Java，就像flliows一样，为什么程序的全局变量仅保留一个实例？

Question

Could the uniqueeInstance have one and only one instance?

public class A {

     private static A uniqueInstance = new A();

     private A() {}

     public static A getInstance() {
            return uniqueInstance;
     }
}

Answer 1

This is a Singleton pattern, it's purpose is to have only one possible instance for a class.

This is why you have a private constructor , so that no other class can attempt to instantiate it directly.

Here are more elaborate thoughts for possible uses of a Singleton :

When to use the Singleton

Answer 2

It is not guaranteed.

By reflection you are easy to get more instances.

The only way to guarantee that you have exactly one instance is to use an enum:

enum Holder {

    INSTANCE;

    //Keep in Mind of A you may still have more instances. if you want to have the
    //guarantee to have only one instance you may need merge the whole class
    //into an enum (which may not be possible)
    public A uniqueInstance = new A();    
}

Other ways like throwing an exception in the constructor are generally also possible. But not completely secure since there are ways to create an Object without calling any constructor.