#10 effective java: item 4

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/Elvis.java

@@ -1,5 +1,7 @@
 package com.example.effectivejava.chapter01.item03.staticfactory;
 
+import java.util.function.Supplier;
+
 // 코드 3-2 정적 팩터리 방식의 싱글턴 (24쪽)
 public class Elvis implements Singer {
     private static final Elvis INSTANCE = new Elvis();
@@ -15,6 +17,8 @@ public class Elvis implements Singer {
         Elvis elvis = Elvis.getInstance();
         elvis.leaveTheBuilding();
 
+        Supplier<Elvis> supplier = Elvis::getInstance;
+
         System.out.println(Elvis.getInstance());
         System.out.println(Elvis.getInstance());
     }

effective-java/src/main/java/com/example/effectivejava/chapter01/item03/staticfactory/MetaElvis.java

@@ -7,7 +7,6 @@ public class MetaElvis<T> {
 
     private MetaElvis() { }
 
-    @SuppressWarnings("unchecked")
     public static <E> MetaElvis<E> getInstance() { return (MetaElvis<E>) INSTANCE; }
 
     public void say(T t) {

effective-java/src/main/java/com/example/effectivejava/chapter01/item04/AbstractUtilityClass.java

@@ -0,0 +1,17 @@
+package com.example.effectivejava.chapter01.item04;
+
+public abstract class AbstractUtilityClass {
+
+    public AbstractUtilityClass() {
+        System.out.println("constructor");
+    }
+
+    public static String hello() {
+        return "hello";
+    }
+
+    public static void main(String[] args) {
+        String hello = AbstractUtilityClass.hello();
+
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item04/DefaultUtilityClass.java

@@ -0,0 +1,9 @@
+package com.example.effectivejava.chapter01.item04;
+
+public class DefaultUtilityClass extends AbstractUtilityClass {
+
+    public static void main(String[] args) {
+        DefaultUtilityClass utilityClass = new DefaultUtilityClass();
+        utilityClass.hello();
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item04/UtilityClass.java

@@ -0,0 +1,24 @@
+package com.example.effectivejava.chapter01.item04;
+
+public class UtilityClass {
+
+    /**
+     * 이 클래스는 인스턴스를 만들 수 없습니다.
+     */
+    private UtilityClass() {
+        throw new AssertionError();
+    }
+
+    public static String hello() {
+        return "hello";
+    }
+
+    public static void main(String[] args) {
+        String hello = UtilityClass.hello();
+
+        UtilityClass utilityClass = new UtilityClass();
+        utilityClass.hello();
+
+
+    }
+}