JAVA学习笔记(十七)单例

先封装

/**目标&＃xff1a;学会使用恶汉单例模式设计单例类*/
public class SingleInstance1 {/**static修饰的成员变量&＃xff0c;静态成员变量&＃xff0c;加载一次&＃xff0c;只有一份*/// public static int onLineNumber &＃61; 21;public static SingleInstance1 instance &＃61; new SingleInstance1();/**1、必须私有构造器&＃xff1a;私有构造器对外不能被访问。*/private SingleInstance1(){}}

public class Test {public static void main(String[] args) {
// SingleInstance1 s1 &＃61; new SingleInstance1();
// SingleInstance1 s2 &＃61; new SingleInstance1();
// SingleInstance1 s3 &＃61; new SingleInstance1();SingleInstance1 s1 &＃61; SingleInstance1.instance;SingleInstance1 s2 &＃61; SingleInstance1.instance;SingleInstance1 s3 &＃61; SingleInstance1.instance;System.out.println(s1);System.out.println(s2);System.out.println(s3);System.out.println(s1 &＃61;&＃61; s2);}
}

先封装

/**目标&＃xff1a;设计懒汉单例*/
public class SingleInstance2 {/**2、定义一个静态的成员变量用于存储一个对象&＃xff0c;一开始不要初始化对象&＃xff0c;因为人家是懒汉*/private static SingleInstance2 instance;/**1、私有构造器啊*/private SingleInstance2(){}/**3、提供一个方法暴露&＃xff0c;真正调用这个方法的时候才创建一个单例对象*/public static SingleInstance2 getInstance(){if(instance &＃61;&＃61; null){// 第一次来拿对象&＃xff0c;为他做一个对象instance &＃61; new SingleInstance2();}return instance;}
}

public class Test2 {public static void main(String[] args) {// 得到一个对象SingleInstance2 s1 &＃61; SingleInstance2.getInstance();SingleInstance2 s2 &＃61; SingleInstance2.getInstance();System.out.println(s1 &＃61;&＃61; s2);}
}