NonEmptyMono

WARNING

Simplification

Summary

Avoid vacuous operations on known non-empty Monos

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("NonEmptyMono") to the enclosing element.

Disable this pattern completely by adding -Xep:NonEmptyMono:OFF as compiler argument. Learn more.

Samples

Replacement

Shows the difference in example code before and after the bug pattern is applied.

 import static com.google.common.collect.ImmutableList.toImmutableList;
 
 import com.google.common.collect.ImmutableList;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 class A {
   void m() {
-    Flux.just(1).collect(toImmutableList()).single();
-    Flux.just(1).collect(toImmutableList()).defaultIfEmpty(ImmutableList.of());
-    Flux.just(1).collect(toImmutableList()).switchIfEmpty(Mono.just(ImmutableList.of()));
+    Flux.just(1).collect(toImmutableList());
+    Flux.just(1).collect(toImmutableList());
+    Flux.just(1).collect(toImmutableList());
 
-    Mono.just(2).hasElement().single();
-    Mono.just(2).hasElement().defaultIfEmpty(true);
-    Mono.just(2).hasElement().switchIfEmpty(Mono.just(true));
+    Mono.just(2).hasElement();
+    Mono.just(2).hasElement();
+    Mono.just(2).hasElement();
   }
 }

Identification

Shows code lines which will (not) be flagged by this bug pattern.
A //BUG: Diagnostic contains: comment is placed above any violating line.

import static com.google.common.collect.ImmutableList.toImmutableList;
import static java.util.function.Function.identity;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

class A {
  void m() {
    Mono.just(1).defaultIfEmpty(2);
    Mono.just(1).single();
    Mono.just(1).switchIfEmpty(Mono.just(2));

    // BUG: Diagnostic contains:
    Flux.just(1).all(x -> true).defaultIfEmpty(true);

    // BUG: Diagnostic contains:
    Flux.just(1).any(x -> true).single();

    // BUG: Diagnostic contains:
    Flux.just(1).collect(toImmutableList()).switchIfEmpty(Mono.just(ImmutableList.of()));
    // BUG: Diagnostic contains:
    Flux.just(1).collect(ArrayList::new, List::add).defaultIfEmpty(new ArrayList<>());

    // BUG: Diagnostic contains:
    Flux.just(1).collectList().single();

    // BUG: Diagnostic contains:
    Flux.just(1).collectMap(identity()).switchIfEmpty(Mono.just(ImmutableMap.of()));
    // BUG: Diagnostic contains:
    Flux.just(1).collectMap(identity(), identity()).defaultIfEmpty(ImmutableMap.of());
    // BUG: Diagnostic contains:
    Flux.just(1).collectMap(identity(), identity(), HashMap::new).single();

    // BUG: Diagnostic contains:
    Flux.just(1).collectMultimap(identity()).switchIfEmpty(Mono.just(ImmutableMap.of()));
    // BUG: Diagnostic contains:
    Flux.just(1).collectMultimap(identity(), identity()).defaultIfEmpty(ImmutableMap.of());
    // BUG: Diagnostic contains:
    Flux.just(1).collectMultimap(identity(), identity(), HashMap::new).single();

    // BUG: Diagnostic contains:
    Flux.just(1).collectSortedList().defaultIfEmpty(ImmutableList.of());
    // BUG: Diagnostic contains:
    Flux.just(1).collectSortedList((o1, o2) -> 0).single();

    // BUG: Diagnostic contains:
    Flux.just(1).count().switchIfEmpty(Mono.just(2L));

    // BUG: Diagnostic contains:
    Flux.just(1).elementAt(0).defaultIfEmpty(1);
    // BUG: Diagnostic contains:
    Flux.just(1).elementAt(0, 2).single();

    // BUG: Diagnostic contains:
    Flux.just(1).hasElement(2).switchIfEmpty(Mono.just(true));

    // BUG: Diagnostic contains:
    Flux.just(1).hasElements().defaultIfEmpty(true);

    // BUG: Diagnostic contains:
    Flux.just(1).last().single();
    // BUG: Diagnostic contains:
    Flux.just(1).last(2).switchIfEmpty(Mono.just(3));

    // BUG: Diagnostic contains:
    Flux.just(1).reduceWith(() -> 0, Integer::sum).defaultIfEmpty(2);

    // BUG: Diagnostic contains:
    Flux.just(1).single().single();
    // BUG: Diagnostic contains:
    Flux.just(1).single(2).switchIfEmpty(Mono.just(3));

    Flux.just(1).reduce(Integer::sum).defaultIfEmpty(2);
    // BUG: Diagnostic contains:
    Flux.just(1).reduce(2, Integer::sum).single();

    // BUG: Diagnostic contains:
    Mono.just(1).defaultIfEmpty(1).switchIfEmpty(Mono.just(2));
    // BUG: Diagnostic contains:
    Mono.just(1).hasElement().defaultIfEmpty(true);
    // BUG: Diagnostic contains:
    Mono.just(1).single().single();
  }
}