Java test helper classes

ConfigurableResponses.java

import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class ConfigurableResponses<T> {
  private final Object response;

  private ConfigurableResponses(Object value) {
    response = value;
  }

  public static <T> ConfigurableResponses<T> empty() {
    return sequence(List.of());
  }

  public static <T> ConfigurableResponses<T> always(T value) {
    return new ConfigurableResponses<>(value);
  }

  public static ConfigurableResponses<?> always(Exception exception) {
    return new ConfigurableResponses<>(exception);
  }

  public static <T> ConfigurableResponses<T> sequence(List<?> values) {
    return new ConfigurableResponses<>(new LinkedList<>(values));
  }

  public static <T> ConfigurableResponses<T> sequence(Object... values) {
    return sequence(List.of(values));
  }

  @SuppressWarnings("unchecked")
  public T next() {
    if (response instanceof Queue<?> q) {
      var v = q.poll();
      if (v == null) {
        throw new IllegalStateException("No more values configured.");
      } else if (v instanceof RuntimeException e) {
        throw e;
      } else {
        return (T) v;
      }
    } else if (response instanceof RuntimeException e) {
      throw e;
    } else {
      return (T) response;
    }
  }

  @SuppressWarnings("unchecked")
  public T tryNext() throws Exception {
    if (response instanceof Queue<?> q) {
      var v = q.poll();
      if (v == null) {
        throw new IllegalStateException("No more values configured.");
      } else if (v instanceof Exception e) {
        throw e;
      } else {
        return (T) v;
      }
    } else if (response instanceof Exception e) {
      throw e;
    } else {
      return (T) response;
    }
  }
}

OutputTracker.java

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

public class OutputTracker<T> {
  private final List<T> data = new ArrayList<>();
  private final Consumer<T> tracker = data::add;
  private final EventEmitter<T> emitter;

  public OutputTracker(EventEmitter<T> emitter) {
    this.emitter = emitter;
    emitter.addListener(tracker);
  }

  public List<T> data() {
    return List.copyOf(data);
  }

  public List<T> clear() {
    var result = data();
    data.clear();
    return result;
  }

  public void stop() {
    emitter.removeListener(tracker);
  }
}