In quite a few cases I have seen myself designing code with listeners and callbacks. It is quite common for a class that emits events, to expose an API to attach listener(s) to it. Those listeners are usually stored in a data structure (see List, Set, Array) and when an event is about to be dispatched the listeners are iterated in a loop and the appropriate callback is called.
Something along the lines of:
public class EventDispatcher {
private final List<Listener> listeners = new ArrayList<>();
public void dispatchEvent() {
final MyEvent event = new MyEvent();
for (Listener listener : this.listeners) {
listener.onEvent(event);
}
}
public void attachListener(final Listener listener) {
this.listeners.add(listener);
}
public void removeListener(final Listener listener) {
this.listeners.remove(listener);
}
}
public static class Listener implements EventListener {
void onEvent(final MyEvent myEvent) {
// do staff
}
}
public static class MyEvent {
}
In many cases I have observed that despite the fact that the class is desinged to accept many listeners, the true is actually that just one listener is attached in the majority of the cases.
Hence I wanted to measure the performance penalty paid in case the class had just one listener vs if the class was initially designed to accept just one listener.
In essence I wanted to check the performance impact on the below two cases.
private Listener listener;
private final List<Listener> singleElementArray = new ArrayList<Listener>(){
{add(new Listener());}
};
public void dispatch() {
this.listener.onEvent(new MyEvent());
}
public void dispatchInLoop() {
for (int i = 0; i < 1; i++) {
this.singleElementArray.get(i).onEvent(new MyEvent());
}
}
Assumptions Made Prior To Testing
Before creating a benchmark for the above, I made some assumptions:
- I assumed the single element (single listener in a data container) loop would be unrolled
- I (wrongly) assumed that the performane cost will not be significant. As effectively with the loop unrolled I would think the native code produced would more or less look close enough
JMH Benchmark
In order to test my assumptions I created the below benchmark:
SingleElementLoopBenchmark.java
Initial Observations
To my surprise I found out that an invocaiton on a single element list was about ~2,5 slower, based on the below throughput numbers:
Benchmark Mode Cnt Score Error Units
SingleElementLoopBenchmark.directInvocation thrpt 10 0.317 ± 0.022 ops/ns
SingleElementLoopBenchmark.singleElementListLoopInvocation thrpt 10 0.114 ± 0.010 ops/ns
I couldn’t really understand why and the above seemed a bit too far from my expecations/assumptions.
The first thing that I verified with JVM argument -XX:+PrintCompilation was that both methods were compiled with C2 compiler, which was the case.
I also tried to print the assembly code with -XX:+PrintAssembly but I couldn’t really read/interpret the assembly code.
Resorting to Social Media
I ended up posting a tweet about my findings and asking some pointer on where/how to look for explanations on what I was observing. The answer I got was to try to find the hot methods by using something like perfasm, which would tie the assembly output to the hottest methods of my benchmark.
Which I did with -prof dtraceasm (The benchmark was running on a Mac that’s why I used dtrace). The output was the below:
Direct Invocation
9.56% ↗ 0x000000010b73c950: mov 0x40(%rsp),%r10
1.00% │ 0x000000010b73c955: mov 0xc(%r10),%r10d ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@1 (line 23)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
0.17% │ 0x000000010b73c959: mov 0xc(%r12,%r10,8),%r11d ; implicit exception: dispatches to 0x000000010b73ca12
11.18% │ 0x000000010b73c95e: test %r11d,%r11d
0.00% │ 0x000000010b73c961: je 0x000000010b73c9c9 ;*invokevirtual performAction {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invoke@5 (line 40)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@5 (line 23)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
10.69% │ 0x000000010b73c963: mov %r9,(%rsp)
0.65% │ 0x000000010b73c967: mov 0x38(%rsp),%rsi
0.00% │ 0x000000010b73c96c: mov $0x1,%edx
0.14% │ 0x000000010b73c971: xchg %ax,%ax
10.08% │ 0x000000010b73c973: callq 0x000000010b6c2900 ; ImmutableOopMap{[48]=Oop [56]=Oop [64]=Oop [0]=Oop }
│ ;*invokevirtual consume {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Listener::performAction@2 (line 53)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invoke@5 (line 40)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@5 (line 23)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
│ ; {optimized virtual_call}
1.44% │ 0x000000010b73c978: mov (%rsp),%r9
0.19% │ 0x000000010b73c97c: movzbl 0x94(%r9),%r8d ;*ifeq {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@30 (line 123)
