Talk about single case mode

java Use a class in , Instantiate only once
The answer is ： The singleton pattern .

The specific code is
public class A{
    private A(){}// Set constructor to private, Other classes can no longer call instantiation
    private static A instance=new A();// The only instance , add static attribute
    public static A getInstance(){// Other class calls A The only access to , It's also static attribute , The calling method is A.getInstance()
        return instance;
    }
}

advantage ：

First of all 、 Control the use of resources , Control the concurrent access of resources through thread synchronization ;

second 、 Control the number of instances generated , To save resources .

Third 、 To use as a medium of communication , That is data sharing , It can be done without a direct connection , Allow multiple unrelated threads or processes to communicate with each other .