt(i), 105k cbuff[i], 62k new[i], 17k field access. (chars/ms)
len = 4: 285k charAt(i), 166k cbuff[i], 114k new[i], 43k field access. (chars/ms)
len = 6: 315k charAt(i), 230k cbuff[i], 162k new[i], 69k field access. (chars/ms)
len = 8: 333k charAt(i), 275k cbuff[i], 181k new[i], 85k field access. (chars/ms)
len = 12: 342k charAt(i), 342k cbuff[i], 222k new[i], 117k field access. (chars/ms)
len = 16: 363k charAt(i), 347k cbuff[i], 275k new[i], 152k field access. (chars/ms)
len = 20: 363k charAt(i), 392k cbuff[i], 289k new[i], 180k field access. (chars/ms)
len = 24: 375k charAt(i), 428k cbuff[i], 311k new[i], 205k field access. (chars/ms)
len = 28: 378k charAt(i), 474k cbuff[i], 341k new[i], 233k field access. (chars/ms)
len = 32: 376k charAt(i), 492k cbuff[i], 340k new[i], 251k field access. (chars/ms)
len = 64: 374k charAt(i), 551k cbuff[i], 374k new[i], 367k field access. (chars/ms)
len = 128: 385k charAt(i), 624k cbuff[i], 415k new[i], 509k field access. (chars/ms)
len = 256: 390k charAt(i), 675k cbuff[i], 436k new[i], 619k field access. (chars/ms)
len = 512: 394k charAt(i), 703k cbuff[i], 439k new[i], 695k field access. (chars/ms)
len = 1024: 395k charAt(i), 718k cbuff[i], 462k new[i], 742k field access. (chars/ms)
len = 2048: 396k charAt(i), 725k cbuff[i], 471k new[i], 767k field access. (chars/ms)
len = 4096: 396k charAt(i), 727k cbuff[i], 459k new[i], 780k field access. (chars/ms)
len = 8192: 397k charAt(i), 712k cbuff[i], 446k new[i], 772k field access. (chars/ms)
服務器模式：
len = 2: 86k charAt(i), 41k cbuff[i], 46k new[i], 80k field access. (chars/ms)
len = 4: 571k charAt(i), 250k cbuff[i], 97k new[i], 222k field access. (chars/ms)
len = 6: 666k charAt(i), 333k cbuff[i], 125k new[i], 315k field access. (chars/ms)
len = 8: 800k charAt(i), 400k cbuff[i], 181k new[i], 380k field access. (chars/ms)
len = 12: 800k charAt(i), 521k cbuff[i], 260k new[i], 545k field access. (chars/ms)
len = 16: 800k charAt(i), 592k cbuff[i], 296k new[i], 640k field access. (chars/ms)
len = 20: 800k charAt(i), 666k cbuff[i], 408k new[i], 800k field access. (chars/ms)
len = 24: 800k charAt(i), 705k cbuff[i], 452k new[i], 800k field access. (chars/ms)
len = 28: 777k charAt(i), 736k cbuff[i], 368k new[i], 933k field access. (chars/ms)
len = 32: 800k charAt(i), 780k cbuff[i], 571k new[i], 969k field access. (chars/ms)
len = 64: 800k charAt(i), 901k cbuff[i], 800k new[i], 1306k field access. (chars/ms)
len = 128: 1084k charAt(i), 888k cbuff[i], 633k new[i], 1620k field access. (chars/ms)
len = 256: 1122k charAt(i), 966k cbuff[i], 729k new[i], 1790k field access. (chars/ms)
len = 512: 1163k charAt(i), 1007k cbuff[i], 676k new[i], 1910k field access. (chars/ms)
len = 1024: 1179k charAt(i), 1027k cbuff[i], 698k new[i], 1954k field access. (chars/ms)
len = 2048: 1184k charAt(i), 1043k cbuff[i], 732k new[i], 2007k field access. (chars/ms)
len = 4096: 1188k charAt(i), 1049k cbuff[i], 742k new[i], 2031k field access. (chars/ms)
len = 8192: 1157k charAt(i), 1032k cbuff[i], 723k new[i], 2048k field access. (chars/ms)
結論：
如您所見，服務器模式要快得多。
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