public static <U> UnmodifiableFuture<U> completedFuture(@Nullable U value) {
        if (value == null) {
            @SuppressWarnings("unchecked")
            final UnmodifiableFuture<U> cast = (UnmodifiableFuture<U>) NIL;
            return cast;
        }

        if (value == Boolean.TRUE) {
            @SuppressWarnings("unchecked")
            final UnmodifiableFuture<U> cast = (UnmodifiableFuture<U>) TRUE;
            return cast;
        }

        if (value == Boolean.FALSE) {
            @SuppressWarnings("unchecked")
            final UnmodifiableFuture<U> cast = (UnmodifiableFuture<U>) FALSE;
            return cast;
        }

        if (value == ImmutableList.of()) {
            @SuppressWarnings("unchecked")
            final UnmodifiableFuture<U> cast = (UnmodifiableFuture<U>) IMMUTABLE_LIST;
            return cast;
        }

        final UnmodifiableFuture<U> future = new UnmodifiableFuture<>();
        future.doComplete(value);
        return future;
    }
Returns an {@link UnmodifiableFuture} which has been completed with the specified {@code value}.
core/src/main/java/com/linecorp/armeria/common/util/UnmodifiableFuture.java