使用 Streams 替换循环

Question

I have the following code:

boolean found = false;
for (Commit commit : commits.values()) {
    if (commit.getLogMessage().compareTo(commitMessageToSearch) == 0) {
        System.out.println(commit.getSha());
        found = true;
    }
}
if (!found) {
    System.out.println("aksdhlkasj");
}

Is there some way to write this succinctly using streams or anything else in Java

Answer 1

You can use Stream#filter along with Stream#findFirst .

System.out.println(commits.values().stream()
    .filter(commit -> commit.getLogMessage().compareTo(commitMessageToSearch) == 0)
    .findFirst().map(Commit::getSha).orElse("aksdhlkasj"));

Answer 2

Just replacing your logic with stream functionality, using lambda expression

    boolean found = false;
    
    commits.values().stream().filter(c -> 
    c.getLogMessage().compareTo(commitMessageToSearch) == 0).forEach(c -> {
    System.out.println(c.getSha());
    found = true;
    });
    
    if (!found) {
        System.out.println("aksdhlkasj");
    }

Answer 3

As there is a "state" that needs to be checked outside the loop, a stream-based solution using Stream::forEach would set found value outside the stream and therefore is not pure but it prints all filtered SHA codes as the for-loop version:

AtomicBoolean found = new AtomicBoolean(); // effectively final container boolean

commits.values().stream()
    .filter(commit -> commit.getLogMessage().compareTo(commitMessageToSearch) == 0)
    .map(Commit::getSha)
    .forEach(sha -> {
        found.set(true); 
        System.out.println(sha);
    });

if (!found.get()) {
    System.out.println("none found");
}

---

Or use short-circuiting Stream::anyMatch to check if the filtered stream contains at least one matching entry:

if (commits.values().stream()
    .map(Commit::getLogMessage) // equals should be equivalent to compareTo == 0
    .anyMatch(commitMessageToSearch::equals))
{
    commits.values().stream()
        .filter(commit -> commit.getLogMessage().compareTo(commitMessageToSearch) == 0)
        .map(Commit::getSha)
        .forEach(System.out::println);
} else {
    System.out.println("none found");
}

But this is definitely more verbose than for-loop solution.

Answer 4

You want this:

commits.values()
       .stream()
       .filter(commit -> commit.getLogMessage().compareTo(commitMessageToSearch) == 0)
       .findFirst()
       .map(Commit::getSha)
       .ifPresentOrElse(
               System.out::println,                      // if present, print out sha
               () -> System.out.println("aksdhlkasj"));  // print some garbage otherwise

Basically, you filter out the unwanted commits. Once you find a correct one, the method Stream#findfirst short-circuits the processing as long as there is no need to search further. For that reason, you might want to add break in your for a loop. Finally, extract sha from the Commit using Optional#map and then Optional#ifPresentOrElse handles both situation if such commit is present or not.