Worrying about my开发者_StackOverflow社区 web application's performances, I am wondering which of "if/else" or switch statement is better regarding performance?
I totally agree with the opinion that premature optimization is something to avoid.
But it's true that the Java VM has special bytecodes which could be used for switch()'s.
See WM Spec (lookupswitch and tableswitch)
So there could be some performance gains, if the code is part of the performance CPU graph.
That's micro optimization and premature optimization, which are evil. Rather worry about readabililty and maintainability of the code in question. If there are more than two if/else blocks glued together or its size is unpredictable, then you may highly consider a switch statement.
Alternatively, you can also grab Polymorphism. First create some interface:
public interface Action {
void execute(String input);
}
And get hold of all implementations in some Map. You can do this either statically or dynamically:
Map<String, Action> actions = new HashMap<String, Action>();
Finally replace the if/else or switch by something like this (leaving trivial checks like nullpointers aside):
actions.get(name).execute(input);
It might be microslower than if/else or switch, but the code is at least far better maintainable.
As you're talking about webapplications, you can make use of HttpServletRequest#getPathInfo() as action key (eventually write some more code to split the last part of pathinfo away in a loop until an action is found). You can find here similar answers:
- Using a custom Servlet oriented framework, too many servlets, is this an issue
- Java Front Controller
If you're worrying about Java EE webapplication performance in general, then you may find this article useful as well. There are other areas which gives a much more performance gain than only (micro)optimizing the raw Java code.
It's extremely unlikely that an if/else or a switch is going to be the source of your performance woes. If you're having performance problems, you should do a performance profiling analysis first to determine where the slow spots are. Premature optimization is the root of all evil!
Nevertheless, it's possible to talk about the relative performance of switch vs. if/else with the Java compiler optimizations. First note that in Java, switch statements operate on a very limited domain -- integers. In general, you can view a switch statement as follows:
switch (<condition>) {
case c_0: ...
case c_1: ...
...
case c_n: ...
default: ...
}
where c_0, c_1, ..., and c_N are integral numbers that are targets of the switch statement, and <condition> must resolve to an integer expression.
If this set is "dense" -- that is, (max(ci) + 1 - min(ci)) / n > α, where 0 < k < α < 1, where
kis larger than some empirical value, a jump table can be generated, which is highly efficient.If this set is not very dense, but n >= β, a binary search tree can find the target in O(2 * log(n)) which is still efficient too.
For all other cases, a switch statement is exactly as efficient as the equivalent series of if/else statements. The precise values of α and β depend on a number of factors and are determined by the compiler's code-optimization module.
Finally, of course, if the domain of <condition> is not the integers, a switch
statement is completely useless.
Use switch!
I hate to maintain if-else-blocks! Have a test:
public class SpeedTestSwitch
{
private static void do1(int loop)
{
int temp = 0;
for (; loop > 0; --loop)
{
int r = (int) (Math.random() * 10);
switch (r)
{
case 0:
temp = 9;
break;
case 1:
temp = 8;
break;
case 2:
temp = 7;
break;
case 3:
temp = 6;
break;
case 4:
temp = 5;
break;
case 5:
temp = 4;
break;
case 6:
temp = 3;
break;
case 7:
temp = 2;
break;
case 8:
temp = 1;
break;
case 9:
temp = 0;
break;
}
}
System.out.println("ignore: " + temp);
}
private static void do2(int loop)
{
int temp = 0;
for (; loop > 0; --loop)
{
int r = (int) (Math.random() * 10);
if (r == 0)
temp = 9;
else
if (r == 1)
temp = 8;
else
if (r == 2)
temp = 7;
else
if (r == 3)
temp = 6;
else
if (r == 4)
temp = 5;
else
if (r == 5)
temp = 4;
else
if (r == 6)
temp = 3;
else
if (r == 7)
temp = 2;
else
if (r == 8)
temp = 1;
else
if (r == 9)
temp = 0;
}
System.out.println("ignore: " + temp);
}
public static void main(String[] args)
{
long time;
int loop = 1 * 100 * 1000 * 1000;
System.out.println("warming up...");
do1(loop / 100);
do2(loop / 100);
System.out.println("start");
// run 1
System.out.println("switch:");
time = System.currentTimeMillis();
do1(loop);
System.out.println(" -> time needed: " + (System.currentTimeMillis() - time));
// run 2
System.out.println("if/else:");
time = System.currentTimeMillis();
do2(loop);
System.out.println(" -> time needed: " + (System.currentTimeMillis() - time));
}
}
My C# standard code for benchmarking
I remember reading that there are 2 kinds of Switch statements in Java bytecode. (I think it was in 'Java Performance Tuning' One is a very fast implementation which uses the switch statement's integer values to know the offset of the code to be executed. This would require all integers to be consecutive and in a well-defined range. I'm guessing that using all the values of an Enum would fall in that category too.
