Java：String 與 StringBuffer 效能比較

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JDK Version: 1.8

跑出來的結果如下：

StringBuffer object 1455 milliseconds.

String object with string literal 9 milliseconds.

String object with String construtor 160 milliseconds.

String +operator 2448 milliseconds.

StringBuffer append method 2 milliseconds.

建立 StringBuffer object 會花比較多的時間，但StringBuffer在串聯字串時的效能遠比String 還快很多，另外還有個StringBuilder可用，詳細的比較內容可參考下列網址：

[1] http://java67.blogspot.tw/2015/05/4-ways-to-concatenate-strings-in-java.html

[2] http://javapapers.com/java/java-string-vs-stringbuilder-vs-stringbuffer-concatenation-performance-micro-benchmark/

本文中的測試程式碼：

public classStringVersusStringBuffer {

   public static void main(String args[]) {

      int ROUNDS = 20000000;

      long startMS, stopMS, elapsedMS;

      startMS = System.currentTimeMillis();

      for(int i = 0; i < ROUNDS; i++) {

          StringBuffer strBuffer = new StringBuffer("Hello");

      }

      stopMS = System.currentTimeMillis();

      elapsedMS = stopMS - startMS;

      System.out.println("Time for creating StringBuffer object:" + elapsedMS);

      startMS = System.currentTimeMillis();

      for(int i = 0; i < ROUNDS; i++) {

          String str = "Hello";

      }

      stopMS = System.currentTimeMillis();

      elapsedMS = stopMS - startMS;

     

      System.out.println("Time for creating String object with string literal:" + elapsedMS);

     

      startMS = System.currentTimeMillis();

      for(int i = 0; i < ROUNDS; i++) {

          String str = new String("Hello"); 

      }

      stopMS = System.currentTimeMillis();

      elapsedMS = stopMS - startMS;

      System.out.println("Time for creating String object with String construtor:" + elapsedMS);

     

      String helloStr = "Hello";

      StringBuffer helloStrBuffer = new StringBuffer("Hello");

     

      ROUNDS = 20000;

     

      startMS = System.currentTimeMillis();

      for(int i = 0; i < ROUNDS; i++) {

          helloStr += "Hello";

      }

      stopMS = System.currentTimeMillis();

      elapsedMS = stopMS - startMS;

      System.out.println("Time for String +operator:" + elapsedMS);

     

      startMS = System.currentTimeMillis();

      for(int i = 0; i < ROUNDS; i++) {

          helloStrBuffer.append("Hello");

      }

      stopMS = System.currentTimeMillis();

      elapsedMS = stopMS - startMS;

      System.out.println("Time for StringBuffer append method:" + elapsedMS);

   }

}