Shulou(Shulou.com)06/02 Report--

< 100; i++) { put(i, "val:" + i); } }}; public static void main(String[] args) throws RunnerException { // 启动基准测试 Options opt = new OptionsBuilder() // 要导入的测试类 .include(HashMapCycleTest.class.getSimpleName()) // 输出测试结果的文件 .output("D:/JAVA/面试/workplace/interview/jmh-hashMap.log") .build(); // 执行测试 new Runner(opt).run(); } /** * 迭代器 EntrySet */ @Benchmark public void entrySet() { // 遍历 Iterator iterator = map.entrySet().iterator(); while (iterator.hasNext()) { Map.Entry entry = iterator.next(); Integer k = entry.getKey(); String v = entry.getValue(); } } /** * ForEach EntrySet */ @Benchmark public void forEachEntrySet() { // 遍历 for (Map.Entry entry : map.entrySet()) { Integer k = entry.getKey(); String v = entry.getValue(); } } /** * 迭代器 KeySet */ @Benchmark public void keySet() { // 遍历 Iterator iterator = map.keySet().iterator(); while (iterator.hasNext()) { Integer k = iterator.next(); String v = map.get(k); } } /** * ForEach KeySet */ @Benchmark public void forEachKeySet() { // 遍历 for (Integer key : map.keySet()) { Integer k = key; String v = map.get(k); } } /** * Lambda 表达式 */ @Benchmark public void lambda() { // 遍历 map.forEach((key, value) ->

{Integer k = key; String v = value;});} / * * Stream API single thread * / @ Benchmark public void streamApi () {/ / single thread traversal map.entrySet () .stream () .forEach ((entry)-> {Integer k = entry.getKey (); String v = entry.getValue () });} / * * Stream API multithreading * this does not need to be tested, it is certain that the performance is the best. * if this is added to the test, the performance should be the worst in theory (it has been tested) * Why do you say that? Because you have already opened several threads to test other methods * it is impossible for you to create a few more threads to improve performance. All the threads are allocated. * Thread context switching will take longer! So we can't test together! * / public void parallelStreamApi () {/ / multithreaded traversal of map.entrySet () .parallelStream () .forEach ((entry)-> {Integer k = entry.getKey (); String v = entry.getValue ();});}} 2.3Test results

Run the program and view the output log!

(1) the first time

(2) the second time

(3) the third time

2.4 Analysis

Explanation of the above figure: test conclusion {test method (Benchmark), test type (Mode), total test times (Cnt), test result (Score), error (Error), unit (Units)}

Where Units means ns/op means execution completion time (in nanoseconds), while Score lists the average execution time, and the ±symbol indicates error.

As you can see from the above results, the performance of Lambda is similar to that of the two EntrySet, followed by Stream API single thread, followed by KeySet, with the worst performance.

From the above results, we can see that the performance of entrySet is twice as high as that of keySet, so we should try to use entrySet to traverse the Map collection. Of course, it is better if we are proficient in Lambda using Lambda. After all, the code is simple.

If you want an in-depth understanding of why performance is so different, it is recommended to look at the bytecode file for analysis. Or decompile using the javap-c class name .class to see the underlying implementation.

The above is the example analysis of 7 traversal methods and performance of HashMap in java. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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