exercicio1Singleton

Singleton.java

package exerciciosingleton;

import javax.swing.*;

public final class Singleton {
    private static Singleton singleton;
    
    public Singleton(){
       JOptionPane.showMessageDialog(null, "Objeto criado");
    }

public static Singleton getSingletonInstance(){
    if(singleton == null){
       singleton = new Singleton();
    }
    return singleton;
   }

}

testeSingleton.java

package exerciciosingleton;
import javax.swing.*;

public class ExercicioSingleton {
   
    public static void main(String[] args) {
       Singleton singleton1;
       Singleton singleton2;
       Singleton secondSingleton;
       
       singleton1 = Singleton.getSingletonInstance();
       singleton2 = Singleton.getSingletonInstance();
       Singleton singleton3 = new Singleton();
      
       if(singleton1==singleton2){
           JOptionPane.showMessageDialog(null,"singleton1 e singleton2 "+ "Estão usando a mesma instância de singleton");
        }
       
        System.err.println("Endereço - Singleton 1 " + singleton1.hashCode());
        System.err.println("Endereço - Singleton 2 " + singleton2.hashCode());
        System.err.println("Endereço - Singleton 3 " + singleton3.hashCode());
               
    }
    
}