I agree with many other posters though... it may be premature to worry about this, unless this is very very hot code.
According to Cliff Click in his 2009 Java One talk A Crash Course in Modern Hardware:
Today, performance is dominated by patterns of memory access. Cache misses dominate – memory is the new disk. [Slide 65]
You can get his full slides here.
Cliff gives an example (finishing on Slide 30) showing that even with the CPU doing register-renaming, branch prediction, and speculative execution, it's only able to start 7 operations in 4 clock cycles before having to block due to two cache misses which take 300 clock cycles to return.
So he says to speed up your program you shouldn't be looking at this sort of minor issue, but on larger ones such as whether you're making unnecessary data format conversions, such as converting "SOAP → XML → DOM → SQL → …" which "passes all the data through the cache".
In my test the better performance is ENUM > MAP > SWITCH > IF/ELSE IF in Windows7.
import java.util.HashMap;
import java.util.Map;
public class StringsInSwitch {
public static void main(String[] args) {
String doSomething = null;
//METHOD_1 : SWITCH
long start = System.currentTimeMillis();
for (int i = 0; i < 99999999; i++) {
String input = "Hello World" + (i & 0xF);
switch (input) {
case "Hello World0":
doSomething = "Hello World0";
break;
case "Hello World1":
doSomething = "Hello World0";
break;
case "Hello World2":
doSomething = "Hello World0";
break;
case "Hello World3":
doSomething = "Hello World0";
break;
case "Hello World4":
doSomething = "Hello World0";
break;
case "Hello World5":
doSomething = "Hello World0";
break;
case "Hello World6":
doSomething = "Hello World0";
break;
case "Hello World7":
doSomething = "Hello World0";
break;
case "Hello World8":
doSomething = "Hello World0";
break;
case "Hello World9":
doSomething = "Hello World0";
break;
case "Hello World10":
doSomething = "Hello World0";
break;
case "Hello World11":
doSomething = "Hello World0";
break;
case "Hello World12":
doSomething = "Hello World0";
break;
case "Hello World13":
doSomething = "Hello World0";
break;
case "Hello World14":
doSomething = "Hello World0";
break;
case "Hello World15":
doSomething = "Hello World0";
break;
}
}
System.out.println("Time taken for String in Switch :"+ (System.currentTimeMillis() - start));
//METHOD_2 : IF/ELSE IF
start = System.currentTimeMillis();
for (int i = 0; i < 99999999; i++) {
String input = "Hello World" + (i & 0xF);
if(input.equals("Hello World0")){
doSomething = "Hello World0";
} else if(input.equals("Hello World1")){
doSomething = "Hello World0";
} else if(input.equals("Hello World2")){
doSomething = "Hello World0";
} else if(input.equals("Hello World3")){
doSomething = "Hello World0";
} else if(input.equals("Hello World4")){
doSomething = "Hello World0";
} else if(input.equals("Hello World5")){
doSomething = "Hello World0";
} else if(input.equals("Hello World6")){
doSomething = "Hello World0";
} else if(input.equals("Hello World7")){
doSomething = "Hello World0";
} else if(input.equals("Hello World8")){
doSomething = "Hello World0";
} else if(input.equals("Hello World9")){
doSomething = "Hello World0";
} else if(input.equals("Hello World10")){
doSomething = "Hello World0";
} else if(input.equals("Hello World11")){
doSomething = "Hello World0";
} else if(input.equals("Hello World12")){
doSomething = "Hello World0";
} else if(input.equals("Hello World13")){
doSomething = "Hello World0";
} else if(input.equals("Hello World14")){
doSomething = "Hello World0";
} else if(input.equals("Hello World15")){
doSomething = "Hello World0";
}
}
System.out.println("Time taken for String in if/else if :"+ (System.currentTimeMillis() - start));
//METHOD_3 : MAP
//Create and build Map
Map<String, ExecutableClass> map = new HashMap<String, ExecutableClass>();
for (int i = 0; i <= 15; i++) {
String input = "Hello World" + (i & 0xF);
map.put(input, new ExecutableClass(){
public void execute(String doSomething){
doSomething = "Hello World0";
}
});
}
//Start test map
start = System.currentTimeMillis();
for (int i = 0; i < 99999999; i++) {
String input = "Hello World" + (i & 0xF);
map.get(input).execute(doSomething);
}
System.out.println("Time taken for String in Map :"+ (System.currentTimeMillis() - start));
//METHOD_4 : ENUM (This doesn't use muliple string with space.)
