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public class UrgSingleton {
    private static UrgSingleton uniqueInstance = new UrgSingleton();
    private UrgSingleton() {}

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

public class DoubleSynSingleton {
    private static volatile DouleSynSingleton uniqueInstance;

    private DoubleSynSingleton() {}
    
    public static DoubleSynSingleton getInstance() {
        if (uniqueInstance == null) {
            synchronized(DoubleSynSingleton.class) {
                if(uniqueInstance == null)
                    uniqueInstance = new DoubleSynSingleton();
            }
        }
        return uniqueInstance;
    }
}