diff --git a/app/src/test/java/ReadFileTest.java b/app/src/test/java/ReadFileTest.java
new file mode 100644
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+++ b/app/src/test/java/ReadFileTest.java
@@ -0,0 +1,49 @@
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@DisplayName("ReadFile class functions test")
+class ReadFileTest {
+
+	@TempDir
+	static Path tempDir;
+
+	@Nested
+	@DisplayName("Testing ReadFile `open()` function")
+	class OpenTest {
+		
+		@Test
+		@DisplayName("Empty file name in `filename` parameter")
+		void emptyFileName() {
+			assertThrowsExactly(IllegalArgumentException.class, () -> ReadFile.open(""));
+		}
+
+		@Test
+		@DisplayName("Empty file name in `filename` parameter")
+		void invalidFileExtension() throws IOException {
+			// Create files with wrong file extensions
+			Files.createFile(tempDir.resolve("emails.csv"));
+			Files.createFile(tempDir.resolve("emails"));
+			Files.createFile(tempDir.resolve("emails.xlsx"));
+			Files.createFile(tempDir.resolve("emails.db"));
+
+			assertThrowsExactly(IllegalArgumentException.class, () -> ReadFile.open("emails.csv"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> ReadFile.open("emails"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> ReadFile.open("emails.xlsx"));
+			assertThrowsExactly(IllegalArgumentException.class, () -> ReadFile.open("emails.db"));
+		}
+		
+		@Test
+		@DisplayName("Trying to open non-existent file")
+		void nonExistentFile() {
+			assertThrowsExactly(RuntimeException.class, () -> ReadFile.open("doesNotExist.txt"));
+		}
+	}
+}
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