CollectionHelper.java

package com.geo.spatial.manager;

import java.util.Collection;
import java.util.Objects;
import java.util.function.Supplier;

public final class CollectionHelper {

  private Collection<?> collection;
  private boolean isEmpty = true;

  private CollectionHelper() {
    new AssertionError("no");
  }

  private void validate() {
    if (Objects.isNull(collection)) {
      new AssertionError("call check first");
    }
  }

  @SuppressWarnings("unchecked")
  private CollectionHelper(Collection<?> collection) {
    Objects.requireNonNull(collection);
    this.collection = collection;
  }

  @SuppressWarnings("unchecked")
  public static CollectionHelper check(Collection<?> collection) {
    return new CollectionHelper(collection);
  }

  @SuppressWarnings("unchecked")
  public CollectionHelper isEmpty() {
    validate();
    isEmpty = ((Collection)collection).isEmpty();
    return this;
  }

  @SuppressWarnings("unchecked")
  public CollectionHelper thenFetch(Collection<?> collection) {
    validate();
    if (isEmpty && !collection.isEmpty()) {
      this.collection = collection;
      isEmpty = collection.isEmpty();
    }
    return this;
  }

  @SuppressWarnings("unchecked")
  public CollectionHelper thenFetch(Supplier<Collection<?>> function) {
    if (!isEmpty)
      return this;
    Collection<?> result = function.get();
    return thenFetch(result);
  }

  public Collection<?> get() {
    validate();
    return collection;
  }
}