创建一个类
public class NewObject {} 测试
public class NewOne {public static void main(String[] args) {NewObject no1= new NewObject();NewObject no2= new NewObject();System.out.println("no1--->"+no1);System.out.println("no2--->"+no2);}
}

public class Singleton {/* * 懒汉式单例:只有在第一次请求实例的时候创建,并且只在第一次创建后,以后不再创建该类的实例 */  // 1.一个私有的指向自己的静态变量  private static Singleton instance;  // 2.私有的构造方法,保证不能从外部创建对象  private Singleton(){}  // 3.公开的静态工厂方法,返回该类的唯一实例(当发现没有实例没有初始化的时候才初始化)  public static Singleton getInstance(){  if(instance == null){  instance = new Singleton();  System.out.println("创建Singleton类的实例");  }else {  System.out.println("实例已经创建,不能再创建");  }  return instance;  }  }  

public class Test {public static void main(String[] args) {  Singleton s = Singleton.getInstance();  System.out.println("第一次---->"+s);  System.out.println("第二次---->"+Singleton.getInstance());  System.out.println("第三次---->"+Singleton.getInstance());  }
}

public class Singleton2 {  /* * 恶汉单例模式:在类加载的时候就创建一个单例模式. */  // 1.私有的构造函数,确保不能在类的外部访问该类的构造函数  private Singleton2(){  System.out.println("构造函数执行了");  }  // 2.私有的唯一的静态实例变量,在类加载的时候就创建好单例对象  private final static Singleton2 instance = new Singleton2();  // 3.公开的静态工厂返回此类的唯一实例  public static Singleton2 getInstance(){  return instance;  }
}  

public class Test {public static void main(String[] args) {     Singleton2 s1 = Singleton2.getInstance();  System.out.println(s1);  System.out.println(Singleton2.getInstance());  System.out.println(Singleton2.getInstance());  }
}   

package com.单列模式;public class Singleton3 {private static Singleton3 instance=null;private Singleton3(){System.out.println("构造函数开始！");}public static synchronized Singleton3 getInstance(){if(instance==null){synchronized(Singleton3.class){if(instance==null){instance=new Singleton3();}}}return instance;}
}

public class Test {public static void main(String[] args) {  Singleton3 s3 =Singleton3.getInstance();System.out.println(s3);System.out.println(Singleton3.getInstance());System.out.println(Singleton3.getInstance());}
}