我們常常用String的split()方法去分割字符串，有兩個地方值得注意：

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1. 當分隔符是句號時(".")，需要轉義：

由于String.split是基于正則表達式來分割字符串，而句號在正則表達式里表示任意字符。

//Wrong: //String[] words = tmp.split(".");//Correct: String[] words = tmp.split("\\.");

所以，假設分隔符在正則表達式里有一定的意義時，需要格外留心，必須將它們轉義才能達到分割的效果。

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2. 假設字符串最后有連續多個分隔符，且這些分隔符都需要被分割的話，需要調用split(String regex,int limit)這個方法：

String abc = "a,b,c,,,"; String[] str = abc.split(",");System.out.println(Arrays.toString(str)+" "+str.length);String[] str2 = abc.split(",",-1);System.out.println(Arrays.toString(str2)+" "+str2.length);

輸出如下：

[a, b, c] 3
[a, b, c, , , ] 6

需要輸出csv文件的時候，尤其需要注意。

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3. 假設需要快速分割字符串，split()并不是最有效的方法。在split()方法內，有如下的實現：

1 public String[] split(String regex, int limit) { 2 return Pattern.compile(regex).split(this, limit); 3 }

頻繁調用split()會不斷創建Pattern這個對象，因此可以這樣去實現，減少Pattern的創建：

1 //create the Pattern object outside the loop 2 Pattern pattern = Pattern.compile(" "); 3 4 for (int i = 0; i < 1000000; i++) 5 { 6 String[] split = pattern.split("Hello World", 0); 7 list.add(split); 8 }

另外split()也往往比indexOf()+subString()這個組合分割字符串要稍慢，詳情可看這個帖子。

我在本機做過測試，感覺indexOf()+subString()比split()快一倍：

1 public static void main(String[] args) { 2 StringBuilder sb = new StringBuilder(); 3 for (int i = 100000; i < 100000 + 60; i++) 4 sb.append(i).append(' '); 5 String sample = sb.toString(); 6 7 int runs = 100000; 8 for (int i = 0; i < 5; i++) { 9 { 10 long start = System.nanoTime(); 11 for (int r = 0; r < runs; r++) { 12 StringTokenizer st = new StringTokenizer(sample); 13 List<String> list = new ArrayList<String>(); 14 while (st.hasMoreTokens()) 15 list.add(st.nextToken()); 16 } 17 long time = System.nanoTime() - start; 18 System.out.printf("StringTokenizer took an average of %.1f us%n", time / runs 19 / 1000.0); 20 } 21 { 22 long start = System.nanoTime(); 23 Pattern spacePattern = Pattern.compile(" "); 24 for (int r = 0; r < runs; r++) { 25 List<String> list = Arrays.asList(spacePattern.split(sample, 0)); 26 } 27 long time = System.nanoTime() - start; 28 System.out.printf("Pattern.split took an average of %.1f us%n", time / runs 29 / 1000.0); 30 } 31 { 32 long start = System.nanoTime(); 33 for (int r = 0; r < runs; r++) { 34 List<String> list = new ArrayList<String>(); 35 int pos = 0, end; 36 while ((end = sample.indexOf(' ', pos)) >= 0) { 37 list.add(sample.substring(pos, end)); 38 pos = end + 1; 39 } 40 } 41 long time = System.nanoTime() - start; 42 System.out 43 .printf("indexOf loop took an average of %.1f us%n", time / runs / 1000.0); 44 } 45 } 46 }

在jdk1.7測試后，結果如下：

StringTokenizer took an average of 7.2 us
Pattern.split took an average of 7.9 us
indexOf loop took an average of 3.5 us

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StringTokenizer took an average of 6.8 us
Pattern.split took an average of 5.4 us
indexOf loop took an average of 3.1 us

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StringTokenizer took an average of 6.0 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.1 us

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StringTokenizer took an average of 5.9 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.1 us

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StringTokenizer took an average of 6.4 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.2 us

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本文完

轉載于:https://www.cnblogs.com/techyc/p/3709182.html

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