BAEL-9391: Abstract Method With Variable List of Arguments (#18805)

Author: samuelnjoki29

core-java-modules/core-java-lang-8/src/main/java/com/baeldung/abstractmethodvarargs/Book.java

@@ -0,0 +1,16 @@
+package com.example.items;
+
+/** A Book item that doesn't need any external data when used. */
+public class Book extends Item<EmptyContext> {
+    private final String title;
+
+    public Book(String title) {
+        this.title = title;
+    }
+
+    @Override
+    public void use(EmptyContext ctx) {
+        // ctx is always EmptyContext.INSTANCE; no data to read from it.
+        System.out.println("You read the book: " + title);
+    }
+}

core-java-modules/core-java-lang-8/src/main/java/com/baeldung/abstractmethodvarargs/EmptyContext.java

@@ -0,0 +1,9 @@
+package com.example.items;
+
+/**
+ * A tiny singleton context object used when no arguments are needed.
+ */
+public final class EmptyContext {
+    public static final EmptyContext INSTANCE = new EmptyContext();
+    private EmptyContext() {}
+}

core-java-modules/core-java-lang-8/src/main/java/com/baeldung/abstractmethodvarargs/Item.java

@@ -0,0 +1,15 @@
+package com.example.items;
+
+/**
+ * Generic abstract base class for items.
+ *
+ * @param <C> the type of the context object that contains parameters
+ *            needed to "use" the item.
+ */
+public abstract class Item<C> {
+    /**
+     * Use the item with a typed context object.
+     * Subclasses will specify C and implement this method.
+     */
+    public abstract void use(C context);
+}

core-java-modules/core-java-lang-8/src/main/java/com/baeldung/abstractmethodvarargs/Key.java

@@ -0,0 +1,16 @@
+package com.example.items;
+
+/** A Key item that needs a KeyContext to operate. */
+public class Key extends Item<KeyContext> {
+    private final String name;
+
+    public Key(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public void use(KeyContext ctx) {
+        System.out.println("Using key '" + name + "' on door: " + ctx.getDoorId());
+        System.out.println("Doors remaining in queue: " + ctx.getDoorsQueue().size());
+    }
+}

core-java-modules/core-java-lang-8/src/main/java/com/baeldung/abstractmethodvarargs/KeyContext.java

@@ -0,0 +1,25 @@
+package com.example.items;
+
+import java.util.Queue;
+
+/**
+ * Context data for Key items.
+ * Immutable fields with getters give clear intent and type safety.
+ */
+public final class KeyContext {
+    private final String doorId;
+    private final Queue<String> doorsQueue;
+
+    public KeyContext(String doorId, Queue<String> doorsQueue) {
+        this.doorId = doorId;
+        this.doorsQueue = doorsQueue;
+    }
+
+    public String getDoorId() {
+        return doorId;
+    }
+
+    public Queue<String> getDoorsQueue() {
+        return doorsQueue;
+    }
+}

core-java-modules/core-java-lang-8/src/test/java/com/baeldung/abstractmethodvarargs/BookUnitTest.java

@@ -0,0 +1,15 @@
+package com.example.items;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+class BookUnitTest {
+
+    @Test
+    void givenBook_whenUseWithEmptyContext_thenNoException() {
+        Book book = new Book("The Hobbit");
+
+        assertDoesNotThrow(() -> book.use(EmptyContext.INSTANCE));
+    }
+}

core-java-modules/core-java-lang-8/src/test/java/com/baeldung/abstractmethodvarargs/KeyUnitTest.java

@@ -0,0 +1,22 @@
+package com.example.items;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import java.util.Queue;
+import java.util.LinkedList;
+
+public class KeyUnitTest {
+
+    @Test
+    void givenKey_whenUseWithKeyContext_thenNoException() {
+        Queue<String> doors = new LinkedList<>();
+        doors.add("front-door");
+        doors.add("back-door");
+
+        Key key = new Key("MasterKey");
+        KeyContext ctx = new KeyContext("front-door", doors);
+
+        assertDoesNotThrow(() -> key.use(ctx));
+    }
+}