java String拼接的方法選擇及效能分析

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String 拼接的方法選擇

在拼接靜態字串時，盡量用 +，因為通常編譯器會對此做最佳化，如：

 String test = "this " + "is " + "a " + "test " + "string"

編譯器會把它視為：

 String test = "this is a test string"

在拼接動態字串時，盡量用 StringBuffer 或 StringBuilder的 append，這樣可以減少構造過多的臨時 String 對象。

 

測試代碼：（按照附錄1修改）

 1 public class teststring { 2     3     public void testPlus() { 4         String s = ""; 5         long ts = System.currentTimeMillis(); 6         for (int i = 0; i < 10000; i++) { 7             s = s + String.valueOf(i); 8         } 9         long te = System.currentTimeMillis();10         System.out.println("+ cost {} ms  "+( te - ts));11     }12     13     public void testConcat() {14         String s = "";15         long ts = System.currentTimeMillis();16         for (int i = 0; i < 10000; i++) {17             s = s.concat(String.valueOf(i));18         }19         long te = System.currentTimeMillis();20         System.out.println("concat cost {} ms  "+ (te - ts));21     }22 23     24     public void testStringBuffer() {25         StringBuffer sb = new StringBuffer();26         long ts = System.currentTimeMillis();27         for (int i = 0; i < 10000; i++) {28             sb.append(String.valueOf(i));29         }30         sb.toString();31         long te = System.currentTimeMillis();32         System.out.println("StringBuffer cost {} ms  "+(te - ts));33     }34     35     public void testStringBuilder() {36         StringBuilder sb = new StringBuilder();37         long ts = System.currentTimeMillis();38         for (int i = 0; i < 100000; i++) {39             sb.append(String.valueOf(i));40         }41         sb.toString();42         long te = System.currentTimeMillis();43         System.out.println("StringBuilder cost {} ms  "+(te - ts));44     }45     46     public static void main(String[] args) {47         teststring a = new teststring();48         a.testConcat();49         a.testPlus();50         a.testStringBuffer();51         a.testStringBuilder();52     }53 }

 

運行結果：

concat cost {} ms 113
+ cost {} ms 195
StringBuffer cost {} ms 2
StringBuilder cost {} ms 9

 

附錄：

1 Java 5種字串拼接方式效能比較 http://blog.csdn.net/kimsoft/article/details/3353849

2 Java 效能最佳化之 String 篇  http://www.ibm.com/developerworks/cn/java/j-lo-optmizestring/

java String拼接的方法選擇及效能分析