9.77% │ 0x000000010b73c984: mov 0x108(%r15),%r10
0.99% │ 0x000000010b73c98b: add $0x1,%rbp ; ImmutableOopMap{r9=Oop [48]=Oop [56]=Oop [64]=Oop }
│ ;*ifeq {reexecute=1 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@30 (line 123)
0.02% │ 0x000000010b73c98f: test %eax,(%r10) ; {poll}
0.28% │ 0x000000010b73c992: test %r8d,%r8d
0.00% ╰ 0x000000010b73c995: je 0x000000010b73c950 ;*aload_1 {reexecute=0 rethrow=0 return_oop=0}
Single Element Loop Invocation
╭ 0x000000011153fa9d: jmp 0x000000011153fad6
0.19% │ ↗ 0x000000011153fa9f: mov 0x58(%rsp),%r13
3.55% │ │ 0x000000011153faa4: mov (%rsp),%rcx
0.09% │ │ 0x000000011153faa8: mov 0x60(%rsp),%rdx
0.22% │ │ 0x000000011153faad: mov 0x50(%rsp),%r11
0.17% │ │ 0x000000011153fab2: mov 0x8(%rsp),%rbx ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
│ │ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
│ │ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ │ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
3.55% │↗│ 0x000000011153fab7: movzbl 0x94(%r11),%r8d ;*goto {reexecute=0 rethrow=0 return_oop=0}
│││ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@35 (line 44)
│││ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.16% │││ 0x000000011153fabf: mov 0x108(%r15),%r10
0.28% │││ 0x000000011153fac6: add $0x1,%rbx ; ImmutableOopMap{r11=Oop rcx=Oop rdx=Oop r13=Oop }
│││ ;*ifeq {reexecute=1 rethrow=0 return_oop=0}
│││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@30 (line 123)
0.19% │││ 0x000000011153faca: test %eax,(%r10) ; {poll}
4.00% │││ 0x000000011153facd: test %r8d,%r8d
│││ 0x000000011153fad0: jne 0x000000011153fbe9 ;*aload_1 {reexecute=0 rethrow=0 return_oop=0}
│││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@33 (line 124)
0.07% ↘││ 0x000000011153fad6: mov 0xc(%rcx),%r8d ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
││ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@1 (line 28)
││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.22% ││ 0x000000011153fada: mov 0x10(%r12,%r8,8),%r10d ;*getfield singleListenerList {reexecute=0 rethrow=0 return_oop=0}
││ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@4 (line 44)
││ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
││ ; implicit exception: dispatches to 0x000000011153ff2a
0.21% ││ 0x000000011153fadf: mov 0x8(%r12,%r10,8),%edi ; implicit exception: dispatches to 0x000000011153ff3e
4.39% ││ 0x000000011153fae4: cmp $0x237565,%edi ; {metadata('com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1')}
││ 0x000000011153faea: jne 0x000000011153fc92
0.33% ││ 0x000000011153faf0: lea (%r12,%r10,8),%r9 ;*invokeinterface size {reexecute=0 rethrow=0 return_oop=0}
││ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@7 (line 44)
││ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
││ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.09% ││ 0x000000011153faf4: mov 0x10(%r9),%r9d
0.14% ││ 0x000000011153faf8: test %r9d,%r9d
╰│ 0x000000011153fafb: jle 0x000000011153fab7 ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
3.98% │ 0x000000011153fafd: lea (%r12,%r8,8),%rdi ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@1 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.06% │ 0x000000011153fb01: xor %r9d,%r9d ;*aload_0 {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@15 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.09% │ 0x000000011153fb04: mov 0x8(%r12,%r10,8),%esi ; implicit exception: dispatches to 0x000000011153ff4e
0.06% │ 0x000000011153fb09: cmp $0x237565,%esi ; {metadata('com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1')}
0.00% │ 0x000000011153fb0f: jne 0x000000011153fcc2
3.93% │ 0x000000011153fb15: lea (%r12,%r10,8),%rax ;*invokeinterface get {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.06% │ 0x000000011153fb19: mov 0x10(%rax),%r10d ;*getfield size {reexecute=0 rethrow=0 return_oop=0}
│ ; - java.util.ArrayList::get@2 (line 458)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.11% │ 0x000000011153fb1d: test %r10d,%r10d
│ 0x000000011153fb20: jl 0x000000011153fcf6 ;*invokestatic checkIndex {reexecute=0 rethrow=0 return_oop=0}
│ ; - java.util.Objects::checkIndex@3 (line 372)
│ ; - java.util.ArrayList::get@5 (line 458)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.28% │ 0x000000011153fb26: cmp %r10d,%r9d
0.00% │ 0x000000011153fb29: jae 0x000000011153fc1c
3.97% │ 0x000000011153fb2f: mov 0x14(%rax),%r10d ;*getfield elementData {reexecute=0 rethrow=0 return_oop=0}
│ ; - java.util.ArrayList::elementData@1 (line 442)
│ ; - java.util.ArrayList::get@11 (line 459)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.05% │ 0x000000011153fb33: mov %r9d,%ebp ;*invokestatic checkIndex {reexecute=0 rethrow=0 return_oop=0}
│ ; - java.util.Objects::checkIndex@3 (line 372)
