gaozhangmin commented on pull request #14346:
URL: https://github.com/apache/pulsar/pull/14346#issuecomment-1044229473
> Don't use `putIfAbsent`. Use `computeIfAbsent`.
>
> For example,
>
> ```java
> private static String newValue() {
> System.out.println("Call newValue");
> return "hello";
> }
>
> public static void main(String[] args) {
> final Map<String, String> map = new ConcurrentHashMap<>();
> System.out.println(map.putIfAbsent("A", newValue()));
> System.out.println(map.putIfAbsent("A", newValue()));
> }
> ```
>
> The output is:
>
> ```
> Call newValue
> null
> Call newValue
> hello
> ```
>
> But if you use
>
> ```java
> System.out.println(map.computeIfAbsent("A", __ -> newValue()));
> System.out.println(map.computeIfAbsent("A", __ -> newValue()));
> ```
>
> The output will be:
>
> ```
> Call newValue
> hello
> hello
> ```
>
> You can see `computeIfAbsent` avoids computing the value if the key
already exists. Though the returned value is always non-null. So I suggested
capturing an atomic boolean in the function of `computeIfAbsent` like:
>
> ```java
> final AtomicBoolean computed = new AtomicBoolean(false);
> map.computeIfAbsent("A", __ -> {
> computed.set(true);
> return newValue();
> });
> ```
We need use putIfAbsent here , as previous logic, Only If the key is existed
before , we register the entry.
If We use computeIfAbsent, the return value will be previous value or
computed value, We cannot judge if the key existed before. @BewareMyPower
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