Include static factory method in design patterns (#49)

Author: mperor

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/factory/stat/method/Category.java

@@ -0,0 +1,18 @@
+package pl.mperor.lab.java.design.pattern.creational.factory.stat.method;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Category {
+    private static final Map<String, Category> CACHE = new HashMap<>();
+
+    private final String name;
+
+    private Category(String name) {
+        this.name = name;
+    }
+
+    static Category of(String name) {
+        return CACHE.computeIfAbsent(name.toLowerCase(), Category::new);
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/creational/factory/stat/method/Pair.java

@@ -0,0 +1,28 @@
+package pl.mperor.lab.java.design.pattern.creational.factory.stat.method;
+
+import java.util.Map;
+
+record Pair<L, R>(L left, R right) {
+
+    private static final Pair EMPTY = new Pair<>(null, null);
+
+    static <L, R> Pair<L, R> of(L left, R right) {
+        return new Pair<>(left, right);
+    }
+
+    static <L, R> Pair<L, R> empty() {
+        return EMPTY;
+    }
+
+    static <L, R> Pair<L, R> left(L left) {
+        return of(left, null);
+    }
+
+    static <L, R> Pair<L, R> right(R right) {
+        return of(null, right);
+    }
+
+    static <L, R> Pair<L, R> fromMapEntry(Map.Entry<L, R> mapEntry) {
+        return of(mapEntry.getKey(), mapEntry.getValue());
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/creational/factory/stat/method/StaticFactoryMethodTest.java

@@ -0,0 +1,35 @@
+package pl.mperor.lab.java.design.pattern.creational.factory.stat.method;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.Map;
+
+public class StaticFactoryMethodTest {
+
+    @Test
+    public void testClearNaming() {
+        Assertions.assertEquals(new Pair<>("one", "two"), Pair.of("one", "two"));
+
+        Assertions.assertNull(Pair.empty().left());
+        Assertions.assertNull(Pair.empty().right());
+
+        var onlyLeft = Pair.left("Only left");
+        Assertions.assertNotNull(onlyLeft.left());
+        Assertions.assertNull(onlyLeft.right());
+    }
+
+    @Test
+    public void testTypeConversion() {
+        var entry = Map.entry(1, 2);
+        var pair = Pair.fromMapEntry(entry);
+        Assertions.assertEquals(pair.left(), entry.getKey());
+        Assertions.assertEquals(pair.right(), entry.getValue());
+    }
+
+    @Test
+    public void testOptimizedObjectReuseAkaCaching() {
+        Assertions.assertSame(Pair.empty(), Pair.empty());
+        Assertions.assertSame(Category.of("Knowledge"), Category.of("Knowledge"));
+    }
+}