The problem

Flags instances of non-API types from being accepted or returned in public APIs.

What it flags

• Primitive Arrays: Methods accepting or returning primitive or boxed-primitive arrays (int[], Integer[], double[], Double[], long[], Long[]). Prefer ImmutableIntArray, ImmutableDoubleArray, or ImmutableLongArray instead.
  • Note: Var-args parameters (e.g. int... rest) are not flagged, as var-args is idiomatic Java for parameter lists.
• Collection Implementations: Accepting or returning concrete collection classes (ArrayList, LinkedList, HashSet, LinkedHashSet, TreeSet, HashMap, LinkedHashMap, TreeMap). Prefer interface types (List, Set, Map).
• Immutable Collections as Parameters: Accepting ImmutableCollection, ImmutableList, ImmutableSet, or ImmutableMap as method parameters. Prefer accepting Collection, List, Set, Map, or Iterable for parameter generality.
• Optional Parameters: Accepting java.util.Optional or com.google.common.base.Optional as method parameters. Prefer method overloading: creating one signature with the parameter and one without (or use @Nullable parameters).
• com.google.common.base.Pair: Passing Pair across API boundaries. Define a well-named class or record instead.
• Iterators & Streams: Returning Iterator (prefer Stream or collecting to an ImmutableList/ImmutableSet) or accepting Stream as a parameter (prefer Iterable or Collection).
  • Returning stateful single-use Iterators limits caller options
• ProtoTime Types: Using com.google.protobuf.Duration, Timestamp, or com.google.type.* types across public APIs instead of standard java.time.* types (Duration, Instant, LocalDate, etc.).
• Flogger Loggers: Passing FluentLogger or GoogleLogger instances across method boundaries; this can break standard per-class logger initialization patterns.

Why

• Type Generality & Interface Abstraction: Methods should accept abstract interface types (e.g., List rather than ArrayList) to give callers flexibility in implementation details.
• Immutability & Safety: Primitive arrays are mutable and expose internal array state directly to callers. Guava’s ImmutableIntArray, ImmutableDoubleArray, and ImmutableLongArray provide immutable, safe, and efficient alternatives.

Suppression

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