add singleton design pattern

README.md

@@ -41,5 +41,3 @@ if (1 ^ n == n + 1) {
 ```
 ---  
 
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src/main/java/org/example/SingletonClass.java

@@ -0,0 +1,114 @@
+package org.example;
+
+/**
+ * 7种单例设计模式
+ * 1.饿汉式
+ * 2.懒汉式
+ * 3.懒汉式+同步方法
+ * 4.Double check
+ * 5.Holder 方式
+ * 6.Enum 方式
+ * 7.Enum Holder
+ */
+public class SingletonClass {
+    //懒汉式单例
+    public static class StarvingSingleton {
+        private static final StarvingSingleton INSTANCE = new StarvingSingleton();
+
+        private StarvingSingleton(){}
+
+        public static StarvingSingleton getInstance(){
+            return INSTANCE;
+        }
+    }
+
+    //懒汉式单例
+    public static class LazyBoneSingleton{
+        private static LazyBoneSingleton INSTANCE;
+
+        private LazyBoneSingleton(){}
+
+        public static LazyBoneSingleton getInstance(){
+            if (INSTANCE == null){
+                INSTANCE = new LazyBoneSingleton();
+            }
+            return INSTANCE;
+        }
+    }
+
+    //懒汉式单例+同步方法
+    public static class LazyBoneSynchronizedSingleton{
+        private static LazyBoneSynchronizedSingleton INSTANCE;
+
+        private LazyBoneSynchronizedSingleton(){}
+
+        public static synchronized LazyBoneSynchronizedSingleton getInstance(){
+            if (null == INSTANCE){
+                INSTANCE = new LazyBoneSynchronizedSingleton();
+            }
+            return INSTANCE;
+        }
+    }
+
+    //Double-check
+    public static class DoubleCheckSingleton{
+        private volatile static DoubleCheckSingleton INSTANCE ;
+
+        private DoubleCheckSingleton(){}
+
+        public static DoubleCheckSingleton getInstance(){
+            if (null == INSTANCE){
+                synchronized (DoubleCheckSingleton.class){
+                    if (null == INSTANCE ){
+                        INSTANCE = new DoubleCheckSingleton();
+                    }
+                }
+            }
+            return INSTANCE;
+        }
+    }
+
+    //Holder
+    public static  class  HolderSingleton{
+        private  HolderSingleton(){}
+
+        private static class Holder{
+            private  static HolderSingleton instance = new HolderSingleton();
+
+            public static HolderSingleton getInstance(){
+                return Holder.instance;
+            }
+        }
+    }
+
+    //Enum
+    public enum EnumSingleton{
+        INSTANCE;
+
+        public static EnumSingleton getInstance(){
+            return INSTANCE;
+        }
+    }
+
+    //Enum Holder
+    public static class EnumHolderSingleton{
+        private EnumHolderSingleton(){}
+
+        private enum EnumHolder{
+            INSTANCE;
+            private EnumHolderSingleton instance;
+
+            EnumHolder(){
+                this.instance = new EnumHolderSingleton();
+            }
+
+            private EnumHolderSingleton getInstance(){
+                return instance;
+            }
+        }
+
+        public static EnumHolderSingleton getInstance(){
+            return EnumHolder.INSTANCE.getInstance();
+        }
+    }
+}