java - 在java类中创建实例

Question

请告诉我两种java构造函数声明方式的区别

  public class A{

    private static A instance = new A();
    public static A getInstance() { return instance;
        }
    public static void main(String[] args) {
          A a= A.getInstance();
        }
 }

和

    public class B{
    public B(){};


     public static void main(String[] args) {
     B b= new B();
     }
}

谢谢

Answer 1

• 类 A 应该是 Singleton ，其中您只能拥有 A 的一个实例。您可以通过调用 getInstance();
检索该单个实例

In software engineering, the singleton pattern is a design pattern used to implement the mathematical concept of a singleton, by restricting the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.

根据您的要求，有几种方法可以解决此问题:

public class A{
    private static A instance = new A();
    private A(){} // private constructor
    public static A getInstance() {return instance;}
}

或者直到第一次调用才创建实例

public class A{
    private static A instance = null;
    private A(){} // private constructor
    public static A getInstance() {
        if(instance == null){
           instance = new A(); // create the one instance.
        }
        return instance;
    }
}

• Class B 是一个带有无参数构造函数的类。您可以通过调用 new B();
创建任意数量的 B 实例