When you create a **FastAPI** *path operation* you can normally return any data from it: a `dict`, a `list`, a Pydantic model, a database model, etc.

If you declare a [Response Model](../tutorial/response-model.md) FastAPI will use it to serialize the data to JSON, using Pydantic.

If you don't declare a response model, FastAPI will use the `jsonable_encoder` explained in [JSON Compatible Encoder](../tutorial/encoder.md) and put it in a `JSONResponse`.

  }

  @CollectionFeature.Require(SERIALIZABLE)
  public void testInverseSerialization() {
    BiMapPair<K, V> pair = new BiMapPair<>(getMap());
    BiMapPair<K, V> copy = SerializableTester.reserialize(pair);
    assertEquals(pair.forward, copy.forward);
    assertEquals(pair.backward, copy.backward);
    assertSame(copy.backward, copy.forward.inverse());
    assertSame(copy.forward, copy.backward.inverse());
  }

# Currently, there is no way to pass multiple cookies with the same name.
# The only way to pass multiple values for cookie params is to serialize them using

    assertEquals(3, function.apply("Three").intValue());

    new EqualsTester()
        .addEqualityGroup(
            function, Functions.forMap(map, 42), SerializableTester.reserialize(function))
        .addEqualityGroup(Functions.forMap(map))
        .addEqualityGroup(Functions.forMap(map, null))
        .addEqualityGroup(Functions.forMap(map, 43))
        .testEquals();
  }

    Predicate<Class<?>> predicate = Predicates.subtypeOf(Integer.class);
    Predicate<Class<?>> reserialized = SerializableTester.reserializeAndAssert(predicate);

    assertEvalsLike(predicate, reserialized, Integer.class);
    assertEvalsLike(predicate, reserialized, Float.class);
    assertEvalsLike(predicate, reserialized, null);
  }

  /*
   * Tests for Predicates.isNull()
   */

If you declare the return type as `AsyncIterable[Item]`, FastAPI will use it to **validate**, **document**, and **serialize** the data using Pydantic.

{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[1:25] hl[10:12,23] *}

/// tip

As Pydantic will serialize it in the **Rust** side, you will get much higher **performance** than if you don't declare a return type.

///

If you declare the return type, FastAPI will use it to **validate** the data, **document** it in OpenAPI, **filter** it, and **serialize** it using Pydantic.

/// tip

As Pydantic will serialize it in the **Rust** side, you will get much higher **performance** than if you don't declare a return type.

///

import static com.google.common.collect.testing.testers.NavigableSetNavigationTester.getHoleMethods;
import static com.google.common.testing.SerializableTester.reserialize;
import static com.google.common.testing.SerializableTester.reserializeAndAssert;
import static com.google.common.truth.Truth.assertThat;

import com.google.common.annotations.GwtCompatible;

    Helpers.testComparator(comparator, ordered);
    assertThat(SerializableTester.reserialize(comparator)).isSameInstanceAs(comparator);

    // The Java implementation.
    Comparator<byte[]> javaImpl = UnsignedBytes.lexicographicalComparatorJavaImpl();
    Helpers.testComparator(javaImpl, ordered);
    assertThat(SerializableTester.reserialize(javaImpl)).isSameInstanceAs(javaImpl);
  }

 * (We have made some trivial local modifications (commented out
 * uncompilable code).)
 */

package com.google.common.util.concurrent;

import static com.google.common.testing.SerializableTester.reserialize;
import static com.google.common.truth.Truth.assertThat;
import static java.util.concurrent.TimeUnit.MILLISECONDS;

import com.google.common.annotations.GwtIncompatible;