单例模式

单例模式确保一个类只创建一个对象，不需要实例化类的对象，提供了一个访问实例的方法。

故构造函数必须是私有的，且有一个返回类型是实例本身的方法

代码演示：

# 创建一个Singleton类

SingleObject.java

饿汉式

SingleObject.java

public class SingleObject {

   //create an object of SingleObject
   private static SingleObject instance = new SingleObject();

   //make the constructor private so that this class cannot be
   //instantiated
   private SingleObject(){}

   //Get the only object available
   public static SingleObject getInstance(){
      return instance;
   }

   public void showMessage(){
      System.out.println("Hello World!");
   }
}

懒汉式

public class SingleObject {

   //create an object of SingleObject
   private static SingleObject instance;

   //make the constructor private so that this class cannot be
   //instantiated
   private SingleObject(){}

   //Get the only object available
   public static SingleObject getInstance(){
       if(instance == null){
           instance = new SingleObject();
           return instance;
       }
       return instance;
   }

   public void showMessage(){
      System.out.println("Hello World!");
   }
}

# 测试并验证输出

从单例类中获取唯一的对象

SingletonPatternDemo.java

public class SingletonPatternDemo {
   public static void main(String[] args) {

      //Get the only object available
      SingleObject object = SingleObject.getInstance();

      //show the message
      object.showMessage();
   }
}

输出结果如下：

Hello World!