如何測試字符串數組搜索的速度？

Question

我編寫了一個Java類的程序，我正在爲一個特定的目標搜索字符串數組。程序從數組的開始到數組的結尾搜索目標，然後從數組的末尾搜索數組的開始。我應該測試兩種搜索的速度，以查看哪一個更快。我如何測試這個？

下面是程序：

public class LinearStringSearch { 

    // Array filled with random Strings 
    String[] randomStrings = {"apple", "yellow", "fire", "wood", "zinc", 
      "ram", "mouse", "fish", "cheese", "dirt"}; 

    // Save target arguments for global access(console printing purposes) 
    String target; 
    String target2; 

    /** 
    * 
    * @param target - the item you want to retrieve from array 
    * @param sa - the name of the array 
    * @return i - the target, otherwise return error code: -1 
    */ 
    int linearStringSearch(String target, String[] sa) { 
      this.target = target; // set class variable for print access 
      for(int i = 0; i < sa.length; ++i) { 
      System.out.println("Searching array position: " + i); 
      if (sa[i].equals(target)) { 
       // System.out.println("Target found! "); 
       return i; 
      } 
      } 
      return -1; 
    } 

    /** 
    * 
    * @param target - the item you want to retrieve from array 
    * @param sa - the name of the array 
    * @return i - the target, otherwise return error code: -1 
    */ 
    int backwardLinearStringSearch(String target, String[] sa) { 
     this.target2 = target; // set class variable for print access 
     for(int i = 9; i < sa.length; --i) { 
      System.out.println("Searching array position: " + i); 
      if (sa[i].equals(target)) { 
       return i; 
      } 
     } 
     return -1; // -1 means that target was not found 
    } 

    /* 
     * The target string is searched from the beginning of the array to the end of the array, then 
     * from the end of the array to the beginning of the array. 
     */ 
    public static void main(String[] args) { 

     LinearStringSearch lss = new LinearStringSearch(); 

     // Search array from beginning to end 
     System.out.println("Linear search: ");       // Print title 
     int index = lss.linearStringSearch("mouse", lss.randomStrings); // Pass arguments 
     System.out.println("The target " + "'" + lss.target + "'" +  // Print to console 
     " found at array index: "+index);  

     // Search array from end to beginning 
     System.out.println("\nBackwards linear search: ");      // Print title 
     int index2 = lss.backwardLinearStringSearch("mouse", lss.randomStrings); // Pass arguments 
     System.out.println("The target " + "'" + lss.target2 + "'" +    // Print to console 
     " found at array index: "+index2); 
    } 
} 

這裏是輸出：

Linear search: 
Searching array position: 0 
Searching array position: 1 
Searching array position: 2 
Searching array position: 3 
Searching array position: 4 
Searching array position: 5 
Searching array position: 6 
The target 'mouse' found at array index: 6 

Backwards linear search: 
Searching array position: 9 
Searching array position: 8 
Searching array position: 7 
Searching array position: 6 
The target 'mouse' found at array index: 6 

Answer 1

JVM是很複雜的，所以如果你想要得到準確和真實的結果，你必須要記住JIT影響。這意味着您需要測試已由JIT優化的代碼，並且在方法計時期間不會更改。所以你必須寫microbenchmark - 你可以使用庫如Caliper或JMH。在這種情況下，卡尺它看起來就像是：

public class MyBenchmark extends Benchmark { 
    public void timeMyOperation(int reps) { 
     for (int i = 0; i < reps; i++) { 
     int index = lss.linearStringSearch("mouse", lss.randomStrings);; 
     } 
    } 
} 

Answer 2

看看Java Performance Testing。 System.currentTimeMillis()或更好getCurrentThreadCpuTime()是你的朋友。如果時差太小，請考慮多次運行每個測試並比較需要多長時間。