Idiomatic way of comparing results of two completableFutures with assertj

Question

There are 2 completable futures cf1 and cf2 defined as follows:

CompletableFuture<Boolean> cf1 = CompletableFuture.completedFuture(true);

CompletableFuture<Boolean> cf2 = CompletableFuture.completedFuture(true);

Technically, one could do:

var result1 = cf1.get();
var result2 = cf2.get();

assertThat(result1).isEqualTo(result2);

For example, if there was only one future, we could do the following:

assertThat(cf1)
    .succeedsWithin(Duration.ofSeconds(1))
    .isEqualTo(true);

Is there a more idiomatic way to compare the two futures against each other? Note that while the example here uses CompletableFuture<Boolean>, the Boolean can be replaced with any class.

It probably depends on what you'd like to verify with cf2. For example, is it just about its value or also that cf2 succeeds within a certain threshold? — Stefano Cordio, Sep 16 '22 at 16:15
@StefanoCordio For the simplest case, just compare for equality. — Bhushan, Sep 16 '22 at 18:13

Stefano Cordio · Answer 1 · 2022-09-17T08:14:38.613

If you are interested only in value comparison, passing cf2.get() as argument of isEqualTo should be enough:

CompletableFuture<Boolean> cf1 = CompletableFuture.completedFuture(true);
CompletableFuture<Boolean> cf2 = CompletableFuture.completedFuture(true);

assertThat(cf1)
  .succeedsWithin(Duration.ofSeconds(1))
  .isEqualTo(cf2.get());

The only downside is that get() can potentially throw ExecutionException and InterruptedException so they need to be declared in the test method signature.

If type-specific assertions are needed, succeedsWithin(Duration, InstanceOfAssertFactory) can help:

assertThat(cf1)
  .succeedsWithin(Duration.ofSeconds(1), InstanceOfAssertFactories.BOOLEAN)
  .isTrue(); // hardcoded check to show type-specific assertion

Thanks, Stefano. The downside that you mention is the exact thing that I'm trying to avoid. I would like to avoid `.get()`. For a simple boolean, hard-coding to `.isTrue()` (or false) is possible, but as I said, I'm comparing two objects here. — Bhushan, Sep 19 '22 at 04:24

michid · Accepted Answer · 2022-09-20T06:09:09.757

What about composing the two futures into one that completes successfully with a value of true if and only if both individual futures complete successfully with the same value? You could e.g. use thenCompose followed by thenApply:

CompletableFuture<Boolean> bothEqual = cf1.thenCompose(b1 -> cf2.thenApply(b2 -> b1 == b2));

If the sequential execution from this solution is problematic, you can parallelize by implementing a helper function alsoApply and use that one instead of thenCompose. See this answer for more details.

Idiomatic way of comparing results of two completableFutures with assertj

2 Answers2