How do their APIs compare?

These two projects aren't really comparable at the level of "API design" in a like-for-like sense — one is an Android compile-time annotation library, the other is a Spring Boot microservices system exposing HTTP REST APIs. Here's how their "APIs" (in the broadest sense of "surface that consumers program against") differ:

jakewharton/butterknife: an annotation-driven, compile-time Java/Kotlin API

Butterknife's public API is a small set of annotations plus a thin static entry-point class, backed by generated code:

• The primary entry point is the static ButterKnife class, which is explicitly non-instantiable (private ButterKnife() { throw new AssertionError(...); }) and exposes only static methods [butterknife/src/main/java/butterknife/ButterKnife.java:L74-L77].
• Binding is done via overloaded bind() methods keyed on the Android component type — bind(Activity), bind(View), bind(Dialog), and overloads that take a separate target object — all returning an Unbinder for later cleanup [butterknife/src/main/java/butterknife/ButterKnife.java:L91-L113].
• There's a parallel implementation in the experimental butterknife-reflect artifact with the same public surface (bind(Activity), bind(View), setDebug, etc.) so it's binary-compatible and swappable at build time depending on whether the annotation processor or reflection is used [butterknife-reflect/src/main/java/butterknife/ButterKnife.java:L49-L88], as also described in the reflect artifact's README [butterknife-reflect/README.md:L36-L62].
• The actual field/view/resource binding surface is annotation-based, not method-based: @BindView, @BindString, @BindBool, @BindInt, @BindColor, @OnClick, @OnItemClick, @OnItemLongClick, @OnPageChange, etc. [butterknife-annotations/src/main/java/butterknife/OnItemLongClick.java:L1-L50, butterknife-annotations/src/main/java/butterknife/OnPageChange.java:L47-L74]. These annotations declaratively describe a listener interface, method name, and parameter types via internal @ListenerClass/@ListenerMethod meta-annotations that drive code generation.
• Supporting runtime helpers like Utils.findRequiredView, Utils.castView, Utils.findOptionalViewAsType throw descriptive IllegalStateExceptions when a bound view is missing or mistyped, giving the API strong fail-fast semantics at bind time [butterknife-runtime/src/main/java/butterknife/internal/Utils.java:L59-L98, L99-L138].
• There's also a build-time API surface: the Gradle plugin generates an R2 class mirroring Android's R class but with resource-type annotations (@IdRes, @StringRes, etc.) so IDs can be validated at compile time [butterknife-gradle-plugin/src/test/resources/fixtures/R2.java:L20-L99].

In short: butterknife's "API" is annotations + one static utility class; there is no object instantiation, no network I/O, and validation happens via reflection/generated code at bind-time with thrown exceptions.

sqshq/piggymetrics: a distributed system of Spring Boot REST APIs

The retrieved data for piggymetrics is mostly limited to the README, which describes it as a microservices architecture — not a library with a stable Java API. Per the README, it's decomposed into independently deployable services organized by business domain, communicating over Spring Cloud infrastructure (README excerpt above). Its "API" would be HTTP/REST endpoints exposed by each service (e.g., account, statistics, notification services), not Java classes you import — the opposite integration model from butterknife.

Key contrasts