Added NoPbFieldsException

Author: rumpfc

src/main/java/com/rumpf/proto/exception/NoPbFieldsException.java

@@ -0,0 +1,19 @@
+package com.rumpf.proto.exception;
+
+public class NoPbFieldsException extends RuntimeException {
+
+    private Class<?> clazz;
+
+    public NoPbFieldsException(Class<?> clazz) {
+        this.clazz = clazz;
+    }
+
+    public Class<?> getObjectClazz() {
+        return clazz;
+    }
+
+    @Override
+    public String getMessage() {
+        return String.format("Class \"%s\" does not contain any fields with @PbField annotation", clazz.getName());
+    }
+}

src/main/java/com/rumpf/proto/mapper/PbObjectMapper.java

@@ -4,6 +4,7 @@
 import com.google.protobuf.CodedOutputStream;
 import com.rumpf.proto.PbField;
 import com.rumpf.proto.exception.DuplicateFieldNumberException;
+import com.rumpf.proto.exception.NoPbFieldsException;
 import com.rumpf.proto.field.MessageField;
 import com.rumpf.proto.field.MessageFieldFactory;
 
@@ -54,6 +55,10 @@ private void initMessageFields(Object object) {
                 messageFields.put(mf.getFieldNumber(), mf);
             }
         }
+
+        if(messageFields.isEmpty()) {
+            throw new NoPbFieldsException(object.getClass());
+        }
     }
 
     ////////////////////////////////////////////////////

src/test/java/com/rumpf/exception/NoPbFieldsTest.java

@@ -0,0 +1,49 @@
+package com.rumpf.exception;
+
+import com.rumpf.proto.exception.NoPbFieldsException;
+import com.rumpf.proto.mapper.PbObjectMapper;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import java.nio.ByteBuffer;
+
+public class NoPbFieldsTest {
+
+    private static PbObjectMapper mapper;
+
+    @BeforeAll
+    public static void setup() {
+        mapper = new PbObjectMapper();
+    }
+
+    private static class SampleClass1 {
+
+        private String a;
+        private String b;
+    }
+
+    private static class SampleClass2 {
+
+    }
+
+    @Test
+    public void duplicateFieldNumberTest1() {
+        NoPbFieldsException ex = Assertions.assertThrows(
+                NoPbFieldsException.class,
+                () -> mapper.write(new SampleClass1(), ByteBuffer.allocate(1))
+        );
+
+        Assertions.assertEquals(SampleClass1.class, ex.getObjectClazz(), "Unexpected Class<?> object returned");
+    }
+
+    @Test
+    public void duplicateFieldNumberTest2() {
+        NoPbFieldsException ex = Assertions.assertThrows(
+                NoPbFieldsException.class,
+                () -> mapper.write(new SampleClass2(), ByteBuffer.allocate(1))
+        );
+
+        Assertions.assertEquals(SampleClass2.class, ex.getObjectClazz(), "Unexpected Class<?> object returned");
+    }
+}