*
     * @param factory The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");
            }
            PROTOCOL_HANDLERS.clear();
            Handler.factory = factory;
        }
    }

     *            The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");
            }
            PROTOCOL_HANDLERS.clear();
            Handler.factory = factory;
        }
    }

        // This test ensures that attempting to set the URLStreamHandlerFactory more than once
        // throws an IllegalStateException as per the contract.
        URLStreamHandlerFactory mockFactory1 = mock(URLStreamHandlerFactory.class);
        URLStreamHandlerFactory mockFactory2 = mock(URLStreamHandlerFactory.class);

        Handler.setURLStreamHandlerFactory(mockFactory1); // First call, should succeed.

import static org.mockito.Mockito.when;

import java.lang.reflect.Field;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLStreamHandler;
import java.net.URLStreamHandlerFactory;
import java.util.ArrayList;
import java.util.List;

import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;