diff -r 627e1c99bcf1 -r a95423d37588 src/test/java/de/uapcore/sudoku/DocumentHandlerTest.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/test/java/de/uapcore/sudoku/DocumentHandlerTest.java	Tue Jul 28 13:48:59 2020 +0200
@@ -0,0 +1,96 @@
+package de.uapcore.sudoku;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.nio.file.Files;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class DocumentHandlerTest {
+
+    @Test
+    void testFilenameManagement() {
+        final var dh = new DocumentHandler();
+        assertFalse(dh.isFilenameSet());
+        dh.setFilename("foo");
+        assertTrue(dh.isFilenameSet());
+        dh.clearFilename();
+        assertFalse(dh.isFilenameSet());
+    }
+
+    private File extractFile(String name) throws IOException {
+        final var f = File.createTempFile("sudoku-test", ".txt");
+        try (var in = DocumentHandlerTest.class.getClassLoader().getResourceAsStream(name);
+             var out = new FileOutputStream(f)) {
+            in.transferTo(out);
+        }
+        return f;
+    }
+
+    private int[] createTestdata() {
+        return new int[]{0,1,0,9,0,0,8,0,0,
+                0,0,0,0,0,8,0,0,4,
+                6,0,5,0,0,0,7,0,0,
+                0,9,0,0,6,0,0,0,8,
+                0,0,0,2,0,7,0,0,0,
+                8,0,0,0,3,0,0,6,0,
+                0,0,2,0,0,0,5,0,3,
+                1,0,0,4,0,0,0,0,0,
+                0,0,6,0,0,2,0,1,0};
+    }
+
+    @Test
+    void load() throws IOException {
+        final var f = extractFile("test-sudoku");
+        final var dh = new DocumentHandler();
+        final var field = new Field();
+        dh.setFilename(f.getAbsolutePath());
+        dh.load(field);
+
+        final var expected = createTestdata();
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                assertEquals(expected[x+9*y], field.getCellValue(x, y));
+            }
+        }
+    }
+
+    @Test
+    void save() throws IOException {
+        final var f = File.createTempFile("sudoku-test", ".txt");
+        final var dh = new DocumentHandler();
+
+        final var field = new Field();
+        final var testdata = createTestdata();
+        for (int x = 0 ; x < 9 ; x++) {
+            for (int y = 0 ; y < 9 ; y++) {
+                field.setCellValue(x, y, testdata[x+9*y]);
+            }
+        }
+
+        dh.setFilename(f.getAbsolutePath());
+        dh.save(field);
+
+        final String[] expected = {
+                "_ 1 _ 9 _ _ 8 _ _",
+                "_ _ _ _ _ 8 _ _ 4",
+                "6 _ 5 _ _ _ 7 _ _",
+                "_ 9 _ _ 6 _ _ _ 8",
+                "_ _ _ 2 _ 7 _ _ _",
+                "8 _ _ _ 3 _ _ 6 _",
+                "_ _ 2 _ _ _ 5 _ 3",
+                "1 _ _ 4 _ _ _ _ _",
+                "_ _ 6 _ _ 2 _ 1 _"
+        };
+
+        final var actual = Files.readAllLines(f.toPath());
+
+        assertEquals(expected.length, actual.size());
+        for (int i = 0 ; i < expected.length ; i++) {
+            assertEquals(expected[i], actual.get(i));
+        }
+    }
+}