return "z";
  }

  @Override
  protected String getValueNotInPopulatedMap() throws UnsupportedOperationException {
    return "26";
  }

  /** Helper assertion comparing two maps */
  private void assertMapsEqual(Map<?, ?> expected, Map<?, ?> map) {
    assertEquals(expected, map);
    assertEquals(expected.hashCode(), map.hashCode());

    assertThat(e).hasMessageThat().contains("toString representation");
  }

  private static void assertErrorMessage(Throwable e, String message) {
    // TODO(kevinb): use a Truth assertion here
    if (!e.getMessage().contains(message)) {
      fail("expected <" + e.getMessage() + "> to contain <" + message + ">");
    }
  }

  /**

import java.util.List;
import java.util.Map.Entry;
import java.util.Set;
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * A series of tests that support asserting that collections cannot be modified, either through
 * direct or indirect means.
 *
 * @author Robert Konigsberg
 */
@GwtCompatible
@NullMarked
public class UnmodifiableCollectionTests {

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import assertk.assertThat
import assertk.assertions.startsWith
import kotlin.test.assertFailsWith
import kotlin.test.fail
import okhttp3.HttpUrl.Companion.toHttpUrl
import okhttp3.internal.idn.Punycode
import okio.Buffer
import okio.ByteString

package jcifs.http;

import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
import static org.mockito.ArgumentMatchers.any;
import static org.mockito.ArgumentMatchers.anyBoolean;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.lenient;
import static org.mockito.Mockito.never;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

}
```

## 🏗️ Architecture & Design Patterns

### Core Design Principles
CoreLib follows a **utility-class pattern** where most functionality is exposed through static methods:

- **Assertion-based validation** - All methods validate inputs using `AssertionUtil.assertArgumentNotNull()` and `AssertionUtil.assertArgumentNotEmpty()`

    // need a long enough string for the buffer to hit 0 remaining before the copy completes
    String string = Strings.repeat("0123456789", 100);
    StringBuilder b = new StringBuilder();
    // the main assertion of this test is here... the copy will fail if the buffer size goes down
    // each time it is not filled completely
    long copied = CharStreams.copy(newNonBufferFillingReader(new StringReader(string)), b);

    // need a long enough string for the buffer to hit 0 remaining before the copy completes
    String string = Strings.repeat("0123456789", 100);
    StringBuilder b = new StringBuilder();
    // the main assertion of this test is here... the copy will fail if the buffer size goes down
    // each time it is not filled completely
    long copied = CharStreams.copy(newNonBufferFillingReader(new StringReader(string)), b);

import org.codelibs.core.exception.EmptyArgumentException;
import org.codelibs.core.exception.NullArgumentException;
import org.codelibs.core.lang.StringUtil;

/**
 * Utility class for assertions.
 *
 * @author shot
 */
public abstract class AssertionUtil {

    /**
     * Do not instantiate.
     */
    protected AssertionUtil() {
    }

    /**

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import assertk.assertThat
import assertk.assertions.isEqualTo
import assertk.fail
import java.io.IOException
import java.net.InetAddress
import java.net.InetSocketAddress
import java.net.Proxy
import java.security.cert.X509Certificate
import java.util.Arrays