[英]Why is if-else for String faster than switch-case for enum?
由于Java 6没有String
switch-case
,我经常使用enum将if-else块更改为switch-case,如下面的代码所示。 然而,当我试图检查两种替代方案的性能时,我发现switch-case比if-else替代方案慢,这与我的预期相反。 以下是我在下面的代码中获得的一些结果
Iterations If-Else Switch-Case
1 11810 1609181
10 8214 1059115
100 24141 1152494
1000 183975 1580605
10000 4452698 8710648
100000 7069243 19457585
package conditionals;
import java.util.Random;
public class StringConditionalCheck {
private static int ifElseCounter = 0;
private static int switchCaseCounter = 0;
private static final String first = "First";
private static final String second = "Second";
private static final String third = "Third";
private static final String fourth = "Fourth";
enum StringOptions {
First, Second, Third, Fourth
}
public static void main(String[] args) {
final int iterations = Integer.parseInt(args[0]);
String[] userInputs = generateUserInputs(iterations);
// Using if-else
long ifelseStartTime = System.nanoTime();
for(int i=0; i<iterations; i++){
useIfElse(userInputs[i]);
}
long ifelseEndTime = System.nanoTime();
long ifElseDuration = ifelseEndTime - ifelseStartTime;
long switchcaseStartTime = System.nanoTime();
for(int i=0; i<iterations; i++){
useSwitchCase(userInputs[i]);
}
long switchcaseEndTime = System.nanoTime();
//just to verify that both options had the same result.
long switchcaseDuration = switchcaseEndTime - switchcaseStartTime;
System.out.println(iterations + " " + ifElseDuration + " " + switchcaseDuration + " " + ifElseCounter + " " + switchCaseCounter);
}
private static String[] generateUserInputs(int numberOfInputs) {
String[] generatedInputs = new String[numberOfInputs];
String[] inputsToChooseFrom = new String[]{first, second, third, fourth};
Random r = new Random();
for (int i = 0; i < numberOfInputs; i++) {
int choice = r.nextInt(4);
generatedInputs[i] = inputsToChooseFrom[choice];
}
return generatedInputs;
}
public static void useSwitchCase(String input) {
StringOptions option = StringOptions.valueOf(input);
switch(option){
case First:
switchCaseCounter += 1;
break;
case Second:
switchCaseCounter += 2;
break;
case Third:
switchCaseCounter += 3;
break;
case Fourth:
switchCaseCounter += 4;
break;
}
}
public static void useIfElse(String input) {
if(input.equals("First")){
ifElseCounter += 1;
}else if(input.equals("Second")){
ifElseCounter += 2;
}else if(input.equals("Third")){
ifElseCounter += 3;
}else if(input.equals("Fourth")){
ifElseCounter += 4;
}
}
}
造成这种差异的原因是什么? 我期待if-else
会变慢,因为平均会有更多的比较。
因为99%的时间都花在StringOptions option = StringOptions.valueOf(input);
,而不是在switch
基于源代码,StringOptions.valueOf调用执行几个复杂的操作,包括每次调用构建HashMap,而String.equals只是循环遍历字符串一次。
将Enum.valueOf (调用Enum.getConstantDirectory)的源代码与String.equals进行比较 。
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