[file-is-empty] Check If a File is Empty in Java (#14319)

* [file-is-empty] Check If a File is Empty in Java

* [file-is-empty] adjust code format

* [file-is-empty] rename test methods

* [file-is-empty] Then->then in method names

core-java-modules/core-java-io-apis-3/src/test/java/com/baeldung/emptyfile/CheckFileIsEmptyUnitTest.java

@@ -0,0 +1,71 @@
+package com.baeldung.emptyfile;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.NoSuchFileException;
+import java.nio.file.Path;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+public class CheckFileIsEmptyUnitTest {
+    @Test
+    void whenTheFileIsEmpty_thenFileLengthIsZero(@TempDir Path tempDir) throws IOException {
+        File emptyFile = tempDir.resolve("an-empty-file.txt")
+          .toFile();
+        emptyFile.createNewFile();
+        assertTrue(emptyFile.exists());
+        assertEquals(0, emptyFile.length());
+    }
+
+    @Test
+    void whenFileDoesNotExist_thenFileLengthIsZero(@TempDir Path tempDir) {
+        File aNewFile = tempDir.resolve("a-new-file.txt")
+          .toFile();
+        assertFalse(aNewFile.exists());
+        assertEquals(0, aNewFile.length());
+    }
+
+    boolean isFileEmpty(File file) {
+        if (!file.exists()) {
+            throw new IllegalArgumentException("Cannot check the file length. The file is not found: " + file.getAbsolutePath());
+        }
+        return file.length() == 0;
+    }
+
+    @Test
+    void whenTheFileDoesNotExist_thenIsFilesEmptyThrowsException(@TempDir Path tempDir) {
+        File aNewFile = tempDir.resolve("a-new-file.txt")
+          .toFile();
+        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () -> isFileEmpty(aNewFile));
+        assertEquals(ex.getMessage(), "Cannot check the file length. The file is not found: " + aNewFile.getAbsolutePath());
+    }
+
+    @Test
+    void whenTheFileIsEmpty_thenIsFilesEmptyReturnsTrue(@TempDir Path tempDir) throws IOException {
+        File emptyFile = tempDir.resolve("an-empty-file.txt")
+          .toFile();
+        emptyFile.createNewFile();
+        assertTrue(isFileEmpty(emptyFile));
+
+    }
+
+    @Test
+    void whenTheFileIsEmpty_thenFilesSizeReturnsTrue(@TempDir Path tempDir) throws IOException {
+        Path emptyFilePath = tempDir.resolve("an-empty-file.txt");
+        Files.createFile(emptyFilePath);
+        assertEquals(0, Files.size(emptyFilePath));
+    }
+
+    @Test
+    void whenTheFileDoesNotExist_thenFilesSizeThrowsException(@TempDir Path tempDir) {
+        Path aNewFilePath = tempDir.resolve("a-new-file.txt");
+        assertThrows(NoSuchFileException.class, () -> Files.size(aNewFilePath));
+    }
+}