SingletonClass.java

public class SingletonClass {

    private static SingletonClass sSoleInstance;

    //private constructor.
    private SingletonClass(){
       
        //Prevent form the reflection api.
        if (sSoleInstance != null){
            throw new RuntimeException("Use getInstance() method to get the single instance of this class.");
        }
    } 

    public static SingletonClass getInstance(){
        if (sSoleInstance == null){ //if there is no instance available... create new one
            sSoleInstance = new SingletonClass();
        }

        return sSoleInstance;
    }
}

Singleton contract is breakable by reflection.
As sSoleInstance not getting initialized when instance is created from reflection only and eveytime it will be not null and new instance created.

https://medium.com/@kevalpatel2106/how-to-make-the-perfect-singleton-de6b951dfdb0