Add Note model object

src/main/java/model/Note.java

@@ -0,0 +1,21 @@
+package model;
+
+import java.util.Objects;
+
+public record Note(int id, String message, String from, String to, long timestamp, int fame) {
+    private static final int PLACEHOLDER_ID = -1;
+
+    public Note {
+        Objects.requireNonNull(message);
+        Objects.requireNonNull(from);
+        Objects.requireNonNull(to);
+    }
+
+    public Note createNormal(String message, String from, String to, long timestamp) {
+        return new Note(PLACEHOLDER_ID, message, from, to, timestamp, 0);
+    }
+
+    public Note createGift(String message, String from, String to, long timestamp) {
+        return new Note(PLACEHOLDER_ID, message, from, to, timestamp, 1);
+    }
+}

src/test/java/model/NoteTest.java

@@ -0,0 +1,28 @@
+package model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class NoteTest {
+
+    @Test
+    void requireNonNullMessage() {
+        assertThrows(NullPointerException.class, () -> new Note(1, null, "from", "to", System.currentTimeMillis(), 0));
+    }
+
+    @Test
+    void requireNonNullFrom() {
+        assertThrows(NullPointerException.class, () -> new Note(2, "message", null, "to", System.currentTimeMillis(), 0));
+    }
+
+    @Test
+    void requireNonNullTo() {
+        assertThrows(NullPointerException.class, () -> new Note(3, "message", "from", null, System.currentTimeMillis(), 0));
+    }
+
+    @Test
+    void createNew() {
+        assertDoesNotThrow(() -> new Note(4, "message", "from", "to", System.currentTimeMillis(), 5));
+    }
+}