Add memento design pattern with test scenario (#57)

Author: mperor

DesignPatterns/README.md

@@ -12,6 +12,7 @@ with practical examples and best practices for using design patterns to create r
 - [Chain of Responsibility](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/chain/of/responsibility) 🔗
 - [Command](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/command) 📝
 - [Execute Around Method (EAM)](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/eam) ⭕
+- [Memento](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/memento) 💾
 - [Observer](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer) 👀
 - [State](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state) 📜
 - [Strategy](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/strategy) 🎯

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/memento/History.java

@@ -0,0 +1,19 @@
+package pl.mperor.lab.java.design.pattern.behavioral.memento;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+class History {
+
+    private final Deque<TextMemento> history = new ArrayDeque<>();
+
+    void save(TextEditor editor) {
+        history.push(editor.save());
+    }
+
+    void undo(TextEditor editor) {
+        if (!history.isEmpty()) {
+            editor.restore(history.pop());
+        }
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/memento/TextEditor.java

@@ -0,0 +1,23 @@
+package pl.mperor.lab.java.design.pattern.behavioral.memento;
+
+class TextEditor {
+
+    private StringBuilder content = new StringBuilder();
+
+    TextEditor write(String text) {
+        content.append(text);
+        return this;
+    }
+
+    TextMemento save() {
+        return new TextMemento(content.toString());
+    }
+
+    void restore(TextMemento memento) {
+        content = new StringBuilder(memento.content());
+    }
+
+    String getContent() {
+        return content.toString();
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/memento/TextMemento.java

@@ -0,0 +1,4 @@
+package pl.mperor.lab.java.design.pattern.behavioral.memento;
+
+record TextMemento(String content) {
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/memento/TextEditorMementoTest.java

@@ -0,0 +1,23 @@
+package pl.mperor.lab.java.design.pattern.behavioral.memento;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class TextEditorMementoTest {
+
+    @Test
+    public void testSavingAndRestoringWithTextMemento() {
+        var editor = new TextEditor();
+        var history = new History();
+
+        editor.write("⭐");
+        Assertions.assertEquals("⭐", editor.getContent());
+        history.save(editor);
+
+        editor.write("1️⃣").write("2️⃣").write("3️⃣");
+        Assertions.assertEquals("⭐1️⃣2️⃣3️⃣", editor.getContent());
+
+        history.undo(editor);
+        Assertions.assertEquals("⭐", editor.getContent());
+    }
+}