│ ; - java.util.ArrayList::get@5 (line 458)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.08% │ 0x000000011153fb36: mov 0xc(%r12,%r10,8),%esi ; implicit exception: dispatches to 0x000000011153ff62
1.27% │ 0x000000011153fb3b: cmp %esi,%ebp
│ 0x000000011153fb3d: jae 0x000000011153fc5a
3.94% │ 0x000000011153fb43: shl $0x3,%r10
0.05% │ 0x000000011153fb47: mov 0x10(%r10,%rbp,4),%r9d ;*aaload {reexecute=0 rethrow=0 return_oop=0}
│ ; - java.util.ArrayList::elementData@5 (line 442)
│ ; - java.util.ArrayList::get@11 (line 459)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
1.71% │ 0x000000011153fb4c: mov 0x8(%r12,%r9,8),%r10d ; implicit exception: dispatches to 0x000000011153ff72
17.85% │ 0x000000011153fb51: cmp $0x237522,%r10d ; {metadata('com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Listener')}
0.00% │ 0x000000011153fb58: jne 0x000000011153fef6 ;*checkcast {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@25 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
3.79% │ 0x000000011153fb5e: mov %rdi,0x18(%rsp)
0.02% │ 0x000000011153fb63: mov %r8d,0x10(%rsp)
0.02% │ 0x000000011153fb68: mov %rbx,0x8(%rsp)
0.19% │ 0x000000011153fb6d: mov %r11,0x50(%rsp)
3.95% │ 0x000000011153fb72: mov %rdx,0x60(%rsp)
0.02% │ 0x000000011153fb77: mov %rcx,(%rsp)
0.03% │ 0x000000011153fb7b: mov %r13,0x58(%rsp)
0.36% │ 0x000000011153fb80: mov %rdx,%rsi
3.78% │ 0x000000011153fb83: mov $0x1,%edx
0.01% │ 0x000000011153fb88: vzeroupper
4.05% │ 0x000000011153fb8b: callq 0x00000001114c2900 ; ImmutableOopMap{[80]=Oop [88]=Oop [96]=Oop [0]=Oop [16]=NarrowOop [24]=Oop }
│ ;*invokevirtual consume {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Listener::performAction@2 (line 53)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@29 (line 45)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
│ ; {optimized virtual_call}
0.98% │ 0x000000011153fb90: mov 0x10(%rsp),%r8d
3.61% │ 0x000000011153fb95: mov 0x10(%r12,%r8,8),%r10d ;*getfield singleListenerList {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@4 (line 44)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.24% │ 0x000000011153fb9a: mov 0x8(%r12,%r10,8),%r9d ; implicit exception: dispatches to 0x000000011153ff9e
0.74% │ 0x000000011153fb9f: inc %ebp ;*iinc {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@32 (line 44)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.04% │ 0x000000011153fba1: cmp $0x237565,%r9d ; {metadata('com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1')}
0.00% │ 0x000000011153fba8: jne 0x000000011153fd36
3.60% │ 0x000000011153fbae: lea (%r12,%r10,8),%r11 ;*invokeinterface size {reexecute=0 rethrow=0 return_oop=0}
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@7 (line 44)
│ ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
│ ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
0.11% │ 0x000000011153fbb2: mov 0x10(%r11),%r9d
0.35% │ 0x000000011153fbb6: cmp %r9d,%ebp
╰ 0x000000011153fbb9: jge 0x000000011153fa9f ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
As I said I am not really able to read/interpret assembly code, but in between the lines I could see that:
- The loop was indeed unrolled
- A penalty was paid to cast the item to the expected type (17.85% of the CPU instructions)
- A penalty was paid to fetch the item from the list, underlying array
In order to get some advice from someone knowledgable on this I posted the below question on StackOverflow. The answer is pretty comprehensive, as the person who answered is one of the most prominent names in JVM community
StackOveflow: Java Method Direct Invocation vs Single Element Loop
Conclusion/Observations
In summary:
- The loop was indeed unrolled, as expected and as seen from the assembly code
- The main penalty paid is for fetching the element from the list and casting it to the expected type
- Some cost is also because of checks performed on the data container itself (i.e. size)
- In general the extra cost been paid is memory access cost, rather than CPU instructions cost
As seen in the SO answer, Andrei makes the point that invoking the object’s method from inside the loop is not ~2,5 times slower, but rather 3 ns slower, if we look it from a perspective of latency (ns/op) rather than throughput (ops/ns). This is a valid point, but I am not sure If i aggree 100%, as in some applications, depending on the nature, that extra cost will actually translate in ~2,5.
Finally, I have added in the JMH Benchmark test, tests for different data container types:
- Array
- List
- Set
Observing the numbers of those, and as expected, an array is faster than the rest. The array is typed, hence the casting cost is not paid. The array underlying an array list is of type Object, hence the need for casting to the list’s type.