private构造器和单例模式

// hiding/Lunch.java
// Demonstrates class access specifiers. Make a class
// effectively private with private constructors:

class Soup1 {
    private Soup1() {}

    public static Soup1 makeSoup() { // [1]
        return new Soup1();
    }
}

class Soup2 {
    private Soup2() {}

    private static Soup2 ps1 = new Soup2(); // [2]

    public static Soup2 access() {
        return ps1;
    }

    public void f() {}
}
// Only one public class allowed per file:
public class Lunch {
    void testPrivate() {
        // Can't do this! Private constructor:
        //- Soup1 soup = new Soup1();
    }

    void testStatic() {
        Soup1 soup = Soup1.makeSoup();
    }

    void testSingleton() {
        Soup2.access().f();
    }
}

为什么private构造方法就不能在别的类new对象了，但是可以在自身类new，原因就是private方法是类级访问的。这就是单例的大部分思路，下面的链接是我之前总结的几种单例实现。

https://juejin.im/post/5cd186dcf265da034e7ea3a2