start = System.currentTimeMillis();
for (int i = 0; i < 99999999; i++) {
String input = "HW" + (i & 0xF);
HelloWorld.valueOf(input).execute(doSomething);
}
System.out.println("Time taken for String in ENUM :"+ (System.currentTimeMillis() - start));
}
}
interface ExecutableClass
{
public void execute(String doSomething);
}
// Enum version
enum HelloWorld {
HW0("Hello World0"), HW1("Hello World1"), HW2("Hello World2"), HW3(
"Hello World3"), HW4("Hello World4"), HW5("Hello World5"), HW6(
"Hello World6"), HW7("Hello World7"), HW8("Hello World8"), HW9(
"Hello World9"), HW10("Hello World10"), HW11("Hello World11"), HW12(
"Hello World12"), HW13("Hello World13"), HW14("Hello World4"), HW15(
"Hello World15");
private String name = null;
private HelloWorld(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void execute(String doSomething){
doSomething = "Hello World0";
}
public static HelloWorld fromString(String input) {
for (HelloWorld hw : HelloWorld.values()) {
if (input.equals(hw.getName())) {
return hw;
}
}
return null;
}
}
//Enum version for betterment on coding format compare to interface ExecutableClass
enum HelloWorld1 {
HW0("Hello World0") {
public void execute(String doSomething){
doSomething = "Hello World0";
}
},
HW1("Hello World1"){
public void execute(String doSomething){
doSomething = "Hello World0";
}
};
private String name = null;
private HelloWorld1(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void execute(String doSomething){
// super call, nothing here
}
}
/*
* http://stackoverflow.com/questions/338206/why-cant-i-switch-on-a-string
* https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-3.html#jvms-3.10
* http://forums.xkcd.com/viewtopic.php?f=11&t=33524
*/
For most switch and most if-then-else blocks, I can't imagine that there are any appreciable or significant performance related concerns.
But here's the thing: if you're using a switch block, its very use suggests that you're switching on a value taken from a set of constants known at compile time. In this case, you really shouldn't be using switch statements at all if you can use an enum with constant-specific methods.
Compared to a switch statement, an enum provides better type safety and code that is easier to maintain. Enums can be designed so that if a constant is added to the set of constants, your code won't compile without providing a constant-specific method for the new value. On the other hand, forgetting to add a new case to a switch block can sometimes only be caught at run time if you're lucky enough to have set your block up to throw an exception.
Performance between switch and an enum constant-specific method should not be significantly different, but the latter is more readable, safer, and easier to maintain.
A good explanation at the link below:
https://www.geeksforgeeks.org/switch-vs-else/
Test(c++17)
1 - If grouped
2 - If sequential
3 - Goto Array
4 - Switch Case - Jump Table
https://onlinegdb.com/Su7HNEBeG
![Interactive visualization of a graph in python [closed]](https://www.devze.com/res/2023/04-10/09/92d32fe8c0d22fb96bd6f6e8b7d1f457.gif)
加载中,请稍侯......
精彩评论