```

`Optional[str]`을 `str` 대신 쓰게 되면, 특정 값이 실제로는 `None`이 될 수도 있는데 항상 `str`이라고 가정하는 상황에서 에디터가 에러를 찾게 도와줄 수 있습니다.

#### Generic(제네릭) 타입

이 타입은 대괄호 안에 매개변수를 가지며, 종류는:

* `List`
* `Tuple`
* `Set`
* `Dict`
* `Optional`
* ...등등

위와 같은 타입은 **Generic(제네릭) 타입** 혹은 **Generics(제네릭스)**라고 불립니다.

### 타입으로서의 클래스

변수의 타입으로 클래스를 선언할 수도 있습니다.

이름(name)을 가진 `Person` 클래스가 있다고 해봅시다.

package com.google.common.collect.testing.testers;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractCollectionTester;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code size()} operations on a collection. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */

// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package kms

import (
	"context"

	"github.com/minio/kms-go/kes"
)

// PolicyManager is the generic interface that handles KMS policy] operations
type PolicyManager interface {
	// DescribePolicy describes a policy by returning its metadata.
	// e.g. who created the policy at which point in time.

import com.google.common.collect.testing.features.CollectionFeature;
import com.google.common.collect.testing.features.CollectionSize;
import java.util.NoSuchElementException;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code remove()} operations on a queue. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible

import com.google.common.collect.testing.WrongType;
import com.google.common.collect.testing.features.CollectionSize;
import com.google.common.collect.testing.features.MapFeature;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code containsValue()} operations on a map. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.MapTestSuiteBuilder}.
 *
 * @author George van den Driessche

import com.google.common.collect.testing.Helpers;
import com.google.common.collect.testing.features.CollectionFeature;
import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code toString()} operations on a collection. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */

  @SuppressWarnings({"unchecked", "DoNotMock"}) // mock
  @Override
  public void setUp() throws Exception {
    super.setUp();
    /*
     * Class parameters must be raw, so we can't create a proxy with generic
     * type arguments. The created proxy only records calls and returns null, so
     * the type is irrelevant at runtime.
     */
    mock = mock(Cache.class);
    forward =

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.features.CollectionFeature;
import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code remove(Object)} operations on a list. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author George van den Driessche
 */

import com.google.common.collect.testing.Helpers;
import com.google.common.collect.testing.features.CollectionFeature;
import java.util.Arrays;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code stream} operations on a collection. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Louis Wasserman
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionFeature;
import com.google.common.collect.testing.features.CollectionSize;
import java.lang.reflect.Method;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests add operations on a set. Can't be invoked directly; please see
 * {@link com.google.common.collect.testing.SetTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible(emulated = true)