Add some test for uncovered functions.

api/src/test/java/net/solarnetwork/io/modbus/test/ModbusFunctionCodeTests.java

@@ -186,6 +186,17 @@ public void valueOf_user() {
 				is(equalTo(new UserModbusFunction((byte) 0x65))));
 	}
 
+	@Test
+	public void valueOf_negative() {
+		// WHEN
+		ModbusFunction fn = ModbusFunctionCode.valueOf((byte) (ModbusFunctionCodes.ERROR_OFFSET
+				+ ModbusFunctionCode.ReadHoldingRegisters.getCode()));
+
+		// THEN
+		assertThat("ModbusFunctionCode %s is decoded from error offset", fn.functionCode(),
+				is(ModbusFunctionCode.ReadHoldingRegisters));
+	}
+
 	@Test
 	public void forCode() {
 		assertThat(ModbusFunctionCode.forCode(ModbusFunctionCodes.READ_COILS),

api/src/test/java/net/solarnetwork/io/modbus/test/ModbusMessageTests.java

@@ -0,0 +1,118 @@
+/* ==================================================================
+ * ModbusMessageTests.java - 17/05/2024 7:15:24 pm
+ *
+ * Copyright 2024 SolarNetwork.net Dev Team
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+ * 02111-1307 USA
+ * ==================================================================
+ */
+
+package net.solarnetwork.io.modbus.test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+import org.junit.jupiter.api.Test;
+import net.solarnetwork.io.modbus.ModbusError;
+import net.solarnetwork.io.modbus.ModbusErrorCode;
+import net.solarnetwork.io.modbus.ModbusFunction;
+import net.solarnetwork.io.modbus.ModbusMessage;
+
+/**
+ * Test cases for the {@link ModbusMessage} class.
+ *
+ * @author matt
+ * @version 1.0
+ */
+public class ModbusMessageTests {
+
+	@Test
+	public void defaultIsNotException() {
+		// GIVEN
+		ModbusMessage msg = new ModbusMessage() {
+
+			@Override
+			public <T extends ModbusMessage> T unwrap(Class<T> msgType) {
+				return null;
+			}
+
+			@Override
+			public boolean isSameAs(ModbusMessage obj) {
+				return false;
+			}
+
+			@Override
+			public int getUnitId() {
+				return 0;
+			}
+
+			@Override
+			public ModbusFunction getFunction() {
+				return null;
+			}
+
+			@Override
+			public ModbusError getError() {
+				return null;
+			}
+		};
+
+		// WHEN
+		boolean result = msg.isException();
+
+		// THEN
+		assertThat("Is not exception if no error provided", result, is(equalTo(false)));
+	}
+
+	@Test
+	public void defaultIsException() {
+		// GIVEN
+		ModbusMessage msg = new ModbusMessage() {
+
+			@Override
+			public <T extends ModbusMessage> T unwrap(Class<T> msgType) {
+				return null;
+			}
+
+			@Override
+			public boolean isSameAs(ModbusMessage obj) {
+				return false;
+			}
+
+			@Override
+			public int getUnitId() {
+				return 0;
+			}
+
+			@Override
+			public ModbusFunction getFunction() {
+				return null;
+			}
+
+			@Override
+			public ModbusError getError() {
+				return ModbusErrorCode.IllegalFunction;
+			}
+		};
+
+		// WHEN
+		boolean result = msg.isException();
+
+		// THEN
+		assertThat("Is exception if error provided", result, is(equalTo(true)));
+	}
+
+}