Java8：流 findFirst 结果

Question

I want to know if there is a way to get rid of the warning at findFirst().get() without using .orElse() when I know 100% that every time there is a result, so I never get a NoSuchElementException .

For example let's see the following code:

    List<String> myList = new ArrayList<>();
    myList.add("Test");
    myList.add("Example");
    myList.add("Sth");

    String fisrstString = myList.stream().findFirst().get(); // here I surely get "Test" 

I don't know how other IDEs treat this, but IntelliJ treats that as a warning

'Optional.get()' without 'isPresent()'

I think probably it doesn't know when can you get NoSuchElementException there and when not, or I have no idea why. I know there are ways to solve this warning( isPresent() check, .orElse(something) ) but with useless code, so simply I don't want to use those solutions because they are so unnecessary.

Do you have any idea what can I do, or explain how is that treated by the IDE?

Answer 1

Well, as for me, the best way is to use functional programing and continue to work with optional. So, for example if you need to pass this string to some service, you can do:

String fisrstString = myList.stream().findFirst().get();
service.doSomething(fisrstString);

But this looks not so good. Instead you can use the pros of functional programing, and do:

myList.stream().findFirst().ifPresent(service::doSomething);

Answer 2

You should make use of the Optional returned by findFirst() instead of trying to get its value (if it's actually present).

myList.stream()
    .findFirst()
    .ifPresent(/* consume the string here, if present */);

The Optional.ifPresent method receives a Consumer that will be used only if the Optional contains a non-null value.

The problem is that we Java developers are so used to the imperative paradigm... In particular, we are used to getting an object and pushing it ie to a method:

String myString = "hello"; // getting an object here

System.out.println(myString); // pushing the object to System.out here
                              // (via the println method)

With the Optional returned by Stream.findFirst() you were doing the same as above:

String myString = myList.stream()
    .findFirst()
    .get(); // getting a string here

System.out.println(myString); // pushing the string here

On the other hand, the functional paradigm (including Optional ) usually works the other way:

myList.stream()
    .findFirst()
    .ifPresent(myString -> System.out.println(myString));

Here, you don't get the string and then push it to some method. Instead, you provide an argument to Optional 's ifPresent operation and let the implementation of Optional push the value to your argument. In other words, you pull the value wrapped by the Optional by means of ifPresent 's argument. ifPresent will then use this Consumer argument, only if the value is present.

This pull pattern is seen a lot in functional programming and is very useful, once you get used to it. It just requires us developers to start thinking (and programming) in a different way.

Answer 3

First you will not get a NPE , but a NoSuchElementException . Second, it's you who might be sure; but other people might come along and don't realize that it will not throw an exception.

For a sandbox project - yes you would not care and can ignore the warning; for production code I would not disable it (even if you could).

And the last point is that if you are so sure, why not throw an exception?

orElseThrow(IAmSureThisWillNotHappenException::new)

Answer 4

you can stream an empty list without a problem, but if you try to get the 1st element on an empty list you will get a NoSuchElementException

the Stream API is aware of that flawless therefore they offer you multiple ways to handle that:

Option1 : orElse you can return a "default" value if no 1st element is found

String firstString = myList.stream().findFirst().orElse("Ups!");

Option2 : orElseGet you can use a Supplier<String> that gives back a String if no 1st element is found

firstString = myList.stream().findFirst().orElseGet(mySupplier);

Option3 : orElseThrow you can throw an exception if no 1st element is found

firstString = myList.stream().findFirst().orElseThrow(WhatTerribleFailException::new);

---

System.out.println(fisrstString);

Answer 5

IF you know your Optional never be empty, you can use @SuppressWarnings annotation as below:

@SuppressWarnings("ConstantConditions") String foo = Optional.of("bar").get();

Sometimes Optional.get will be raising a NullPointerException , for example:

Optional<String> it = Optional.empty();
String foo = it.get();
          //   ^--- throws NullPointerException when this method is invoked

SO when this expression is used Intellij will reporting a warnning inspection.

IF you want to disable all contract inspection you can do the following actions: Settings -> Inspections -> unchecked the Constant condition & exceptions option -> don't forget to click Apply button at bottom to saving your settings.

IF you don't want to disable all contract inspection except Optional.get() warnnings you can do the following actions: Settings -> Inspections -> checked the Constant condition & exceptions option -> at the right bottom side there is a frame to configure Optional.get() warnnings -> don't forget to click Apply button at bottom to saving your settings.